NCHurricane2009's Blog

2012 Atlantic Hurricane Season Birdseye Discussion #120

By: NCHurricane2009, 5:47 PM GMT on September 29, 2012

...SEPTEMBER 29 2012...1:50 PM EDT...
An outage persists with GOES-E satellite imagery in the last five and a half days. GOES-W has been extended to cover much of the view in the two birdseye charts below. However...the east edge of the temporary GOES-W scan has a bias for showing cold cloud tops that are not actually present. Therefore...I have patched the east side of the atmospheric birdseye chart with Meteosat-9 grafts. The east side of the thermodynamics birdseye chart is left unrepaired...so be mindful that the moisture content on the east side of this chart has a positive bias due to the false illusion of cold cloud tops.

Tropical Storm Nadine became a hurricane yesterday...subsequently weakening back to a strong tropical storm in the early morning. Even though I was previously forecasting Nadine to begin weakening today (special update #119A and full discussion #119)...the tenacious cyclone has instead regained hurricane strength as of 11 AM EDT. The cyclone is expected to track slowly north in the Atlantic subtropics for the next few days. See Nadine special feature section below for details.

As paragraph P1 in the mid-latitudes discussion highlights...the next upper trough in the mid-latitude westerlies is driving the formation of an impressive surface frontal low over Texas. In the next 12 hours...weather from this feature will begin spreading into the Gulf of Mexico...but no tropical development is expected. However...similar to Invest 93-L in discussion #110...this system for the next days could be capable of significant weather across the US Gulf coast...SE US...and along the east US coast. See details of 93-L in discussion #110 for the kinds of impacts a system like this could deliver.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0729Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE NADINE...
Nadine's previous intensification episode ended when the east side of the paragraph P6 C Atlc upper trough imparted southerly vertical shear across the storm...causing her to weaken to a strong tropical storm this morning. I was previously expecting the shear to continue until dissipation (my previous dissipation time was 11 PM Mon). Clearly in Figure 1...I am now keeping the cyclone alive well past 11 PM Mon...because of her recent re-intensification into hurricane strength at 11 AM EDT this morning. Despite being over waters less than 26 deg C...satellite imagery in Figure 2 shows Nadine with impressive t-storms at her core with a small eye. Her more symmetrical satellite appearance suggests the shear is reducing as her latent heat release is causing the shearing C Altc upper trough to amplify into an upper vortex. The paragraph P3 upper trough is about to pass to the north and NE of Nadine. This suggests enhanced upper outflow channels into the developing upper vortex to the SW and upper trough to the NE in the next 24 hours. This extremely favorable upper wind configuration causes me to forecast bullish strengthening into a strong category 1 hurricane in the next 24 hrs...quiet more bullish than the 11 AM EDT NHC forecast. The last time we saw rapid intensification at this location was category 2 Hurricane Gordon during discussion #82...but that was over waters above 26 deg C. With Nadine being above waters cooler than 26 deg C...I prefer to stay away from suggesting category 2 strength.

The 12Z GFS model shows Nadine's favorable upper outflow becoming squashed out by rather hostile...zonal shearing upper westerlies on the SW side of the paragraph P3 upper trough by 60 hours (11 PM Mon)...so I forecast a brisk weakening rate by that time. I slow the weakening rate by 84 hours (11 PM Tue) as the GFS shows a shortwave upper ridge ahead of the paragraph P2 weather system coming in from the NW...so I speculate that the upper ridge would reduce shear...and or Nadine taking some advantage of split flow upper divergence between the flow around the upper ridge and departing zonal upper westerlies. By the end of the forecast...I agree with the NHC that Nadine will become a non-tropical remnant low supported by the eastern divergence of the incoming paragraph P2 upper trough. I show no weakening between 11 PM Tue and 11 PM Wed due to the support from the upper trough's divergence.


Figure 1: My forecast for Hurricane Nadine generated early this afternoon.


Figure 2: Infrared satellite image of Hurricane Nadine taken during the late morning by Meteosat-9 (1200Z)

Nadine is currently located southwest of the Azores and further north than what my previous forecasts predicted. And given that Nadine's forward speed has doubled from 7 to 14 mph since I last forecasted 36 hrs ago...my updated forecast track in Figure 1 is a northward shift from my previous. Nadine is moving northward into a developing narrow ridge weakness associated with the 990 mb frontal cyclone in paragraph P3...between paragraph P4 ridge the NE and what is now the 1025 mb paragraph P3 ridge coming in from the NW. I agree with stalling the motion of Nadine by late Sunday and all of Monday as she becomes trapped between these two ridges. I do not agree with the GFS model's and NHC's SW drifting motion by that time...as such a motion in my opinion will be blocked by a third ridge to the west supported by the W convergence of paragraph P6 C Atlc upper trough. After the 1025 mb ridge passes by to the north...the NHC and models show Nadine reacting to a strong frontal cyclone supported by the incoming paragraph P2 upper trough. Because of the SW drift shown by the NHC and GFS model...Nadine is further south and hence travels more east in those solutions while getting caught on the south side of the frontal cyclone. Because I don't agree with the SW drift...my solution would have Nadine getting caught in the east side of the frontal cyclone...which would swing Nadine more north than east as I show in Figure 1.

Impact swath in Figure 1 is based on extrapolating the 11 AM EDT tropical storm wind radius along my forecast track...then shrinking the swath a bit to represent the weakening later in the forecast.

...MID-LATITUDES DISCUSSION...
P1...In the last 36 hours...deep-layered vortex over SE Wyoming has merged with next upper trough in the mid-latitude westerlies. This upper trough is already splitting...with the northern half driving a W Canada frontal cyclone (cyclone's warm front marked in top-center of above atmo chart) whose warm air advection is driving an upper anticyclone over Manitoba. Southern half of upper trough is over the western US...which has sheared off a great bulk of moisture from Eastern Pacific Tropical Cyclone Norman into the SW US upper ridge. Latent heat release from this moisture amplified the SW US upper ridge while the upper ridge has shifted across Texas and N Mexico. Along the front trailing from the NE US 1012 mb low (mentioned in paragraph P2)...split flow upper divergence between this amplified upper ridge and west US upper trough is driving a new and impressive frontal cyclone over Texas. With the immense moisture from Norman wrapped into this frontal cyclone...flood watches are posted across south Texas...spreading into Louisiana and Mississippi. For the next couple of days...potential exists for flood watches to continue spreading across the US Gulf coast region and SE US. Southerly flow ahead of the frontal cyclone will be in directional vertical shear with respect to westerly flow across the upper ridge...so if enough instability develops from daytime heating...severe t-storm and or tornado watches may also be required in the next couple of days.

P2...Shortwave upper trough moving across Hudson Bay in the previous discussion has become amplified to the east of the Manitoba upper anticyclone mentioned in paragraph P1 above. The upper trough is now moving into E Canada and the NE US. The surface cyclone it supported has dissipated. Instead...it has intensified the central US 1018 mb frontal depressions (paragraph P1 discussion #119) into a 1012 mb frontal depression that has moved into the NE US. Western convergence of this upper trough now supports a chunk of a former Great Lakes surface ridge (paragraph P3 discussion #119)...which are the 1021 to 1022 mb centers over the central US in the above atmo chart.

P3...Upper trough over eastern North America has finally shifted into the NW Atlantic and is headed eastward for the Atlantic high seas. The upper trough's eastern divergence continues supporting a frontal cyclone...which has moved offshore from Canada and past the south tip of Greenland while intensifying from 996 to 990 mb in the last 36 hours (it bottomed to 988 mb 12 hours ago). Split flow upper divergence between SW flow ahead of this upper trough and NW flow entering the central Atlantic upper trough (paragraph P6) supports a relatively new 1020 mb frontal depression in the NW Atlantic. The upper trough's western convergence supports a chunk of a former Great Lakes surface ridge (paragraph P3 discussion #119)...which are the 1025 mb centers over Atlantic Canada in the above atmo chart. Western convergence of this upper trough used to drive an eastern US surface ridge...but what is left of this surface ridge is now a 1017 mb center over the Florida panhandle now supported by eastern convergence of the Texas/N Mexico upper ridge in paragraph P1 above. To the east of that upper ridge...cut-off upper trough over the E Gulf to W Caribbean now stretches from coastal NC to the W Caribbean.

P4...Deep-layered ridge in the NE Atlantic...featuring a surface 1028 mb center over the Azores...persists.

P5...Amplified upper trough just offshore of Europe has finally exited the picture in the last 36 hours...with its eastern divergence supporting an impressive surface cyclone that has moved into Spain and Portugal.

...TROPICAL BELT DISCUSSION...
P6...Upper vorticity over the SE Caribbean...and upper ridge cell over the W Atlantic and Puerto Rico...have rapidly de-amplified into zonal upper westerlies across the entire Caribbean and W tropical Atlantic. De-amplification of the upper flow was due to coastal NC-to-W Caribbean cut-off upper trough (paragraph P3) pushing in. Central Atlantic upper trough persists SW of Hurricane Nadine...and its eastern divergence drove an impressive t-storm cluster and new surface trough in the central tropical Atlantic in the last 36 hours. However...SW vertical shear ahead of this upper trough will prevent this t-storm cluster from developing into a tropical cyclone. In relatively higher pressures southeast of this upper trough...upper ridge has built from the W coast of Africa as evidenced by upper-level cloud motions around paragraph P11 and P12 tropical waves.

P7...Surface trough in Bay of Campeche has dissipated in last 36 hours thanks to eastern convergence of paragraph P1 Texas/N Mexico upper ridge. This upper convergence supports a slot of dry air across the W edge of the Caribbean...SE Mexico...and northern Central America seen in the above thermo chart.

P8...Tropical wave moving across the central Caribbean Sea in the previous discussion is now in the W Caribbean. It is producing scattered t-storms over the central Caribbean....Jamaica...Cuba...and the E Bahamas thanks to eastern upper divergence of paragraph P3 coastal NC-to-W Caribbean upper trough. This same upper divergence mechanism is producing a new surface trough just north of the Bahamas.

P9...Surface trough NE of the Lesser Antilles in the previous discussion is now passing north of Puerto Rico...and is being affected by the immense shear of the W tropical Atlantic zonal upper westerlies mentioned in paragraph P6.

P10...Eastern Atlantic surface ridge just NW of the Cape Verde Islands has eroded and dissipated thanks to Nadine's low-level low pressure field to the north.

P11...Tropical wave W of the Cape Verde Islands is entering the tropical waters midway between the Cape Verdes and Lesser Antilles. It continues producing a t-storm cluster while enhanced by upper outflow of upper ridge building from W Africa mentioned in paragraph P6.

P12...Satellite imagery shows that a tropical wave with cyclonic turning and large amounts of t-storms has emerged from W Africa into the Atlantic tropics in the last 36 hours. NHC TAFB positioned this tropical wave just east of the Cape Verde Islands as of 0600Z TAFB this morning. Similar to the tropical wave in paragraph P11...this tropical wave is enhanced by upper outflow of upper ridge building from W Africa mentioned in paragraph P6.

2012 Atlantic Hurricane Season Birdseye Discussion #119A (Special Update)

By: NCHurricane2009, 9:30 AM GMT on September 28, 2012

...SEPTEMBER 28 2012...5:30 AM EDT...
This special update is written concerning Tropical Storm Nadine. Nadine strengthened while I was writing full discussion #119. Given the amount of recent strengthening...I have decided to upgrade my intensity forecast I just recently released in discussion #119. As such...here are my updated intensity forecast points:

5 AM Fri Sep 28 2012...70 mph...initial
11 PM Fri Sep 28 2012...75 mph...21 hr
11 PM Sat Sep 29 2012...65 mph...42 hr
11 PM Sun Sep 30 2012...50 mph...66 hr
11 PM Mon Oct 01 2012...40 mph (Dissipating)...90 hr

This means I now forecast Nadine to gain hurricane strength (75+ mph max winds) later today. I am making no changes to my Nadine track forecast at this time. If this verifies...Nadine will become a hurricane for the second time...as Nadine was previously a hurricane on September 15 and 16. Return to full discussion #119 for an assessment on the rest of the Atlantic tropics.

2012 Atlantic Hurricane Season Birdseye Discussion #119

By: NCHurricane2009, 8:42 AM GMT on September 28, 2012

...SEPTEMBER 28 2012...4:45 AM EDT...
An outage persists with GOES-E satellite imagery in the last 96 hours. GOES-W has been extended to cover much of the view in the two birdseye charts below. However...the east edge of the temporary GOES-W scan has a bias for showing cold cloud tops that are not actually present. Therefore...I have patched the east side of the atmospheric birdseye chart with Meteosat-9 grafts. The east side of the thermodynamics birdseye chart is left unrepaired...so be mindful that the moisture content on the east side of this chart has a positive bias due to the false illusion of cold cloud tops.

Tropical Storm Nadine expected to track erratically in the Atlantic subtropics for the next few days. See Nadine special feature section below for details.

Beginning in 48 hours...next upper trough in the mid-latitude westerlies (not yet in the scope of this discussion) is expected to produce high upper divergence with respect to SW US upper ridge in paragraph P1. This will drive the formation of a NE tracking surface low from the Gulf of Mexico...which could become quiet vigorous similar to Invest 93-L in discussion #110. Similar to 93-L...I expect this system to be non-tropical...but could still be capable of significant weather across the US Gulf coast...SE US...and along the east US coast. See details of 93-L in discussion #110 for the kinds of impacts a system like this could deliver.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1926Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
Nadine has strengthened just a tad more than my intensity forecast showed in special update #118A. Strengthening continued as she has developed favorable upper anticyclonic outflow...becoming enhanced to the NE by departing paragraph P5 upper trough. It was speculated a few times that the upper vortex just SW of Nadine (currently mentioned in paragraph P6) would enhance the outflow to the SW...but it appears this feature has always been too close to Nadine and instead is providing some outflow blockage to the SW as evidenced by the storm canopy and upper outflow bias to the NE half of the storm seen on infrared satellite. This outflow blockage is why I maintain my special update #118A intensity forecast...despite Nadine being currently a little stronger than that intensity forecast. My intensity forecast shows weakening after 11 PM Fri (tomorrow night)...as Nadine turns northward over even cooler waters...and as the paragraph P6 C Atlc upper trough merges with mid-latitude westerlies...hence turning eastward toward Nadine and imparting vertical shear across Nadine with its east side. My current intensity forecast (shown in Figure 1) is in agreement with the 11 PM EDT NHC's for the first 24 hrs...then my weakening rate is more aggressive than the NHC's after that time. In fact...I have Nadine becoming a remnant low by 96 hours while the NHC asserts Nadine being a tropical cyclone thru the next 120 hours due to forecast uncertainty in the longer range.


Figure 1: My forecast for Tropical Storm Nadine generated this early morning.

Nadine is currently located west of the Canary Islands and south of the Azores...and has near-perfectly followed the previous short-term NHC (and my) track forecast. Curiously though...the NHC has done a slight rightward shift in the short-term track...while I choose to copy-paste my previous forecast points since the previous track forecast is doing so well. I also do not agree that Nadine will turn northward quiet as sharp as the NHC currently shows because the paragraph P4 ridge will be too close to Nadine from the NE...based on how the 00Z GFS model shows this ridge in its surface-level output this morning. All of this means I have a slight left bias in my forecast track for a period as seen in Figure 1. Northward hook by the end of the forecast is related to evolution of current E Canada 996 mb frontal cyclone and 1026 mb low-level ridge over the Great Lakes (both mentioned in paragraph P3). The straight north track eventually happens as Nadine gets pulled into the 996 mb frontal cyclone's narrow ridge weakness...located between paragraph P4 ridge by then located to the NE and what is now the Great Lakes ridge coming in from the NW. My straight north track remains slower than the NHC's current...and slower than the even faster 00Z GFS. These faster north-track solutions do not make sense to me as the two ridges are shown to be of equal strength and opposing influence. I think the GFS sees Nadine becoming a non-tropical cyclone supported by the eastern divergence of incoming paragraph P6 C Altc upper trough...in which case Nadine indeed would move faster to the north (and then potentially east into the Azores) while being coupled to and traveling with this upper trough. My vote for now is to dissipate Nadine in intense shear generated by this upper trough while she becomes trapped between the two low-level ridges.

Impact swath in Figure 1 is based on extrapolating the 11 PM EDT tropical storm wind radius along my forecast track...then shrinking the swath away assuming Nadine follows my forecast for dissipation by 96 hrs. Impact statement (b) highlights that surf will linger on the shores of the Azores..due to the large wind radius of Nadine capable of stirring a large amount of water. However..the Azores surf this week will be less intense compared to last week...as Nadine should be further away from the islands this time around.

...MID-LATITUDES DISCUSSION...
P1...Cut-off upper vortex from Utah..mentioned in paragraph P1 of previous discussion #118...has become deep-layered while stacking with broad surface vorticity its eastern divergence supports. Deep-layered vortex is positioned over SE Wyoming this evening...with its eastern upper divergence supporting a 1017 mb surface low in the W Dakotas and 1018 mb central US frontal depressions located along cold front trailing from 996 mb frontal cyclone in paragraph P3. In relatively higher pressures SE of this deep-layered vortex...SW US upper ridge (paragraph P1 of previous discussion #118) persists.

P2...Shortwave upper trough...and surface 1010 mb frontal cyclone supported by the eastern divergence of the upper shortwave...has entered the above atmo birdseye chart from Hudson Bay.

P3...Weather system remains anchored by upper trough over eastern North America gradually shifting into the NW Atlantic. The upper trough's eastern divergence continues driving E Canada frontal depression (cyclone)...which has intensified from 1007 to 996 mb in the last 24 hours. Upper trough's western convergence is driving intensifying Great Lakes surface ridge...which has strengthened from 1022 to 1026 mb in the last 24 hours. Western convergence of this upper trough also drives eastern US and NW Atlantic low-level ridge (which is now 1022 mb). Cut-off upper trough over the central Gulf of Mexico has shifted into the eastern Gulf and W Caribbean.

P4...Deep-layered ridge in the Atlantic...featuring an impressive 1030 mb center...persists. This deep-layered ridge is moving into the NE Atlantic while passing north of Tropical Storm Nadine.

P5...Amplified upper trough remains just offshore of Europe.

...TROPICAL BELT DISCUSSION...
P6...Upper vorticity persists across the SE half of the Caribbean Sea. Upper ridge cell over W Atlantic...Cuba...and Bahamas has shifted east in advance of both upper troughs mentioned in paragraph P3...with the anticyclonic center of the cell now over Puerto Rico. Upper vortex just SW of Nadine (paragraph P5 discussion #118) has merged with upper vortex just NE of the Lesser Antilles (paragraph P3 discussion #118)...resulting in a central Atlantic upper trough. In relatively higher pressures southeast of this upper trough...upper ridge has built from the W coast of Africa as evidenced by upper-level cloud motions around paragraph P11 tropical wave.

P7...Surface troughing persists in Bay of Campeche...supported by split flow upper divergence between paragraph P3 east Gulf cut-off upper trough and paragraph P1 SW US upper ridge. Scattered t-storms across Central America are supported by this same upper divergence mechanism.

P8...Tropical wave continues across the central Caribbean Sea. It is producing scattered t-storms over the south-central Caribbean....Jamaica...and Cuba thanks to eastern upper divergence of paragraph P3 cut-off upper trough moving into the W Caribbean.

P9...Surface trough persists NE of the Lesser Antilles...which remains quiet enhanced by split flow upper divergence between the two merging upper vortices mentioned in paragraph P6.

P10...Eastern Atlantic surface ridge with a 1018 mb center persists NW of the Cape Verde Islands...albeit it has become eroded in its horizontal expanse thanks to Nadine's low-level low pressure field pushing in from the north. I currently speculate that this surface ridge is supported by upper convergence between northerlies from W Africa upper ridge (paragraph P6) and westerlies from paragraph P6 central Atlantic upper trough.

P11...Tropical wave W of the Cape Verde Islands continues producing a t-storm cluster while enhanced by upper outflow of upper ridge building from W Africa mentioned in paragraph P6.

2012 Atlantic Hurricane Season Birdseye Discussion #118A (Special Update)

By: NCHurricane2009, 10:45 AM GMT on September 27, 2012

...SEPTEMBER 27 2012...6:45 AM EDT...
This special update is written concerning Tropical Storm Nadine. Nadine strengthened while I was writing full discussion #118. In that full discussion...I had slightly downgraded my intensity forecast from what I showed in discussions #117 and #116 because it appears Nadine will feel vertical shear a little sooner than thought earlier. However...given the amount of recent strengthening...I have decided to return to the intensity forecasts I showed in discussions #117 and #116. As such...here are my updated intensity forecast points:

5 AM Thu Sep 27 2012...60 mph...initial
11 PM Thu Sep 27 2012...60 mph...21 hr
11 PM Fri Sep 28 2012...70 mph...42 hr
11 PM Sat Sep 29 2012...60 mph...66 hr
11 PM Sun Sep 30 2012...50 mph...90 hr
11 PM Mon Oct 01 2012...40 mph (Dissipating)...114 hr

I am making no changes to my Nadine track forecast at this time. Return to full discussion #118 for an assessment on the rest of the Atlantic tropics.

2012 Atlantic Hurricane Season Birdseye Discussion #118

By: NCHurricane2009, 9:54 AM GMT on September 27, 2012

...SEPTEMBER 27 2012...5:58 AM EDT...
An outage persists with GOES-E satellite imagery in the last 72 hours. GOES-W has been extended to cover much of the view in the two birdseye charts below. However...the east edge of the temporary GOES-W scan has a bias for showing cold cloud tops that are not actually present. Therefore...I have patched the east side of the atmospheric birdseye chart with Meteosat-9 grafts. The east side of the thermodynamics birdseye chart is left unrepaired...so be mindful that the moisture content on the east side of this chart has a positive bias due to the false illusion of cold cloud tops.

Tropical Storm Nadine expected to track erratically in the Atlantic subtropics for the next five days. See Nadine special feature section below for details.

Beginning in 72 hours...next upper trough in the mid-latitude westerlies (not yet in the scope of this discussion) is expected to produce high upper divergence with respect to SW US upper ridge in paragraph P1. This will drive the formation of a NE tracking surface low from the Gulf of Mexico...which could become quiet vigorous similar to Invest 93-L in discussion #110. Similar to 93-L...I expect this system to be non-tropical...but could still be capable of signficant weather across the US Gulf coast...SE US...and along the east US coast. See details of 93-L in discussion #110 for the kinds of impacts a system like this could deliver.

A disturbance has developed from persistent surface troughing east-northeast of the Lesser Antilles interacting with split flow upper divergence. This disturbance was introduced into the NHC Tropical Weather Outlook in the last 24 hours. No sign of tropical development is occuring in this area. See paragraph P8 for updated statement on this area.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0126Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
In the last 48 hrs...Nadine has perfectly followed my previous two intensity forecasts. These forecasts show gradual re-strengthening of Nadine while she is developing favorable upper anticyclonic outflow...becoming enhanced to the SW by retrograding paragraph P5 E Atlc upper vortex...and becoming enhanced to the NE by departing paragraph P4 upper trough. These two forecasts then show weakening at the end of the forecast...as Nadine turns northward over even cooler waters...and as the paragraph P5 E Atlc upper vortex merges with mid-latitude westerlies...hence turning eastward toward Nadine and imparting vertical shear across Nadine with its east side. Despite my previous two intensity forecasts verifying well so far...my updated intensity forecast in Figure 1 below is slightly less than my previous two...because of the westward adjustment in the track forecasts in the last 24 hrs. This westward adjustment means Nadine will feel the vertical shear from the incoming paragraph P5 upper vortex a little sooner. I dissipate Nadine by 96 and 120 hrs as I think the cool waters and high vertical shear will be the final straw that finally kills this long-lasting tropical cyclone.


Figure 1: My forecast for Tropical Storm Nadine generated this early morning.

Nadine is currently located west of the Canary Islands and south of the Azores...but is currently a tad further west than the previous NHC forecasts showed. Therefore the NHC forecast track is a bit further west than the previous...and also a bit faster than the previous. For the short-term...I agree with the NHC's westward adjustment. I also agree with the slightly quickened westward pace as the 00Z GFS shows a gradually dissolving paragraph P9 low-level ridge to the south for the next 72 hours...only leaving the paragraph P2 ridge to the north to push Nadine a little faster to the west.

Northward hook by the end of the forecast is related to evolution of current E Canada 1007 mb frontal cyclone and 1022 mb low-level ridge over the Great Lakes (both mentioned in paragraph P1). I have a more leftward lean in my hook between 11 PM Fri & 11 PM Sat compared to my previous forecast displays...because a closer look at 00Z GFS shows me that the eastward-moving paragraph P2 ridge is still too close to Nadine in my opinion to show a straight north track for that time. The straight north track eventually happens as Nadine gets pulled into the 1007 mb frontal cyclone's narrow ridge weakness...located between paragraph P2 ridge by then located to the NE and what is now the Great Lakes ridge coming in from the NW. My straight north track is slower than the NHC's current...and slower than the even faster 00Z GFS. These faster north-track solutions do not make sense to me as the two ridges are shown to be of equal strength and opposing influence. I think the GFS sees Nadine becoming a non-tropical cyclone supported by the eastern divergence of incoming paragraph P5 E Altc upper vortex...in which case Nadine indeed would move faster to the north (and then eventually east into the Azores) while being coupled to and traveling with this upper vortex. My vote for now is to dissipate Nadine in intense shear generated by this upper vortex while she becomes trapped between the two low-level ridges.

Impact swath in Figure 1 is based on extrapolating the 11 PM EDT tropical storm wind radius along my forecast track...then shrinking the swath away assuming Nadine follows my forecast for dissipation by 120 hrs. Impact statement (b) highlights that surf will linger on the shores of the Azores..due to the large wind radius of Nadine capable of stirring a large amount of water. However..the Azores surf this week will be less intense compared to last week...as Nadine should be further away from the islands this time around.

...MID-LATITUDES DISCUSSION...
P1...Weather system remains anchored by upper trough over eastern North America. Hudson Bay surface cyclone once supported by this upper trough has weakened from 990 to 1006 mb in the last 24 hrs while losing touch with the eastern divergence of the upper trough...and has now shifted onto the east coast of Canada. The upper trough's eastern divergence now drives SW US frontal depression which has zoomed rapidly ENE across the Great Lakes and into E Canada at 1007 mb...while the upper trough's western convergence is driving the a new 1022 mb ridge over the Great Lakes. Along the W Atlantic cold front...NW Atlantic frontal depression has intensified (also with supportive eastern divergence of the upper trough) from 1000 mb to below 996 mb in the last 24 hrs while shooting NNE to the east of Greenland (and hence exiting the above atmo chart). There is probably a shortwave upper trough carved by local cool air advection of this exiting feature...with western upper convergence of shortwave supporting the eastern US and NW Atlantic low-level ridge (which is now 1022 to 1027 mb). Cut-off upper trough over the W Gulf of Mexico has shifted into the central Gulf. Upper ridge wave over the SW US remains built behind this cut-off upper trough. Utah upper vortex (formtion of which discussed in paragraph P1 of discussion #117) persists. Split flow upper divergence between this upper vortex and SW US upper ridge supports broad area of low surface pressures over the SW US...and supports 1013 mb surface low at the Wyoming-Montana border.

P2...Deep-layered ridge across the central Atlantic...featuring a greater-than-1028 mb center...persists.

P3...Upper vortex SE of Bermuda in the previous discussion has been pushed SE by amplifying paragraph P5 upper ridge cell from Cuba/Bahamas. This upper vortex is now just NE of the Lesser Antilles.

P4...Amplified upper trough remains just offshore of Europe. Study of extended Meteosat-9 infrared imagery suggests that repeated cool air advection from surface frontal cyclones/depressions diving into the upper trough re-enforces the upper trough...similar to what is happening with eastern North America upper trough in paragraph P1.

...TROPICAL BELT DISCUSSION...
P5...Large-scale upper vorticity persists across the SE half of the Caribbean Sea. Upper ridge cell remains amplified across Cuba...the Bahamas...and W Atlantic due to low-level warm air advection ahead of the paragraph P1 weather system. Easterly flow across north side of SE Caribbean upper vorticity is converging with northerly flow from this upper ridge cell to produce a patch of dry air across the central Caribbean. In the eastern tropical Atlantic...upper vortex just SW of Nadine persists. In relatively higher pressures southeast of this upper vortex...upper ridge has built from the W coast of Africa as evidenced by upper-level cloud motions around paragraph P10 tropical wave.

P6...Surface troughing persists in Bay of Campeche...now supported by split flow upper divergence between paragraph P1 central Gulf cut-off upper trough and paragraph P1 SW US upper ridge. Scattered t-storms across northern Central America and SE Mexico are supported by this same upper divergence mechanism.

P7...Tropical wave crossing the eastern Caribbean Sea in the previous discussion is now moving into the central Caribbean. It is producing a t-storm cluster over Haiti and vicinity of SE Bahamas supported by split flow upper divergence between paragraph P5 W Atlantic upper ridge and paragraph P5 SE Caribbean upper vortex.

P8...Surface troughing persists ENE of the Lesser Antilles...which has become quiet enhanced by split flow upper divergence between paragraph P3 upper vortex and E tropical Atlc upper vortex mentioned in paragraph P5.

P9...Eastern Atlantic surface ridge with a 1018 mb center persists NW of the Cape Verde Islands. I currently speculate that this surface ridge is now supported by upper convergence between northerlies from W Africa upper ridge (paragraph P5) and westerlies from paragraph P5 E Atlc upper vortex and paragraph P3 upper vortex.

P10...Suspect tropical wave E of the Cape Verde Islands I have been analyzing for the last days has finally been added to NHC TAFB surface maps...at a location just W of the islands as of 0000Z. It is producing scattered t-storms in the area while supported by enhanced upper outflow of upper ridge building from W Africa mentioned in paragraph P5.

2012 Atlantic Hurricane Season Birdseye Discussion #117

By: NCHurricane2009, 9:13 AM GMT on September 26, 2012

...SEPTEMBER 26 2012...5:15 AM EDT...
An outage persists with GOES-E satellite imagery in the last 48 hours. GOES-W has been extended to cover much of the view in the two birdseye charts below. However...the east edge of the temporary GOES-W scan has a bias for showing cold cloud tops that are not actually present. Therefore...I have patched the east side of the atmospheric birdseye chart with Meteosat-9 grafts. The east side of the thermodynamics birdseye chart is left unrepaired...so be mindful that the moisture content on the east side of this chart has a positive bias due to the false illusion of cold cloud tops.

Tropical Storm Nadine expected to track erratically in the Atlantic subtropics for the next five days. See Nadine special feature section below for details.

A currently disorganized disturbance in the vicinity of the Bahamas has been upgraded to Invest 95-L. The disturbance is not mentioned in the NHC Tropical Weather Outlook due to its current lack of impressiveness. A check with computer models (GFS...CMC...ECMWF...NOGAPS) does not suggest tropical cyclone formation in this area for the next days. Therefore I am not considering this area a special feature on this blog. See paragraph P6 for details on the weather in this area.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 1922Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
In the last 24 hrs...Nadine has followed my previous & more conservative intensity forecast which predicted no strengthening during that time. I speculated that an eastern arm of the paragraph P5 E Atlc upper vortex would be parked south of Nadine and provide continued easterly shear. Its hard to say if this eastern arm currently exists...but it is clear that Nadine has not been under easterly shear like I thought it would be as their has been a symmetric ring of t-storm clouds about the center thru much of the last 24 hrs. Perhaps instead its the lack of instability over cool waters below 26 deg C that has prevented Nadine from strengthening. Eventual re-strengthening still appears imminent as models still show favorable upper anticyclonic outflow becoming symmetrical about the storm...becoming enhanced to the SW by retrograding paragraph P5 E Atlc upper vortex...and becoming enhanced to the NE by departing paragraph P4 upper trough. I then show weakening at the end of the forecast...as the paragraph P5 E Atlc upper vortex merges with mid-latitude westerlies...hence turning eastward toward Nadine and imparting southerly vertical shear across Nadine with its east side. My intensity forecast in Figure 1 below is a copy-paste of my previous as my previous has been accurate in the last 24 hrs. My intensity forecast is similar to the NHC's 11 PM EDT forecast.


Figure 1: My forecast for Tropical Storm Nadine generated this early morning.

Nadine is currently located west of the Canary Islands and south of the Azores. She has almost perfectly followed the NHC's short-term previous track forecast which predicted she would first turn southward while ramming into the brickwall that is the paragraph P2 deep-layer ridge to the west. The previous short-term track forecast also agreed the south track would be slow as it has been...thanks to resistance from the paragraph P9 low-level ridge. Because the NHC southward short-term track forecast has not changed in the last 24 hrs...and because that short-term track forecast has done well...I continue to agree with it.

In the longer-term solution (just after 24 hrs)...the paragraph P2 ridge passes to the north...which should force Nadine westward by 48 hours. Previously for the timeframe that is 48 hours (11 PM Thu)...I had a westward bias relative to NHC...as the 00Z GFS shows Nadine's low-level low pressure field having eroded the paragraph P9 ridge to the south such that I see only an impressive paragraph P2 ridge to the north that should in my opinion push Nadine more west than the NHC showed for this time. The NHC in the last day has adjusted leftward more in alignment with my thoughts for this time...but I am still a tad more left. As Nadine re-curves northward late in the forecast...00Z GFS re-develops the paragraph P9 ridge to the south such that Nadine gets trapped again between the paragraph P2 and P9 ridges...which is why I re-agree with a slower track by 72 to 120 hrs.

My northward hook in the forecast track in the longer-term is to the right of the NHC's latest track forecast...and the hook is related to how the paragraph P1 upper trough evolves late in the forecast. By the latter part of the forecast...the upper trough has split into two impulses...with the eastern divergence of the eastern impulse driving a frontal cyclone from Atlantic Canada...and the western convergence of the eastern impulse driving yet another low-level ridge. The frontal cyclone's low-level ridge weakness is not particularly wide...so I show Nadine drifting northward toward the weakness (between the paragraph P2 ridge now to the east and other low-level ridge to the NW). I currently expect this weakness to leave behind Nadine...so we will have to wait for another frontal cyclone in the very long range that will ultimately accelerate Nadine northward to her demise. Some of this morning's models (GFS and CMC) show Nadine moving northward faster than what I showed in Figure 1 to the degree it threatens the western Azores...but this solution does not make sense to me as the paragraph P2 low-level ridge to the east and other low-level ridge to the NW are shown to be of equal strength and opposing influence. It would take a paragraph P2 low-level ridge to be a lot stronger than the other ridge for something like the CMC and GFS to verify.

Impact swath in Figure 1 is based on extrapolating the 11 PM EDT tropical storm wind radius along my forecast track. Impact statement (b) highlights that surf will linger on the shores of the Azores..due to the large wind radius of Nadine capable of stirring a large amount of water. However..the Azores surf this week will be less intense compared to last week...as Nadine should be further away from the islands this time around.

...MID-LATITUDES DISCUSSION...
P1...Weather system remains anchored by upper trough over the eastern United States. The Hudson Bay surface cyclone once supported by this upper trough has whirled cyclonically and weakened from 980 to 990 mb in the last 24 hrs while losing touch with the eastern divergence of the upper trough. It has split from its cold front...which projects into the W Atlantic...where a NW Atlantic frontal depression has intensified (with supportive eastern divergence of the upper trough) from 1013 to 1000 mb in the last 24 hrs while shooting NNE across Atlantic Canada. Western convergence of the upper trough supports a 1023 to 1026 mb surface ridge that has slid into the eastern US and NW Atlantic. SW fragment of upper trough remains cut-off over the W Gulf of Mexico. Upper ridge wave over the SW US remains built behind this cut-off upper trough...whose W divergence supports a broad 1005 to 1009 mb frontal depression in the area (and in turn local cool air advection behind the broad surface depression has created a cut-off upper vortex over Utah). A second frontal cyclone (now 999 mb) and its upper trough has dived southward across Hudson Bay around the south side of the 990 mb cyclone...with this second cyclone becoming absorbed by the 990 mb cyclone.

P2...Deep-layered ridge across the central Atlantic...featuring an impressive 1028 mb center...persists.

P3...Upper vortex remains stationary to the south of amplified paragraph P5 upper ridge cell from Cuba/Bahamas...located SE of Bermuda.

P4...Amplified upper trough remains just offshore of Europe. Study of extended Meteosat-9 infrared imagery suggests that repeated cool air advection from surface frontal cyclones/depressions diving into the upper trough re-enforces the upper trough...similar to what is happening with eastern US upper trough in paragraph P1.

...TROPICAL BELT DISCUSSION...
P5...Large-scale upper vorticity persists across the SE half of the Caribbean Sea. Upper ridge cell remains built across SE Mexico as paragraph P1 upper trough to the north loses a bit of ground...and remains amplified across Cuba...the Bahamas...and W Atlantic due to low-level warm air advection ahead of the paragraph P1 weather system. Easterly flow across north side of SE Caribbean upper vorticity is converging with northerly flow from this upper ridge cell to produce sinking dry air across the central Caribbean (alternatively Westerly flow across the south side of the SE Caribbean upper vorticity diverges with the same upper ridge cell to produce enhanced t-storms over Central America). In the eastern tropical Atlantic...upper troughing just SW of Nadine has consolidated into an upper vortex retrograding about the paragraph P2 ridge. In relatively higher pressures southeast of this upper vortex...upper ridge is building from the W coast of Africa as evidenced by widely scattered eastern tropical Atlantic t-storm clouds whose outflow is enhanced by this upper ridge.

P6...Surface troughing still extends from the Bay of Campeche to the waters offshore of the SE US. A good chunk of this surface troughing and widespread t-storms have been overran by south end of paragraph P1 cold front...and persists in favorable split flow upper divergence between the SE Mexico paragraph P5 upper ridge cell and incoming paragraph P1 upper trough. However...this whole area continues to be bombarded by unfavorable SW vertical shear from paragraph P1 upper trough as to prevent tropical development.

P7...Tropical wave crossing the Lesser Antilles in the previous discussion is now in the eastern Caribbean Sea. Zonal shearing upper westerlies across the tropical wave appear to be re-amplifying into a more favorable upper ridge wave due to the tremendous latent heat release in the t-storm clouds.

P8...Surface troughing persists ENE of the Lesser Antilles...which now appears to be supported by upper divergence on the SE edges of the paragraph P3 upper vortex...and E Atlc upper vortex mentioned in paragraph P5.

P9...Relatively new eastern Atlantic surface ridge of 1018 mb persists NW of the Cape Verde Islands. I currently speculate that this surface ridge is now supported by upper convergence between northerlies from W Africa upper ridge (paragraph P5) and westerlies from paragraph P5 E Atlc upper vortex and paragraph P3 upper vortex.

P10...During the last days...satellite imagery suggested a suspect tropical wave with a low pressure spin stalled along the W coast of Africa. The stalled motion is due to absence of low-level steering easterlies thanks to ridge weakness from paragraph P4 weather system and Tropical Storm Nadine. Based on one cluster of widely scattered t-storms in eastern Atlantic described in paragraph P5...I continue to speculate it is just offshore of W Africa and east of the Cape Verde Islands.

2012 Atlantic Hurricane Season Birdseye Discussion #116

By: NCHurricane2009, 8:04 AM GMT on September 25, 2012

...SEPTEMBER 25 2012...4:04 AM EDT...
An outage has developed on GOES-E satellite imagery in the last 24 hours. GOES-W has been extended to cover much of the view in the two birdseye charts below. However...the east edge of the temporary GOES-W scan has a bias for showing cold cloud tops that are not actually present. Therefore...I have patched the east side of the atmospheric birdseye chart with Meteosat-9 grafts. The east side of the thermodynamics birdseye chart is left unrepaired...so be mindful that the moisture content on the east side of this chart has a positive bias due to the false illusion of cold cloud tops.

Tropical Storm Nadine expected to track erratically in the Atlantic subtropics for the next five days. See Nadine special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0127Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
In the last 36 hrs...Nadine has weakened while ingesting the paragraph P2 dry air to her south...and while experiencing some unpredicted easterly vertical shear produced by north side of E Atlc upper trough/vortex just SW of her (paragraph P5). This weakening was not predicted by the previous NHC intensity forecast...and therefore the newer NHC intensity forecasts are dampened down from the previous. 00Z GFS this morning shows an eastern arm of the shearing upper trough persisting just south of Nadine thru the next 24 hrs...so in Figure 1 I prefer to be slightly below the 11 PM EDT NHC intensity guidance while delaying the re-strengthening of Nadine until after 24 hrs. This re-strengthening appears imminent as models still show favorable upper anticyclonic outflow becoming symmetrical about the storm...becoming enhanced to the SW by retrograding paragraph P5 E Atlc upper trough...and becoming enhanced to the NE by departing paragraph P4 upper trough. These very favorable upper winds suggest the need to show aggressive re-strengthening. But one has to remember that Nadine will be above waters cooler than 26 deg C if she follows the forecast track in Figure 1. I agree with the NHC's weakening shown at the end of the forecast...as the paragraph P5 E Atlc upper trough merges with mid-latitude westerlies...hence turning eastward toward Nadine and imparting southerly vertical shear across Nadine with its east side.


Figure 1: My forecast for Tropical Storm Nadine generated this early morning.

Nadine is currently located west of the Canary Islands and south of the Azores. SW side of 1019 mb low-level ridge that built over the Canary Islands in the previous Nadine discussion imparted more of a northward turn than I thought it would...and so Nadine will soon have to turn southward while ramming into the brickwall that is the paragraph P2 deep-layer ridge to the west. I agree with the NHC on an initial slow southward track as it incurs resistance from new paragraph P9 low-level ridge. Eventually the paragraph P2 ridge passes to the north...which should force Nadine westward by 72 hours. It is by 72 hours I begin disagreeing with the NHC forecast with me showing a westward bias...as the 00Z GFS shows Nadine's low-level low pressure field having eroded the paragraph P9 ridge to the south such that I see only an impressive paragraph P2 ridge to the north that should in my opinion push Nadine more west than the NHC shows for this time. As Nadine re-curves northward late in the forecast...00Z GFS re-develops the paragraph P9 ridge to the south such that Nadine gets trapped again between the paragraph P2 and P9 ridges...which is why I re-agree with a slower track by 96 and 120 hrs.

My northward hook in the forecast track in the longer-term is related to how the paragraph P1 upper trough evolves late in the forecast. By 120 hrs...the upper trough has split into two impulses...with the eastern divergence of the eastern impulse driving a frontal cyclone over Atlantic Canada...and the western convergence of the eastern impulse driving yet another low-level ridge. The frontal cyclone's low-level ridge weakness is not particularly wide...so I show Nadine drifting northward toward the weakness (between the paragraph P2 ridge now to the east and other low-level ridge to the NW). I currently expect this weakness to leave behind Nadine...so we will have to wait for another frontal cyclone in the very long range that will ultimately accelerate Nadine northward to her demise.

Impact swath in Figure 1 is based on extrapolating the 11 PM EDT tropical storm wind radius along my forecast track. Impact statement (b) highlights that surf will linger on the shores of the Azores..due to the large wind radius of Nadine capable of stirring a large amount of water. However..the Azores surf this week will be less intense compared to last week...as Nadine should be further away from the islands this time around.

...MID-LATITUDES DISCUSSION...
P1...Weather system remains anchored by upper trough over eastern North America. The Hudson Bay surface cyclone strengthened from 993 to 980 mb in the last 24 hours while sliding into favorable eastern divergence of the upper trough. Cold front projecting from the cyclone extends into the W edge of the Atlantic...where a new 1013 mb frontal depression has formed in the NW Atlantic while supported by eastern divergence of the upper trough. Western convergence of the upper trough supports a 1022 to 1023 mb surface ridge that has slid into the eastern US. SW fragment of upper trough has become cut-off over Louisiana and W Gulf of Mexico. Upper ridge wave over the SW US has built behind this cut-off upper trough..whose W divergence supports a broad 1006 to 1007 mb frontal depression in the area. A second frontal cyclone (now 995 mb) and its upper trough has dived southward across Hudson Bay on the back side of the 980 mb cyclone.

P2...Deep-layered ridge across the central Atlantic...featuring an impressive greater-than-1028 mb center...persists. South side of this deep-layered ridge is wafting dry air from Africa's desert westward across the eastern Atlantic tropics. This dry air in this area may be under-represented due to the unrepaired thermo birdseye chart as explained in the intro statement of this full discussion.

P3...Deep-layered subtropical low east of Bermuda...formerly Invest 94-L...has become vertically decoupled. The surface vortex continued northward while becoming absorbed by NW Atlantic 1013 mb frontal depression in paragraph P1. The upper vortex meanwhile remains held back by amplifying paragraph P5 upper ridge cell from Cuba/Bahamas...located stationary SE of Bermuda.

P4...Amplified upper trough is moving into Europe according to Meteosat-9 infrared satellite imagery.

...TROPICAL BELT DISCUSSION...
P5...Large-scale upper vorticity persists across the SE half of the Caribbean Sea. Upper ridge cell remains built across SE Mexico as paragraph P1 upper trough to the north loses a bit of ground...and remains amplified across Cuba...the Bahamas...and W Atlantic due to low-level warm air advection ahead of the paragraph P1 weather system. Easterly flow across north side of SE Caribbean upper vorticity is converging with northerly flow from this upper ridge cell to produce sinking dry air across much of the Caribbean (alternatively Westerly flow across the south side of the SE Caribbean upper vorticity diverges with the same upper ridge cell to produce enhanced t-storms over Costa Rica and Panama). In the eastern tropical Atlantic...upper troughing just SW of Nadine has consolidated and is retrograding about the paragraph P2 ridge. In relatively higher pressures southeast of this upper trough...upper ridge is building from the W coast of Africa as evidenced by an increase in eastern tropical Atlantic t-storm clouds whose outflow is enhanced by this upper ridge.

P6...Tropical wave ex-92L has exited the picture from SE Mexico. Extending NE from the tropical wave...surface troughing still extends from the Bay of Campeche to the waters offshore of the SE US. This surface troughing and widespread t-storms have been overran by paragraph P1 cold front (where there is a 1015 mb frontal depression E of Florida)...and persists in favorable split flow upper divergence between the SE Mexico paragraph P5 upper ridge cell and incoming paragraph P1 upper trough. However...this whole area continues to be bombarded by unfavorable SW vertical shear from paragraph P1 upper trough as to prevent tropical development.

P7...Tropical wave nearing the Lesser Antilles in the previous discussion is now crossing those islands. Its t-storm activity was enhanced by favorable upper ridge cell overhead 36 hrs ago...but now is enhanced by divergence on the SE edge of paragraph P5 SE Caribbean upper vorticity. Meanwhile...the upper ridge cell overhead has de-amplified into zonal shearing upper westerlies thanks to paragraph P5 E Atlc upper trough moving in from the NE.

P8...Eastern Atlantic surface trough continues west and persists ENE of the Lesser Antilles (its formation discussed in paragraph P9 of previous discussion #115).

P9...New eastern Atlantic surface ridge of 1016 mb has formed just NW of the Cape Verde Islands. Based on 00Z GFS model's 200 mb (upper) wind initialization...it appears this surface ridge is supported by upper convergence between northerlies from W Africa upper ridge (paragraph P5) and westerlies from paragraph P4 upper trough.

P10...During the last days...satellite imagery suggested a suspect tropical wave with a low pressure spin stalled along the W coast of Africa. The stalled motion is due to absence of low-level steering easterlies thanks to ridge weakness from paragraph P4 weather system and Tropical Storm Nadine. Due to the increase in eastern Atlantic t-storms described in paragraph P5...there is no longer a distinct area of t-storms to mark this suspect tropical wave...so without it being acknowledged by NHC TAFB...I speculate it is just offshore of W Africa and east of the Cape Verde Islands.

2012 Atlantic Hurricane Season Birdseye Discussion #115

By: NCHurricane2009, 9:01 PM GMT on September 23, 2012

...SEPTEMBER 23 2012...5:05 PM EDT...
Nadine regains tropical storm status..and is expected to turn westward across the Atlantic subtropics from its current eastern Atlantic location. As it does so...it is likely to regain hurricane status. See Nadine special feature section below for details.

Tropical wave moving into the Lesser Antilles mentioned in paragraph P8 will bring squalls of t-storms while it remains under favorable upper winds through the next 24 hours. Upper winds will become less favorable by 48 and 72 hours...so no tropical cyclone formation is currently expected here.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1159Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
In the last 24 hrs...upper troughing over Nadine has amplified with a small upper vortex over the storm thanks to paragraph P2 deep-layered ridge moving in from the west. This left Nadine under heavy split flow upper divergence between the NE quad of the upper vortex and westerlies streaming into the paragraph P5 upper trough. The upper divergence dramatically increased the t-storm intensity within Nadine's circulation...the latent heat release from which appears to have re-established a warm core upper anticyclone at least over the NE half of the storm. The warm core upper outflow is evident by some cirrus outflow clouds along the north edge of the storm. Therefore...Nadine is once again a tropical storm as of 11 AM EDT.

This afternoon's 12Z GFS shows the favorable anticyclonic outflow becoming symmetrical about the storm thru the forecast period...becoming enhanced to the SW by retrograding paragraph P6 E Atlc upper trough...and becoming enhanced to the NE by departing paragraph P5 upper trough. Moreover...her upper outflow is likely to merge with the tremendous upper anticyclonic flow of paragraph P2 deep-layered ridge. These very favorable upper winds suggest the need to show aggressive strengthening. But one has to remember that Nadine will be above waters cooler than 26 deg C if she follows the forecast track in Figure 1...and also there is plenty of paragraph P2 dry air lurking to the south that could be ingested by Nadine's low-level circulation at any time. Therefore I prefer to agree with the more modest intensification rate to a category 1 hurricane of 80 mph max winds as shown by NHC at 11 AM EDT. This sort of intensity forecast assumes the waters are not warm enough to support anything stronger. Thru the forecast period...Nadine will be below warm core upper outflow as opposed to a cold core upper vortex...so their will be no cold upper temps to destabilize things and help Nadine get stronger than usual over these mild water temps.


Figure 1: My forecast for Tropical Storm Nadine generated this afternoon

Nadine is currently located west of the Canary Islands and south of the Azores. What prevented the storm from going east into the Canary Islands is a new 1019 mb low-level ridge that built over the islands. I speculate this ridge formed due to upper convergence between southerlies from paragraph P6 upper cell over the W coast of Africa...and westerlies on south side of paragraph P5 upper trough. Although the NHC stated that Nadine is drifting west as of 11 AM EDT...this afternoon's visible satellite suggests to me a stalled motion with Nadine trapped between the 1019 mb low-level ridge to the NE and paragraph P2 1030 mb low-level ridge to the NW. The 00Z GFS shows the 1019 mb ridge persisting for another 12 hours...so I keep Nadine stalled during this time. After 12 hours...I agree with the NHC on a westward track as the 12Z GFS shows the 1030 mb ridge passing to the north. Initially I am slower than the NHC forecast to account for the initial stalled motion...then I become faster than the NHC forecast due to the strength of the 1030 mb ridge. The NHC shows a sinusoidal west track where Nadine first goes NW then goes SW. I see no reason to show a NW bend in the short-term due to the strength of the 1030 mb ridge...nor do I want to show a SW bend as the 12Z GFS does not show this.

In the longer-term...I show a hook to the north and Nadine becoming nearly stalled once again. This is related to how the paragraph P1 upper trough evolves late in the forecast. By 108 and 132 hrs (my last two forecast points in Figure 1)...the upper trough has split into two impulses...with the eastern divergence of the eastern impulse driving a frontal cyclone over Atlantic Canada...and the western convergence of the eastern impulse driving another low-level ridge. The frontal cyclone's low-level ridge weakness is not particularly wide...so I show Nadine drifting northward toward the weakness (between the 1030 mb low-level ridge to the east and other low-level ridge to the NW). I currently expect this weakness to leave behind Nadine...so we will have to wait for another frontal cyclone in the very long range that will ultimately accelerate Nadine northward to her demise.

Impact swath in Figure 1 is based on extrapolating the 11 AM EDT tropical storm wind radius along my forecast track. Impact statement (b) highlights that surf will linger on the shores of the Azores..due to the large wind radius of Nadine capable of stirring a large amount of water. However..the Azores surf this week will be less intense compared to last week...as Nadine should be further away from the islands this time around.

...MID-LATITUDES DISCUSSION...
P1...Weather system (surface frontal cyclone supported by upper trough) is moving from central North America to eastern North America. The Hudson Bay surface cyclone strengthened from 1004 to 993 mb in the last 24 hours while sliding into favorable eastern divergence of the upper trough. Now that the surface cyclone is tucked below the less-divergent upper trough axis...further strengthening should be halted. Both fronts projecting from the cyclone have consolidated into a single cold front extending across the NE US...NW edge of the Atlantic...and then across the southern US. Western convergence of the upper trough supports dry air across the southern edge of the US and a 1028 mb surface ridge that has moved from the western US to the central US.

P2...Deep-layered ridge across the central Atlantic...featuring an impressive 1030 mb center...persists. Formation of this deep-layered ridge is discussed in paragraph P1 of previous discussion #114. South side of this deep-layered ridge is wafting dry air from Africa's desert westward across the eastern Atlantic tropics.

P3...Decaying surface front in the NW Atlantic (paragraph P2 of previous discussion #114) is becoming absorbed by cold front mentioned in paragraph P1.

P4...Deep-layered subtropical low east of Bermuda...formerly Invest 94-L...is becoming vertically decoupled. The surface vortex (marked by 1014 mb low in above atmo birdseye chart) is moving northward at a location NE of Bermuda while steered about west side of paragraph P2 deep-layered ridge...and will eventually become absorbed or overrun by paragraph P1 cold front as it moves into Atlantic Canada. The upper vortex meanwhile is being held back by amplifying paragraph P6 upper ridge cell from Cuba/Bahamas...but will eventually become absorbed by the paragraph P1 upper trough.

P5...Amplified upper trough approaching Europe persists. Eastern divergence of the upper trough supports frontal cyclone moving NE into NW France.

...TROPICAL BELT DISCUSSION...
P6...Large-scale upper vorticity persists across the SE half of the Caribbean Sea. Upper ridge cell is building across SE Mexico as paragraph P1 upper trough to the north loses a bit of ground...and remains amplified across Cuba...the Bahamas...and W Atlantic due to low-level warm air advection ahead of the paragraph P1 weather system. Easterly flow across north side of SE Caribbean upper vorticity is converging with northerly flow from this upper ridge cell to produce sinking dry air across much of the Caribbean. In the eastern tropical Atlantic...upper troughing over Nadine has consolidated and is about to begin a SW retrograding track about the paragraph P2 ridge. As this upper trough retrogrades SW...the central tropical Atlantic upper ridge cell has shifted SW over the tropical wave in paragraph P8. In relatively higher pressures east of this upper trough...a new upper ridge is building over the W coast of Africa.

P7...Tropical wave ex-92L continues moving westward across SE Mexico...and this will be the final statement on this tropical wave on this blog as it will likely be in the eastern Pacific by the release time of the next full discussion. Extending NE from the tropical wave...surface troughing reaching W Bahamas and waters offshore of the SE US persists. This surface troughing persists in favorable split flow upper divergence between the SE Mexico paragraph P6 upper ridge cell and incoming paragraph P1 upper trough...but is in unfavorable hostile SW vertical shear delivered by the upper trough.

P8...Tropical wave midway between the Cape Verdes and Lesser Antilles in the previous discussion is now nearing the Lesser Antilles. Its t-storm activity has continued to increase due to favorable shear-reducing outflow-enhancing paragraph P6 upper ridge cell shifting overhead. This afternoon's 12Z GFS model run suggests this favorable upper wind configuration should last for another 24 hrs...so expect t-storm squalls to spread across the Lesser Antilles. By 48 and 72 hrs...the tropical wave will be nearing the less favorable environment of the SE Caribbean upper vorticity (paragraph P6)...and the eastern Atlantic upper trough (also in paragraph P6) should have retrograded SW to the degree it de-amplifies the favorable upper ridge cell over the tropical wave such that unfavorable westerly shear across the tropical wave increases.

P9...A new eastern Atlantic surface trough between the tropical wave in paragraph P8 and Tropical Storm Nadine is marked as of 1200Z TAFB. Based on above atmo birdseye chart 200 mb wind barbs...it appears to be supported by split flow upper divergence between paragraph P2 deep-layered ridge to the NW and paragraph P6 eastern tropical Atlantic upper trough to the SE.

P10...Satellite imagery suggests a potent tropical wave with a low pressure spin remains stalled out along the W coast of Africa. The stalled motion is due to absence of low-level steering easterlies thanks to ridge weakness from paragraph P5 weather system and Tropical Storm Nadine. In fact...satellite imagery suggests the tropical wave drifting northward along the coast while becoming pulled into the ridge weakness.

2012 Atlantic Hurricane Season Birdseye Discussion #114

By: NCHurricane2009, 8:38 PM GMT on September 22, 2012

...SEPTEMBER 22 2012...4:50 PM EDT...
This is the first day since August 15 with no active tropical cyclones in the Atlantic basin.

In the last 36 hrs...Nadine has transitioned from a tropical storm to a subtropical storm...then into a remnant low. See Nadine special feature section below. If Nadine stops showing potential to recurve southward then southwestward back into the Atlantic tropics...then I will cancel it as a special feature section on this blog.

Tropical wave moving across the Caribbean Sea...formerly Invest 92-L...leaves behind a disturbance offshore of the SE US while the wave itself enters hostile upper winds while moving into southeastern Mexico. The disturbance and the wave do not show signs of tropical development...and therefore have been cancelled as a special feature on this blog. See paragraph P7 for latest statement on this system.

Cut-off deep-layered vortex east of Bermuda...Invest 94-L....no longer expected to become a subtropical depression or storm...and therefore is no longer a special feature on this blog. See paragraph P3 for latest statement on this system.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0723Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...REMNANT LOW OF NADINE...
Nadine's central pressure has been slowly rising in the last 36 hrs...and correspondingly its winds have gradually been weakening. Previously Nadine had been maintaining strength from supportive divergence directly over Nadine (supplied by east side of paragraph P4 upper trough)...and supportive divergence just NE of Nadine (supplied by east side of upper trough over Nadine). The current weakening appears to be Nadine losing touch with these upper divergence sources while becoming tucked entirely below the less-divergent upper trough axis above it. Nadine continues to have the characteristics of a subtropical low with no upper anticyclonic outflow...indicating a shallower warm core system below cold core upper troughs. This is why the NHC downgraded Nadine to a subtropical storm last afternoon before cancelling advisories altogether last night. Cancellation in advisories came from lack of intense enough storm bands in Nadine's circulation to consider it a subtropical cyclone.

Nadine is currently located west of the Canary Islands and south of the Azores. The central Atlantic strong and deep-layered ridge to the west (described in paragraph P1) has been pushing Nadine southward until recently...when the new paragraph P4 surface cyclone offshore of Portugal and Spain has begun dragging Nadine eastward. It remains to be seen if Nadine continues eastward...or if it resumes a southward track once the surface cyclone lifts out to the NE. Currently the center of Nadine is over the 26 deg C sea-surface temp isotherm. If the track resumed on a southward course...the instability will increase with more low-level heat content coupled with cold temps of upper troughing above...potentially allowing for t-storms to regenerate...hence allowing Nadine to regenerate into a subtropical storm.

...MID-LATITUDES DISCUSSION...
P1...Weather system (surface frontal cyclone supported by upper trough) in the mid-latitudes persists across central Canada and the central US. The surface cyclone has weakened from 995 mb to a diffuse 1004 mb center over Hudson Bay in the last 36 hrs while stalled beneath the less-divergent upper trough axis. The central US front extending from the cyclone has moved into the eastern US...while the cyclone drives a second front from Canada and into the central US. Western convergence of the upper trough supports dry air across the southern US/northern Gulf of Mexico and a building surface ridge over the western US. Southerly flow ahead of this weather system supports a W Atlantic upper ridge formerly supported by southerly flow ahead of the paragraph P2 system. This upper ridge has shifted east into the central Atlantic...and remains stacked with a strong central Atlantic surface ridge (mentioned in paragraph P4) to produce a deep-layered ridge.

P2...Upper trough moving across Greenland has entered the NE Atlantic and merged with paragraph P4 upper trough in the last 36 hours. Upper trough leaves behind surface cold front stretched across the north Atlantic and into the NW Atlantic...while the remainder of the front leaves behind a pair of surface troughs mentioned in paragraph P7. Days ago...western convergence of this upper trough used to support a low-level ridge across the eastern US...but since the the upper trough is now part of the paragraph P4 upper trough...this surface ridge has been weakening and is becoming replaced by building western US low-level ridge mentioned in paragraph P1.

P3...Deep-layered subtropical low east of Bermuda...Invest 94-L...is beginning to move NW about the deep-layered ridge mentioned in paragraph P1. It will soon pass NE of Bermuda....then curve and accelerate northward toward Atlantic Canada in advance of the paragraph P1 mid-latitude system. At the surface...the subtropical low should be merging with one of the cold fronts of the paragraph P1 system...so it will likely be non-tropical by the time it reaches Atlantic Canada. The upper vortex will eventually become absorbed by the paragraph P1 upper trough.

P4...Upper trough approaching Europe has stalled while becoming energized in amplitude thanks to amplifying central Atlantic upper ridge to the west (paragraph P1) and thanks to upper trough from the NW (paragraph P2) merging with it. Eastern divergence of the amplifying upper trough has quickly spun up a surface frontal cyclone offshore of Portugal and Spain. NW flow on the back side of this upper trough converges with W flow on the north side of the upper ridge...supporting a greater-than-1032 mb strong surface ridge that has moved from the W Atlantic to central Atlantic in the last 36 hrs.

P5...What is left of the eastern Atlantic surface ridge has merged with the central Atlantic surface ridge mentioned in paragraph P4. Associated dry air in the eastern tropical Atlantic is now being wafted westward by the south side of the paragraph P4 central Atlantic surface ridge...and by south side of the paragraph P1 central Atlantic upper ridge.

...TROPICAL BELT DISCUSSION...
P6...SE Mexico upper ridge has been eroded by amplifying paragraph P1 upper trough to the north. Yesterday's retrograding upper vorticity crossing the Lesser Antilles has spread across the SE half of the Caribbean Sea. Upper ridge cell over tropical wave ex-92L (paragraph P7) remains amplified across Cuba...the Bahamas...and W Atlantic in advance of paragraph P1 upper trough. Easterly flow across north side of SE Caribbean upper vorticity is converging with northerly flow from this upper ridge cell to produce sinking dry air in the E half of the Caribbean. Tropical upper ridge cell in the central tropical Atlantic remains. In the vicinity of the Cape Verde Islands...upper troughing over Nadine appears to be cutting-off to the east of this central tropical Atlantic upper ridge cell.

P7...Tropical wave ex-92L in the western Caribbean in the previous discussion is now moving into SE Mexico. Flanking the tropical wave is a surface trough in the Bay of Campeche and another over the W Bahamas...originating from the decaying surface front in paragraph P2. Outflow of the paragraph P6 upper ridge cell overhead and surface forcing of W Bahamas surface trough is creating a disturbance offshore of the SE US. The tropical wave itself and Bay of Campeche surface trough under hostile westerly shear from south side of paragraph P1 upper trough.

P8...The tropical wave nearing the Lesser Antilles in the previous discussion is dissipating while moving into the NE Caribbean.

P9...Tropical wave west of the Cape Verde Islands in the previous discussion is now midway between the Cape Verdes and Lesser Antilles. It has overcome the paragraph P5 dry air in the last 36 hours thanks to enhanced poleward upper outflow streaming into the deep-layered low in paragraph P3.

P10...Satellite imagery suggests a potent tropical wave with a low pressure spin has stalled out along the W coast of Africa. The stalled motion is due to absence of low-level steering easterlies thanks to ridge weakness from paragraph P4 weather system and remnant low of Nadine.

2012 Atlantic Hurricane Season Birdseye Discussion #113

By: NCHurricane2009, 9:14 AM GMT on September 21, 2012

...SEPTEMBER 21 2012...5:15 AM EDT...
Tropical Storm Nadine beginning to move southward away from the Azores. Tropical storm warnings remain in effect across the islands...but could be dropped later today if Nadine moves sufficiently away. The future of Nadine remains uncertain...but there is a possibility that the storm could recurve southward then southwestward back into the Atlantic tropics in the next 48 to 72 hours. Currently it appears there will be upper troughing over the cyclone thru that time...so its re-emergence into the tropical belt is more likely as a subtropical storm rather than a tropical storm. See Nadine special feature section below for details.

Tropical wave moving across the Caribbean Sea...formerly Invest 92-L...is producing disturbed weather over Cuba and or the Bahamas as expected previously. South Florida is also covered by some of this activity. If the disturbed weather does not show signs of tropical development...then I will be finally dropping this system as a special feature on this blog. For now...see the ex-92L special feature section below for details on this wave.

Cut-off deep-layered vortex has west of Nadine and east of the Bermuda...Invest 94-L....still has the potential to become a subtropical depression or storm. See Invest 94-L special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0131Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
Nadine has actually slightly strengthened in the last 24 hours...which I diagnose is due to supportive divergence directly over Nadine (supplied by east side of paragraph P3 upper trough)...and supportive divergence just NE of Nadine (supplied by east side of upper trough over Nadine which has recently amplfied further into an upper vortex). With Nadine in the last day not showing upper anticyclonic outflow in the 200 mb wind barbs in the above atmo birdseye chart...nor showing cirrus outflow clouds on satellite...this indicates to me a shallower warm core system that is supported by divergence from cold core upper troughs (i.e. a subtropical storm). However...I have never heard of a case where the NHC downgrades a tropical cyclone to a subtropical cyclone operationally. Some examples of systems transitioning from tropical to subtropical (according to NHC post-storm analysis) are Lee 2011...Gordon 1994...and Allison 2001. The recent NHC discussions state that they may soon downgrade Nadine into an extratropical (non-tropical) low as the cold front of the paragraph P3 system to the north penetrates the circulation. However...I currently observe Nadine maintaining independence from this cold front cloud band while firing a well-curved t-storm band in her south half...so I prefer to maintain Nadine as a tropical or subtropical cyclone thru the forecast period.

Due to the W Atlantic deep-layered ridge to the west (described in paragraph P2)...Nadine is beginning to turn southward according to the most recent segment of the NHC recorded storm track in Figure 1...and according to infrared satellite loops. With the surface component of the deep-layered ridge forecasted by 00Z GFS to be greater than 1030 mb for the next days...I am not shy to show a track that curves south then southwest (based on the low-level flow around the ridge shown in 00Z GFS). However...I show my south and SW track at a slower pace than the previous in case I am wrong...because the NHC official forecast has not budged toward that solution. I also am not forecasting beyond 48 hours in case the scenario I posed in Figure 1 is incorrect. My forecast track is still further south than the 00Z GFS itself...as the GFS prefers to slow the southward track of Nadine by 48 hours than curve her slowly ENE by 72 hours. I suppose the GFS gives more credence to stagnating swirling upper flow of the upper troughing above Nadine...and or gives more credence to the paragraph P3 upper trough passing by to the north (which will become very impressive to the NE of Nadine as it amplifies to the east of the deep-layered ridge). The GFS solution of an ENE track by 72 hours however does not make sense to me...as it simultaneously shows the trough de-amplifying during that time (I would expect the ENE turn would be caused by the trough amplifying further or at least maintaining amplitude).


Figure 1: My forecast for Tropical Storm Nadine created this morning.

Previously my thoughts on Nadine continuing SW into the tropics (i.e. following my forecast track in Figure 1) is that it would eventually get blasted by northerly shear while sliding beneath the west side of the upper troughing. But now the GFS this morning shows a portion of the upper troughing splitting off while retrograding about the deep-layered ridge. Now if Nadine follows my forecast track...this retrograding upper troughing would keep the vertical shear over Nadine low. Therefore thru the forecast period...I show Nadine maintaining strength. I do not declare it non-tropical by 24 hours like the NHC does...as I model Nadine diving southward into increasing instability as she re-enters warm waters above 26 deg C (instability further enhanced by cold core upper troughing above the cyclone).

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius...which I maintain through the forecast period based on the intensity forecasts from me and the NHC. The swath is kept symmetrical through the forecast as she remains under low shear directly under the upper troughing.

...SPECIAL FEATURE...TROPICAL WAVE EX-92L...
The tropical wave in the central Caribbean in the previous discussion is now moving into the western Caribbean. The paragraph P5 upper ridge cell over the area has become amplified over Cuba and the Bahamas by warm air advection ahead of the paragraph P1 and P2 weather systems...the outflow of which supports scattered t-storm activity across south Florida...Cuba...and the Bahamas this morning. The surface frontal zone of the paragraph P2 system may also contribute. Currently watching to see if a tropical disturbance emerges due to surface forcing of incoming frontal zone and the tropical wave...coupled with the favorable upper outflow of the amplified upper ridge cell.

...SPECIAL FEATURE...SUBTROPICAL LOW INVEST 94-L...
Deep-layered subtropical low east of Bermuda persists. Sea-surface temps remain in the the 27 to 28 deg C range...the upper vortex above is keeping the surface spin in favorable low shear...and it appears the upper vortex is cold enough for instability above these waters (as evidenced by a continued ring of t-storms around the surface spin). I was expecting this to have become a subtropical depression or storm by now. I still forecast this to briefly become a subtropical cyclone before the conditions become less favorable to do so in the next 48 hours.

Currently the deep-layered subtropical low is trapped in meandering steering currents below its upper vortex...and is blocked from any W or NW progression thanks to W Atlantic deep-layered ridge (paragraph P2) hugging the system. In the next 48 hours...the deep-layered ridge should pass to the north...steering the upper vortex and subtropical cyclone W then NW such that is passes NE of Bermuda. It should then curve and accelerate northward toward Atlantic Canada in advance of the paragraph P1 and P2 mid-latitude systems. At the surface...the subtropical cyclone should be merging with the cold front of the paragraph P2 system (if not...then the cold front of the paragraph P1 system)...so it will likely be non-tropical by the time it reaches Atlantic Canada. The upper vortex will become absorbed by the paragraph P1 upper trough.

...MID-LATITUDES DISCUSSION...
P1...Weather system (surface frontal cyclone supported by upper trough) in the mid-latitudes has dived southeast while absorbing a large portion of paragraph P2 upper trough. The surface cyclone is currently at 995 mb over southern Hudson Bay while the surface front is across the central US. Western convergence of the upper trough supports dry air across the southern US and a building surface ridge over the western US.

P2...Major upper trough from eastern Canada to the Gulf of Mexico has split in the last 24 hours. The portion over the US and Gulf of Mexico has been absorbed by paragraph P1 upper trough. The east Canada portion has shifted east across Greenland. The associated surface cold front is stretched from Greenland...across the W Atlantic...and into the Gulf of Mexico. North end of the front is anchored by a surface low that weakened from 981 to 992 mb in the last 24 hours while shooting NE into Greenland from eastern Canada. This surface low remains whirled beneath the less-divergent axis of the upper trough...and therefore may continue to weaken. Southerly flow ahead of the 992 mb low is supporting a W Atlantic upper ridge via warm air advection. This upper ridge has stacked with the W Atlantic surface ridge in paragraph P3 to produce a deep-layered ridge. Finally...western convergence of this upper trough used to support a low-level ridge across the eastern US...but since the main portion of the upper trough is now over Greenland...this surface ridge is weakening and becoming replaced by building western US low-level ridge mentioned in paragraph P1.

P3...Upper trough continues progressing eastward across the Atlantic high seas and is nearing Europe. Western convergence of the upper trough supports a strong low-level ridge over the W Atlantic that currently features a greater-than-1030 mb center.

P4...Eastern Atlantic surface ridge...currently with a 1020 mb center...persists. It is supported as NW upper flow from upper troughing over Nadine and SW upper flow from paragraph P5 upper ridging convergence. As noted in the last days...dry air in the eastern tropical Atlantic is still noted with this surface ridge...perhaps as it south side wafts Africa desert dry air westward. The dry air could is also enhanced by the aforementioned upper convergence.

...TROPICAL BELT DISCUSSION...
P5...Central America and SE Mexico upper ridge cell is now just over SE Mexico...with W Caribbean upper trough moving into Central America while peristing to the east of that. Yesterday's retrograding upper vortex nearing the Lesser Antilles is about to cross the islands. Two upper ridge cells (one to the east and the other to the west) continue flanking this upper vortex...one of which is over tropical wave ex-92L...the other which has become vertically stacked with 1020 mb center of paragraph P4 surface ridge...in essence creating a deep-layered 1020 mb center. Inverted upper trough west of the Cape Verde Islands continues retrograding westward about what is now the 1020 mb deep-layered center. Another inverted upper trough is forming just NW of the Cape Verde Islands...perhaps a fracture of the upper troughing over Nadine that will soon retrograde SW then W about the 1020 mb deep-layered center.

P6...The tropical wave nearing the Lesser Antilles has lost its moisture and activity while becoming suppressed by the upper convergence of the paragraph P5 upper vortex nearby. The upper vortex is not cold enough to destabilize the atmosphere for t-storms to re-fire with the wave.

P7...Tropical wave over and south of the Cape Verde Islands in the previous discussion is now west of the islands. It remains suppressed by the paragraph P4 dry air.

2012 Atlantic Hurricane Season Birdseye Discussion #112

By: NCHurricane2009, 9:09 AM GMT on September 20, 2012

...SEPTEMBER 20 2012...5:15 AM EDT...
Tropical Storm Nadine producing gusty winds for the western and central Azores. Tropical storm warnings remain in effect across the islands. The future of Nadine is uncertain...but there are increasing indications that the storm could recurve southward then southwestward back into the Atlantic tropics in the next 72 hours. Currently it appears there will be upper troughing over the cyclone thru that time...so its re-emergence into the tropical belt is more likely as a subtropical storm rather than a tropical storm. See Nadine special feature section below for details.

Tropical wave moving across the Caribbean Sea...formerly Invest 92-L...could produce a disturbance over Cuba and or the Bahamas in the next 72 hours...and therefore I still am not eliminating it as a special feature on this blog. See the ex-92L special feature section below for details.

Anticipated cut-off deep-layered vortex has formed west of Nadine and east of the Bermuda and has been upgraded to Invest 94-L due to its potential in becoming a subtropical depression or storm. See Invest 94-L special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0123Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
As long anticipated...the paragraph P3 mid-latitude system and its upper trough has approached Nadine from the west while decomposing into two features. The first is a cut-off deep-layered vortex west of Nadine (special feature Invest 94-L). The second is an upper trough passing by to the north. What I did not anticipate is Nadine maintaining strength in the last 24 hours...which I diagnose is due to supportive divergence directly over Nadine (supplied by east side of paragraph P3 upper trough)...and supportive divergence just NE of Nadine (supplied by east side of what I previously described in the last two Nadine special feature sections as a "pesky" upper trough W of Nadine...which is now over Nadine). With Nadine no longer showing upper anticyclonic outflow in the 200 mb wind barbs in the above atmo birdseye chart...this indicates to me a shallower warm core system that is supported by divergence from cold core upper troughs (i.e. a subtropical storm). However...I have never heard of a case where the NHC downgrades a tropical cyclone to a subtropical cyclone operationally...we shall see. Some examples of systems transitioning from tropical to subtropical (according to NHC post-storm analysis) are Lee 2011...Gordon 1994...and Allison 2001.

Due to developing deep-layered ridge to the NW (described in paragraph P2)...Nadine is beginning to accelerate southeastward according to infrared satellite loops. The track on infrared satellite seems both faster and slightly left of the 11 PM EDT NHC forecast...so my track forecast in Figure 1 has a slight left bias and is faster with respect to NHC from the get-go. With the surface component of the deep-layered ridge forecasted by 00Z GFS to be greater than 1030 mb for the next 72 hours...I am not shy to show a track that curves south then southwest (based on the low-level flow around the ridge shown in 00Z GFS). My forecast track is even faster than the 00Z GFS itself...as the GFS prefers to slow the southward track of Nadine by 48 and 72 hours. I suppose the GFS gives more credence to stagnating swirling upper flow of the upper troughing above Nadine...and or gives more credence to the paragraph P3 upper trough passing by to the north (which will become very impressive to the NE of Nadine as it amplifies to the east of the deep-layered ridge). I do not forecast beyond 72 hours in case my aggressive south and SW track is wrong.


Figure 1: My forecast for Tropical Storm Nadine created this morning.

It is worth noting that if Nadine continued on my forecasted brisk pace shown in Figure 1...it would enter hostile northerly shear on the west side of the upper troughing by 120 hrs. But for the next 72 hours...I show Nadine maintaining strength. I do not declare it non-tropical by 24 hours like the NHC does...as I model Nadine diving southward into increasing instability as she re-enters warm waters above 26 deg C (instability further enhanced by cold core upper troughing above the cyclone).

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius...which I maintain through the forecast period based on my new intensity forecast. The swath is kept symmetrical through the forecast as she remains under low shear directly under the upper troughing. This swath covers the western and central Azores...but all of the Azores should continue to experience outer rain squalls from Nadine's north side in the next 24 hours...these squalls containing wind gusts that could reach tropical storm force (my impact swath represent tropical storm sustained winds as opposed to gusts).

...SPECIAL FEATURE...TROPICAL WAVE EX-92L...
The tropical wave in the eastern Caribbean in the previous discussion is now moving west across the central Caribbean. The tropical wave is sliding out of the paragraph P5 upper ridge cell....and into the less favorable environment beneath the W Caribbean upper trough (also mentioned in paragraph P5). However...there are t-storm clouds (albeit scattered) persisting beneath the favorable upper ridge cell over Hispaniola and Jamaica. Moreover for the next 72 hours...there is potential for the favorable upper ridge cell to become amplified over Cuba and the Bahamas by warm air advection ahead vigorous paragraph P2 weather system. The surface frontal zone of the paragraph P2 system may also contribute. Therefore...expect the t-storm activity to spread into Cuba and the Bahamas...where a tropical disturbance could emerge beneath the favorable upper ridge cell.

...SPECIAL FEATURE...SUBTROPICAL LOW INVEST 94-L...
Paragraph P3 in the mid-latitudes discussion best describes the formation of this deep-layered subtropical low currently east of Bermuda. Sea-surface temps are in the 27 to 28 deg C range...the upper vortex above is keeping the surface spin in favorable low shear...and it appears the upper vortex is cold enough for instability above these waters (as evidenced by a symmetric ring of t-storms around the surface spin). I currently expect a 100% chance that this will become a subtropical depression or subtropical storm by early tomorrow morning (if not sooner).

Currently the developing subtropical cyclone is trapped in meandering steering currents below its upper vortex...and is blocked from any W or NW progression thanks to developing deep-layered ridge (paragraph P2) hugging the system. In the next 72 hours...the deep-layered ridge should pass to the north...steering the upper vortex and subtropical cyclone W then NW such that is passes NE of Bermuda. It should then curve and accelerate northward toward Atlantic Canada in advance of the paragraph P2 mid-latitude system. At the surface...the subtropical cyclone should be merging with the cold front of the paragraph P2 system...so it will likely be non-tropical by the time it reaches Atlantic Canada. The upper vortex will become absorbed by the paragraph P2 upper trough.

...MID-LATITUDES DISCUSSION...
P1...The next weather system (surface frontal cyclone supported by upper trough) in the mid-latitudes is diving southeast while attracted to the low pressure field of the immense weather system in paragraph P2 below. The surface cyclone is currently at 998 mb over south-central Canada with its front across the central US.

P2...Vigorous system in the mid-latitude westerlies anchored by a major upper trough remains across the eastern US and eastern Canada...and its south extent has reached the Gulf of Mexico. The associated surface cold front is also stretched across this region and slowly moving into the W Atlantic. Along the front...the remnant low of former disturbance Invest 93-L dissipated at 1014Z yesterday in the vicinity of NE NC/SE VA. Most impressive feature along the front is a surface low that intensified from 991 to 981 mb in the last 24 hours while shooting northward into eastern Canada from the NE US. This surface low has whirled beneath the less-divergent axis of the upper trough...and therefore may start to weaken. Southerly flow ahead of the 981 mb low is supporting a building W Atlantic upper ridge via warm air advection. This upper ridge will soon stack with W Atlantic surface ridge in paragraph P3 to produce a deep-layered ridge. Finally...western convergence of the major upper trough supports plenty of dry air across the southern US and a low-level ridge across the eastern US.

P3...Upper trough in the NW Atlantic in the previous discussion is now shooting eastward across the Atlantic high seas. Eastern divergence of the upper trough supported the formation of a new surface low east of Bermuda...and now this surface low is deep-layered as the south end of this upper trough became a cut-off upper vortex over the surface low. This deep-layered low has a chance of becoming a subtropical depression or storm. See Invest 94-L special feature section above for details. Meanwhile... western convergence of the upper trough supports dry air and a strong low-level ridge over the W Atlantic. The weakening surface vortex being steered into Spain and Portugal by this system was along the coats of Portugal at 1014 mb as of 0000Z TAFB.

P4...Eastern Atlantic surface ridge...currently with a 1019 mb center...persists. It is supported as NW upper flow from paragraph P3 upper trough (and upper trough over Nadine noted in Nadine special feature section) and SW upper flow from paragraph P5 upper ridging converge. As noted in the last 72 hrs...increased dry air in the eastern tropical Atlantic is still noted with this surface ridge...perhaps as it south side wafts Africa desert dry air westward. The dry air could is also enhanced by the aforementioned upper convergence.

...TROPICAL BELT DISCUSSION...
P5...Central America and SE Mexico upper ridge cell persists...with Caribbean upper trough (now in the W Caribbean) persisting to the east of that. Yesterday's retrograding upper trough midway between the Cape Verdes and Lesser Antilles (mentioned in paragraph P5 of discussion #111) is nearing the Lesser Antilles as an upper vortex. Two upper ridge cells (one to the east and the other to the west) continue flanking this upper vortex ...one of which is over tropical wave ex-92L. Inverted upper trough near the Cape Verde Islands is now west of the Cape Verde Islands. Upper ridge cell toward Africa persists in relatively higher pressures NE of this inverted upper trough and SE of the upper trough over Nadine (noted in Nadine special feature section).

P6...The tropical wave midway between the Cape Verdes and Lesser Antilles in the previous discussion is nearing the Lesser Antilles while still producing a batch of moisture in between the paragraph P4 dry air to the east and dry air of nearby paragraph P5 retrograding upper trough/vortex to the west. This moisture is helped by split flow upper divergence at the boundary between this retrograding upper trough/vortex and one of the upper ridge cells in paragraph P5.

P7...Suspect tropical wave emerging from Africa in the previous discussion has been added into NHC TAFB maps in the last 24 hours...positioned over and south of the Cape Verde Islands as of 0000Z TAFB. It is suppressed by the paragraph P4 dry air.

2012 Atlantic Hurricane Season Birdseye Discussion #111

By: NCHurricane2009, 7:52 AM GMT on September 19, 2012

...SEPTEMBER 19 2012...4:00 AM EDT...
Tropical Storm Nadine still heading northeastward toward the Azores. In the last 24 hours...a tropical storm warning has been issued for various islands across the Azores...so interests in the Azores should be prepared. See the Nadine special feature section below for additional details. Visit www.nhc.noaa.gov and click on the latest public advisory under the "Nadine" section for the latest watches and warnings for the Azores.

Tropical wave that has moved into the Caribbean Sea...formerly Invest 92-L...could encounter more favorable upper winds in the next days...and so I maintain it as a special feature on this blog. See the second special feature section below for details.

As paragraph P3 of the mid-latitudes discussion highlights...a 1015 mb frontal low has formed east of Bermuda. This is the beginning of the cut-off deep-layered vortex forming west of Nadine (as highlighted in the Nadine special feature section). This location features water temps in the 27 to 28 deg C range...and coupled with what should be a vigorous cold upper vortex aloft...the atmosphere could become unstable enough for subtropical cyclone development. I may upgrade this to a special feature during my next full discussion.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1925Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
Nadine has tracked more eastward than northward in the last 24 hours than previously thought...which requires me to adjust my forecast track in Figure 1 more southward than the previous. With that said...I am still more north (or left) of the NHC forecast. One reason is that the latest infrared loop shows the swirl center of Nadine tracking faster to the north than the current NHC forecast. Second...I think Nadine will wiggle NW in the next 12 hrs toward a low-level ridge weakness associated with what is now the 1005 mb low in paragraph P3. As the weakness passes by to the north between 12 and 24 hrs...I believe Nadine will then wiggle back NE while trying to link with that weakness.

The aforementioned 1005 mb system and its upper trough approach Nadine while decomposing into two upper vortices...thanks to strong and deep-layered ridge developing in warm air advection ahead of the paragraph P2 weather system. The first is a cut-off deep-layered vortex west of Nadine (which could become a subtropical cyclone as mentioned in the intro of this full discussion). The second is a gradually developing upper vortex to the north. Tonight's 00Z GFS shows this developing upper vortex sliding SE in a way that puts Nadine in its SW side as opposed to its south side...and therefore I show a more SE track for the longer-term as opposed to a straight east track like I showed earlier. The 00Z GFS still shows the developing upper vortex supporting a new surface non-tropical cyclone to the NE of Nadine that could ultimately absorb what is left of Nadine later on.


Figure 1: My forecast for Tropical Storm Nadine created this morning.

Intensity-wise...Nadine in the last 24 hrs has weakened as I have forecasted previously...so I maintain the weakening rate shown in my previous forecast. Pesky shortwave upper trough just west of Nadine...mentioned in the Nadine special feature of previous discussion #110...persists I believe due to relatively lower pressures just southwest of Nadine's warm core upper outflow structure. This shortwave upper trough continues imparting southerly shear...and its western convergence is producing dry air...all of which currently prevent Nadine from developing t-storms in her south half. Another factor contributing to weakening is Nadine being over waters below 26 deg C. In the next 24 hours...an increase in SW vertical shear should develop...delivered by the two upper vortices mentioned in the above track forecast discussion. Tonight's 00Z GFS shows the developing northern upper vortex parking its SW side over Nadine by 48 hours. Although this would reduce vertical shear...I do not think it will deliver enough cold upper air to destabilize things (considering the cool waters Nadine is over)...therefore acting as a cap that suppresses Nadine's upper outflow. I think this is the final straw that will dissipate Nadine into a remnant low by 48 hours.

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius...which I evolve to represent a system that loses its tropical storm force winds in the next 48 hours (on the presumption Nadine follows my track and intensity forecast). My dissipating swath barely covers the western and central Azores...but all of the Azores should continue to experience outer rain squalls from Nadine's north side...these squalls containing wind gusts that could reach tropical storm force (my impact swath represent tropical storm sustained winds as opposed to gusts).

...SPECIAL FEATURE...TROPICAL WAVE EX-92L...
The tropical wave crossing the Lesser Antilles last evening is now in the eastern Caribbean Sea. The easterly vertical shear delivered by south side of paragraph P5 upper ridging appears to be diminishing as the wave axis catches up to the more concentrated storm activity in the NE Caribbean. Relaxation in shear is thanks to upper ridge cell developing over this tropical wave as mentioned in paragraph P5. Moreover...there is potential for the favorable upper ridge to get enhanced by warm air advection ahead of what is now the vigorous paragraph P2 weather system as this tropical wave crosses the Caribbean Sea. Therefore...despite the current feebleness of this tropical wave...interests in the Caribbean should continue monitoring this tropical wave. However...development could be obstructed by one of three things:

(1) If the tropical wave becomes better organized about the NE Caribbean t-storm activity...in which case it would be disrupted by land interaction while traveling across the northern Caribbean Islands.

(2) If the tropical wave gets too close to the less favorable Caribbean upper trough mentioned in paragraph P5...

(3) If the tropical wave later on gets too close to the highly-amplified upper trough associated with the paragraph P2 weather system.

...MID-LATITUDES DISCUSSION...
P1...The next weather system (surface frontal cyclone supported by upper trough) in the mid-latitudes has barely entered the upper-left corner of the above birdseye charts. The system is sweeping southeast while attracted to the low pressure field of the immense weather system in paragraph P2 below. The warm front of the frontal cyclone has reached the Dakotas and northern Nebraska...as it pushes out cold air recently advected in by the back side of the paragraph P2 weather system.

P2...Vigorous system in the mid-latitude westerlies is anchored by "super" upper trough across the eastern US and eastern Canada...the formation of which was mentioned in paragraph P1 of previous discussion #110. The associated surface cold front is also streteched across this region. Along the front is a 1006 mb surface low centered over Georgia as of 1800Z...which is the remnant of former tropical disturbance 93-L. While ex-93L continues to be supported by immense eastern divergence of the "super" upper trough...the more impressive feature along the cold front is the rapid genesis of a 991 mb low taking advantage of this upper divergence maximum...which has pulled a lot of ex-93L's moisture and activity into the NE US and eastern Canada in the last 24 hours. Southerly flow ahead of ex-93L and the 991 mb low is supporting a building W Atlantic upper ridge via warm air advection. This upper ridge will soon stack with 1026 mb W Atlantic surface ridge in paragraph P3 to produce a deep-layered ridge. Finally...western convergence of the "super" upper trough supports plenty of dry air and a 1025 mb low-level ridge across the western US.

P3...Upper vortex over eastern Canada has de-amplified into an upper trough as it moves across the NW Atlantic...and the 994 mb surface cyclone below its axis has weakened to 1005 mb in the last 24 hours. Further weakening of this surface cyclone is possible as long as it remains below the non-divergent axis of the upper trough. Eastern divergence of the upper trough has supported the formation of a new 1015 mb low east of Bermuda and along the cyclone's cold front in the last 24 hours. Western convergence of the upper trough supports dry air and a 1026 mb low-level ridge over the W Atlantic. Warm air advection ahead of the 1005 mb cyclone continues supporting a north Atlantic upper ridge wave which has overspread Tropical Storm Nadine. Elsewhere...upper trough is still sliding eastward toward Europe while driving weakening surface vortex (marked by two red Ls in upper-right of above atmo birdseye chart) into Portugal and Spain. Western convergence of this upper trough supports NE Atlantic surface ridge...which has become tucked under the upper ridge wave over Nadine to create a deep-layered ridge center NE of Nadine.

P4...Eastern Atlantic surface ridge...currently with a 1018 and 1019 mb center...persists. As noted in the last 48 hrs...increased dry air in the eastern tropical Atlantic is still noted with this surface ridge...perhaps as it south side wafts Africa desert dry air westward.

...TROPICAL BELT DISCUSSION...
P5...Central America and SE Mexico upper ridge cell persists...with central Caribbean upper trough and associated dry air persisting to the east of that. Yesterday's upper trough retrograding about central Atlantic upper ridge cell (mentioned in paragraph P6 of discussion #110) is midway between the Cape Verdes and Lesser Antilles with associated dry air...and this feature has now split the central Atlantic upper ridge cell into two cells...one of which is over tropical wave ex-92L. Upper vortex over the Cape Verde Islands has weakened into an inverted upper trough that looks to soon retrograde westward. Upper ridge cell toward Africa persists in relatively higher pressures east of this inverted upper trough.

P6...The tropical wave midway between the Cape Verdes and Lesser Antilles is producing a batch of moisture in between the paragraph P4 dry air to the east and dry air of nearby paragraph P5 retrograding upper trough to the west. This moisture is helped by split flow upper divergence at the boundary between this retrograding upper trough and one of the upper ridge cells in paragraph P5.

P7...Satellite imagery suggests the next tropical wave is about to emerge from Africa (lower-right corner of above atmo birdseye chart). While it is entering the favorable low shear and enhanced upper outflow environment beneath the upper ridge cell toward Africa (paragraph P5)...it may soon have to combat the unfavorable paragraph P4 dry air.

2012 Atlantic Hurricane Season Birdseye Discussion #110

By: NCHurricane2009, 6:21 AM GMT on September 18, 2012

...SEPTEMBER 18 2012...2:30 AM EDT...
Tropical Storm Nadine persisting while heading northeastward toward the Azores. The cyclone will be in the vicinity of the Azores within the next 5 days...so interests in the Azores should continue to monitor the progress of this system and be ready to make preparations in case the storm makes a direct impact. See the Nadine special feature section below for additional details.

Tropical wave approaching the Caribbean Sea...Invest 92-L...is crossing the Lesser Antilles tonight while earlier this afternoon producing a more concentrated storm cluster sheared westward into the NE Caribbean. Due to potential for more favorable upper winds in the next days...I maintain this as a special feature on this blog at this time. See the second special feature section below for details.

Tropical disturbance Invest 93-L already becomes non-tropical while moving NE into the southeastern United States from Louisiana. It is merging with the frontal system in paragraph P1 of the mid-latitudes discussion...becoming quiet energized and producing widespread impacts as it does so. See paragraph P1 in the mid-latitudes discussion for details on this situation.

As the Nadine special feature section highlights...a cut-off deep-layered vortex is forecast to form west of the storm and east of Bermuda by 48 hours. This location features water temps in the 28 deg C range...and coupled with what should be a vigorous cold upper vortex...the atmosphere could become unstable enough for subtropical cyclone development beneath the cold core upper vortex beginning by that time.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1925Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
Nadine's northward deflection in the last 24 hours has been a bit more north than thought by the previous NHC forecasts. Therefore I am adjusting my updated forecast points in Figure 1 with a northward shift from the previous.

The northward deflection in the track is the result of the paragraph P5 low-level ridge bridging with the NE Atlantic low-level ridge in paragraph P2...the bridge forming at a location east of Nadine. I begin disagreeing with the NHC track forecast in between 24 and 36 hours....when I think Nadine will wiggle NW toward a low-level ridge weakness associated with what is now the 994 mb low in paragraph P2. As the weakness passes by to the north between 36 and 48 hrs...I believe Nadine will then wiggle back NE while trying to link with that weakness.

The aforementioned 994 mb system and its upper trough approach Nadine while decomposing into into two upper vortices...thanks to strong and deep-layered ridge developing in warm air advection ahead of the paragraph P1 weather system. The first is a cut-off deep-layered vortex west of Nadine (which could become a subtropical cyclone as mentioned in the intro of this full discussion). The second is a gradually developing upper vortex to the north which still appears to have the potential to drag Nadine eastward between 48 and 72 hrs. Tonight's 00Z GFS handles this northern upper vortex differently than the previous night's...which shot the upper vortex southward across Nadine due to the strength of the deep-layer ridge. Now the 00Z GFS shows the upper vortex moving more east than south...and creating a new surface non-tropical cyclone NE of Nadine. I now prefer to continue dragging Nadine eastward beyond 72 hours while it gets sucked into this non-tropical cyclone. In what is a highly uncertain situation where the model consensus looks like a messy spiderweb...this is putting a lot of faith in the GFS model fields...but the NHC stated at 11 PM EDT that the GFS has done well with Nadine so far.


Figure 1: My forecast for Tropical Storm Nadine created this morning.

Intensity-wise...Nadine in the last 24 hrs has weakened more than I expected...now down to 60 mph max sustained winds as of the earlier 11 PM EDT NHC advisory. I was thinking that the less shearing (more favorable) north Atlantic upper ridge wave (propped up by warm air advection ahead of the paragraph P2 994 mb low) would prevent Nadine from weakening in the previous 24 hrs. I believe the unexpected weakening is due to a pesky shortwave upper trough just west of Nadine...mentioned in paragraph P4 of previous discussion #109. I was expecting the aforementioned warm air advection to de-amplify this shortwave...but instead it persists I believe due to relatively lower pressures just southwest of Nadine's warm core upper outflow structure. This shortwave upper trough is imparting southerly shear...and its western convergence is producing dry air...all of which currently prevent Nadine from developing t-storms in her south half.

Her more northward than expected current position has already caused her center to cross the 26 deg C isotherm into cooler waters. I generally adopt my weakening rate from my previous forecast...which also accounts for an increase in SW vertical shear after 24 hours delivered by the two upper vortices mentioned in the above track forecast discussion. This is very different from the much more bullish NHC forecast (which shows no weakening for the next 5 days). I suppose the NHC sees a gradual transition into a non-tropical low supported by divergence from the shearing SW jet...but alternatively I see Nadine in unfavorable upper convergence on the SW and south sides of the northern upper vortex. Tonight's 00Z GFS shows the developing northern upper vortex dropping off a weak and mini upper vortex over Nadine by 72 hours. Although this mini upper vortex will reduce vertical shear...it will be nowhere near strong (i.e. cold) enough for destabilizing the atmosphere over the cool waters Nadine will be over...therefore acting as a cap that suppresses Nadine's upper outflow. I think this is the final straw that will dissipate Nadine into a remnant low by 72 hours.

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius...which I lean rightward with respect to the storm track after 24 hrs to account for a forecasted increase in SW vertical shear for that timeframe. The swath is also progressively shrunken to represent the weakening Nadine I show. My dissipating swath barely covers the western and central Azores...but all of the Azores should be vigilant in case Nadine follows the more bullish NHC intensity guidance while simultaneously following my forecast track. I also rolled back the time of arrival of direct impacts to the Azores in impact statement (a) of Figure 1...to account for the more northward than expected initial position of the storm.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 92-L...
The tropical wave approaching the Caribbean Sea is crossing the Lesser Antilles tonight while earlier this afternoon producing a more concentrated storm cluster sheared westward into the NE Caribbean. The shear is driven by upper easterlies on the SW quad of the paragraph P6 central Atlantic upper ridge cell...but this quadrant of the upper ridge cell remains generally supportive with split flow upper divergence.

The Caribbean upper trough (paragraph P6) continues to retrograde westward as to allow the SW quad of the central Atlantic upper ridge cell to split off into a favorable shear-reducing and outflow-enhancing upper anticyclone directly over this tropical wave. Moreover...there is potential for the favorable upper anticyclone to get enhanced by warm air advection ahead of what is now the vigorous paragraph P1 weather system as this tropical wave crosses the Caribbean Sea. Therefore...despite the current feebleness of this tropical wave...interests in the Caribbean should continue monitoring this tropical wave over the next days. However...development could be obstructed if the tropical wave gets too close to the less favorable retrograding Caribbean upper trough...or later on gets too close to what is forecast by models to be a highly-amplified upper trough associated with the paragraph P1 weather system.

...MID-LATITUDES DISCUSSION...
P1...Next system in the mid-latitude westerlies continues entering from the upper-left corner of the above birdseye charts...with the upper trough pushing into central Canada and the central US...and the associated surface frontal zone across the central US and eastern Canada. This upper trough is amplifying due to cool air advection from the surface cyclone it supports at the north end of the front. Simultaneously...the SW US upper trough is amplifying due to cool air advection behind disturbance Invest 93-L moving NE from the Gulf of Mexico and into the SE US. As the two amplifying upper troughs merge...a "super" upper trough is forming with vast eastern upper divergence that is causing Invest 93-L to bomb out as it moves across the SE US. 93-L is already producing heavy flooding rain...and severe weather is possible as its east half drives warm air advection and an amplifying upper ridge wave. Upper westerlies across this upper ridge wave are in directional shear with respect to low-level southerlies in 93-L's east half...providing helicity for possible tornadoes...especially in areas with instability triggered by last afternoon's or tomorrow afternoon's low-level daytime heating (tornado warnings were issued in east-central Tennessee last afternoon...and a tornado watch is in effect for parts of Georgia...upstate SC...SW NC...and SE Tennessee as of this writing). There are strong upper-level SW winds across the east side of the "super" upper trough (and above 93-L)...so even in areas with low helicity but high instability...a severe t-storm could mix down these upper winds into surface-level damaging straight line winds. Residents across the southeastern United States...and along the east US coast...should visit www.nws.noaa.gov and search for their local city for up to the minute forecasts and information on what 93-L might deliver.

P2...Cut-off upper trough over the SW US and associated surface disturbance Invest 93-L have merged with the paragraph P1 system (see paragraph P1 above for details on this system). Elsewhere...upper vortex over eastern Canada persists...and the surface cyclone it supports with its eastern divergence has intensified further to 994 mb but has also whirled beneath the center of the upper vortex. While now beneath the non-divergent upper vortex center...this surface 994 mb cyclone will now begin gradual decay. The western convergence of the upper vortex supports a 1020 mb low-level ridge that has moved offshore from the eastern US and into the W Atlantic. Warm air advection ahead of the 994 mb cyclone supports a north Atlantic upper ridge wave. Shortwave upper trough is still sliding eastward toward Europe and has absorbed long-lasting paragraph P4 deep-layer vortex south of the Azores. What's left is a 1008 mb surface vortex moving quickly ENE toward Portugal and Spain while steered and supported by this upper trough. Western convergence of this upper trough supports a low-level ridge moving into the NE Atlantic.

P3...Cut-off upper trough in the Gulf of Mexico continues de-amplifying thanks to warm air advection southeast and ahead of the paragraph P1 frontal system. This upper trough has moved onto the east coast of Florida and is merging with north end of retrograding Caribbean upper trough mentioned in paragraph P6.

P4...Deep-layered vortex south of the Azores has been absorbed by upper trough system heading toward Europe mentioned towards the end of paragraph P2 above. What's left of this system is mentioned in that paragraph.

P5...Eastern Atlantic surface ridge...currently with a 1018 mb center... persists. As noted 24 hrs ago...increased dry air in the eastern tropical Atlantic is still noted with this surface ridge.

...TROPICAL BELT DISCUSSION...
P6...Upper ridging across the tropical Atlantic persists. A Central America and SE Mexico upper ridge cell is split from the central Atlantic upper ridge cell via central Caribbean upper trough retrograding still slowly retrograding west. Dry air persists below this retrograding upper trough. Embedded upper vortex NW of the Cape Verde Islands has split into two entities...an upper trough retrograding westward about the central Atlantic upper ridge cell...and an upper vortex over the Cape Verde Islands themselves. Upper ridge cell toward Africa persists in relatively higher pressures east and south of these upper vorticity features.

P7...The tropical wave SW of the Cape Verde Islands in the previous discussion is moving into the waters midway between the Cape Verdes and Lesser Antilles. Despite being below low shear and favorable upper outflow beneath the paragraph P6 upper ridge cell toward Africa...it appears suppressed by adjacent dry air mentioned in paragraph P5.

2012 Atlantic Hurricane Season Birdseye Discussion #109

By: NCHurricane2009, 7:20 AM GMT on September 17, 2012

...SEPTEMBER 17 2012...3:30 AM EDT...
Hurricane Nadine weakenes to a tropical storm as I forecasted 24 hours ago. The cyclone will be in the vicinity of the Azores by 5 days out...so interests in the Azores should monitor the progress of this system. The worst case scenario at this time appears to be a high wind and heavy rain non-tropical cyclone for the islands. See the Nadine special feature section below for additional details.

Tropical wave Invest 92-L approaching the Lesser Antilles and Caribbean Sea appears quiet fragile at this time. Due to potential for more favorable upper winds in the next days...I maintain this as a special feature on this blog at this time. See the second special feature section below for details.

Disturbed weather Invest 93-L in the western Gulf of Mexico...the formation of which was described in special update #108A...remains unimpressive from a tropical perspective and therefore is still not considered a special feature on this blog. See paragraph P2 for details on this system.

As the Nadine special feature section highlights...a cut-off deep-layered vortex is forecast to form west of the storm and east of Bermuda by 72 hours. This location features water temps in the 28 deg C range...and coupled with what should be a vigorous cold upper vortex...the atmosphere could become unstable enough for subtropical cyclone development beneath the cold core upper vortex by that time.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0126Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
Nadine's eastward motion continues to be slightly faster than the previous forecasts even though those forecasts upped the pace of the eastward track. Therefore it would seem my updated forecast in Figure 1 should be another rightward adjustment for all forecast points. However...my previous forecasts had a southward bias for the timeframe that is the next 24 hours as I saw no reason to bend the track to the north until later on...but Nadine is turning more northward already. Since the NHC has adjusted their track a bit southward...I suppose it is time for me to adjust my track northward and meet the NHC in the middle...therefore agreeing with the NHC forecast track for the first 48 hours. In the end...my northward adjustment does not require me to have a rightward adjustment despite the initial faster than expected eastward motion. Also...Nadine's eastward track is finally slowing down.

The developing slowdown in Nadine's eastward track is the result of resistance from the Atlantic high seas low-level ridge (end of paragraph P2) entering NE Atlantic. The developing northward deflection in the track is the result of the paragraph P5 low-level ridge already bridging with the NE Atlantic low-level ridge...the bridge forming at a location east of Nadine. I begin disagreeing with the NHC track forecast in between 48 and 60 hours....when I think Nadine will wiggle left toward a low-level ridge weakness associated with what is now the 1001 mb low in paragraph P2. As the weakness passes by to the north between 60 and 72 hrs...I believe Nadine will then wiggle back rightward while trying to link with that weakness.

The aforementioned 1001 mb system and its upper trough approach Nadine while decomposing into into two upper vortices...thanks to strong and deep-layered ridge developing in warm air advection ahead of the paragraph P1 frontal system. The first is a cut-off deep-layered vortex west of Nadine (which could become a subtropical cyclone as mentioned in the intro of this full discussion). The second is an upper vortex to the north which now appears to have the potential to drag Nadine eastward between 72 and 96 hrs. The strength of the deep-layered ridge in the 00Z GFS causes it to shoot this second upper vortex southward such that Nadine would whirl counter-clockwise about the upper vortex in between 96 and 120 hrs.


Figure 1: My forecast for Tropical Storm Nadine created this morning.

Intensity-wise...Nadine in the last 24 hrs has weakened to a 70 mph max wind tropical storm as I previously forecasted...so I maintain the weakening rate I showed in my previous forecast. The developing slowdown in track for the next 24 hours should allow the less shearing (more favorable) north Atlantic upper ridge wave...propped up by warm air advection ahead of the paragraph P2 1001 mb low...to catch up to and overspread Nadine...so I flatten the weakening rate during this time. This warm air advection should cause the new and unfavorable shortwave upper trough W of Nadine to diminish as remarked in paragraph P4. I accelerate the weakening rate (faster than the NHC shows) by 48 to 96 hrs as Nadine's forecast track crosses the 26 deg C isotherm into cooler waters...and experiences unfavorable SW vertical shear delivered by the two upper vortices mentioned in the above track forecast discussion. I suppose the NHC shows a slower weakening rate as they expect Nadine to more gradually transition into a non-tropical low supported by divergence from this shearing SW jet...but alternatively I see Nadine in unfavorable upper converence on the SW and south sides of the northern upper vortex. In fact...I don't see Nadine receiving supportive upper divergence until it enters the east side of the northern upper vortex in between 96 and 120 hrs...which is finally when I flatten the weakening rate. I agree with the NHC on a non-tropical Nadine by 120 hrs.

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius...which I lean rightward with respect to the storm track by 48 to 96 hrs to account for a forecasted increase in SW vertical shear for that timeframe. The swath is also shrunken to represent a weakening Nadine in the latter part of the forecast. My swath only covers the western and central Azores at this time...but could easily cover all of the Azores if the radius of the counter-clockswise loop track at 96 to 120 hrs becomes larger...or if Nadine's wind field actually grows or maintains size if the more vigorous above-discussed NHC intensity forecast verifies.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 92-L...
The tropical wave approaching the Lesser Antilles and Caribbean Sea is struggling despite a vast area of supportive split flow upper divergence on the SW quad of paragraph P6 central Atlantic upper ridge cell. Perhaps the upper divergence region is too vast...creating a large area of east-west pressure falls as visible satellite imagery last afternoon suggested an east-west elongated system. With an elongated surface center (instead of a single tight center)...the surface convergence to drive t-storms is not focused enough for tropical development. Perhaps the Caribbean dry air mentioned in paragraph P6 is also not helping this system.

Computer models still insist that the Caribbean upper trough (paragraph P6) would continue to retrograde westward as to allow the SW quad of the central Atlantic upper ridge cell to split off into a favorable shear-reducing and outflow-enhancing upper anticyclone directly over this tropical wave. Moreover...model runs suggest this favorable upper anticyclone becoming enhanced by warm air advection ahead of what is now the paragraph P1 frontal system as this tropical wave crosses the Caribbean Sea. Therefore...interests in the Caribbean should monitor this tropical wave over the next days. However...development could be obstructed if the tropical wave gets too close to the less favorable retrograding east Caribbean upper trough...or later on gets too close to what is forecast by models to be a highly-amplified upper trough associated with the paragraph P1 frontal system.

...MID-LATITUDES DISCUSSION...
P1...Next system in the mid-latitude westerlies continues entering from the upper-left corner of the above birdseye charts...with the upper trough over western Canada...and the associated surface frontal zone across the NW US...north-central US...and along the east coast of Hudson Bay.

P2...Cut-off upper trough over the SW US has re-amplified into an upper vortex...its eastern divergence formerly supporting a frontal low that has dissipated over the SE US (although there was some residual rain from this system in the last 24 hrs). The eastern divergence of this upper vortex now supports a new surface trough Invest 93-L in the western Gulf of Mexico...the origin of this surface trough mentioned in special update #108A. In advance of this upper trough (and the upper trough in paragraph P1)...it is expected in the next 60 hours that Invest 93-L will be steered NE into the northern US Gulf coast and across the SE US while it enhances rainfall. I currently think SW vertical shear ahead of the upper troughing will prevent 93-L from becoming a subtropical or tropical cyclone. Elsewhere...shortwave upper trough over eastern Canada has amplified into an upper vortex due to the cool air advection of the intensifying 1001 mb frontal low it supports with its eastern divergence. The western convergence of the upper vortex supports a 1020 mb low-level ridge across the eastern US. Warm air advection ahead of the 1001 mb low supports a north Atlantic upper ridge wave...and this 1001 mb system has absorbed all layers of western Atlantic low pressure system in paragraph P3. Shortwave upper trough is still sliding eastward toward Europe...with its western convergence supporting a low-level ridge moving eastward across the Atlantic high seas.

P3...Cut-off upper trough and associated 1007 mb frontal low over the W Atlantic has been absorbed by the 1001 mb frontal low and its upper vortex in paragraph P2. Cut-off upper trough in the Gulf of Mexico is de-amplifying thanks to warm air advection southeast and ahead of the paragraph P1 frontal system.

P4...Deep-layered vortex persists south of the Azores...but is becoming pushed eastward as the north Atlantic upper ridge wave (supported by warm air advection ahead of the 1001 mb frontal low in paragraph P2) pushes in from the west. A portion of this deep-layered vortex has split off into an upper trough west of Nadine...the western convergence of this upper trough increasing the dry air to the west of Nadine. This upper trough west of Nadine could easily de-amplify thanks to the warm air advection ahead of the 1001 mb frontal low in paragraph P2.

P5...Eastern Atlantic surface ridge...currently with a 1018 mb center... persists with support from upper convergence as the northerly flow from the paragraph P6 central Atlantic upper ridge cell clashes with southerly flow from the paragraph P6 upper ridge cell located toward Africa. This upper convergence has also increased the dry air in the eastern tropical Atlantic in the last 24 hours.

...TROPICAL BELT DISCUSSION...
P6...Upper ridging across the tropical Atlantic persists. A western Caribbean and SE Mexico upper ridge cell is split from the central Atlantic upper ridge cell via eastern Caribbean upper trough retrograding into the central Caribbean. Dry air has collected below this retrograding upper trough. Embedded upper vortex NW of the Cape Verde Islands persists...with the remainder of the upper ridging located toward west Africa in relatively higher pressures SE of this upper vortex.

P7...The tropical wave south of the Cape Verde Islands in the previous discussion is now SW of the Cape Verde Islands. Despite being below low shear and favoarable upper outflow beneath the paragraph P6 upper ridge cell toward Africa...it appears suppressed by adjacent dry air mentioned in paragraph P5.

2012 Atlantic Hurricane Season Birdseye Discussion #108A (Special Update)

By: NCHurricane2009, 8:14 PM GMT on September 16, 2012

...SEPTEMBER 16 2012...4:15 PM EDT...
A surface trough in the western Gulf of Mexico has been introduced into the National Hurricane Center Tropical Weather Outlook and is upgraded to tropical disturbance Invest 93-L. The surface trough appears to have originated from the decaying upper trough and surface frontal system driven by the remnant of Leslie days ago:

Paragraph P2 in mid-latitudes discussion...Discussion #104
Paragraph P2 in mid-latitudes discussion...Discussion #105
Paragraph P2 in mid-latitudes discussion...Discussion #106
Paragraph P2 in mid-latitudes discussion...Discussion #107
Paragraph P3 in mid-latitudes discussion...Discussion #108

It should be noted that by discussion #106...the decaying front had dissappeared from the Gulf of Mexico in surface anaylyses. However...analyses had added a surface trough in the western Gulf of Mexico by discussion #108 as seen in the atmospheric birdseye chart of that discussion. It now appears in the last 36 hours that this surface trough broke off from the main portion of the decaying frontal boundary while steered westward by the eastern US low-level ridge mentioned in paragraph P2 of discussion #108.

Currently this surface trough is under hostile westerly vertical shear and has unimpressive t-storm activity...but is becoming increasingly supported by the SW US upper trough mentioned in paragraph P2 of discussion #108 as the upper westerly flow is becoming increasingly divergent in advance of this upper trough. A check with the 12Z GFS model provides a 72-hour outlook. The SW US upper trough during this timeframe will be merging with the paragraph P1 upper trough...with the system moving northeastward into the northern US Gulf coast in deep-layer southwesterly flow developing ahead of the paragraph P1 frontal system. While eastern divergence ahead of the merged upper trough will provide a mechanism for increased t-storm activity...the merged upper trough is not forecast to develop a cut-off upper vortex that moves over the disturbance to reduce westerly shear for subtropical development. Instead of a fully-tropical solution...I prefer a more subtropical solution due to the interaction with the upper trough...but again without a shear-reducing cut-off upper vortex...I expect no development in this repsect either. Therefore...I am forecasting this to simply be a rainfall-enhancer for the northern US Gulf coast and much of the SE US...and will not be introducing this as a special feature in my next full discussion.

In the meantime...return to full discussion #108 for an assessment on the rest of the Atlantic tropics.

2012 Atlantic Hurricane Season Birdseye Discussion #108

By: NCHurricane2009, 8:11 AM GMT on September 16, 2012

...SEPTEMBER 16 2012...4:11 AM EDT...
Hurricane Nadine continues to truck quickly eastward in the last 24 hours. The tropical cyclone will be south of the Azores by 5 days out...so interests in the Azores should monitor the progress of this system until it is clear if the tropical cyclone will move toward or away from the islands. See Nadine special feature section below for further details.

The tropical wave approaching the Lesser Antilles and Caribbean Sea has been upgraded to Invest 92-L in the last 24 hours. Due to potential for more favorable upper winds in the next days...I have upgraded this to a special feature section on this blog. See the second special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0127Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE NADINE...
Nadine has continued accelerating eastward toward the paragraph P5 deep-layer vortex in the last 24 hours...and her eastward motion is still slightly faster than the previous forecasts even though those forecasts upped the pace of the eastward track. Therefore my updated forecast in Figure 1 is a rightward adjustment from the previous in all forecast points to account for the initial faster-than-expected eastward motion.

I support an increasingly slower eastward progression after 24 hours due to resistance from a NE Atlantic low-level ridge...which is currently the 1026 mb ridge entering the Atlantic high seas (paragraph P2) expected to zoom eastward to that location. This slow down allows the paragraph P5 deep-layered vortex to slip away from Nadine...leaving Nadine trapped in a narrowing gap between the paragraph P6 low-level ridge to the south and NE Atlantic low-level ridge. As the two low-level ridges bridge to the east of Nadine...I support a northward turn between 48 and 72 hrs. In between 72 and 84 hrs...I believe Nadine will wiggle left toward a low-level ridge weakness associated with what is now the 1008 mb low in paragraph P2. As the weakness passes by to the north between 84 and 96 hrs...I believe Nadine will then wiggle back rightward while trying to link with that weakness. However...it looks as though Nadine will fail to link with this weakness...which I predict will cause her to loop clockwise in track between 96 and 120 hrs as she comes under the influence of an impressive deep-layered ridge coming in from the NW that develops ahead of the paragraph P1 upper trough.


Figure 1: My forecast for Hurricane Nadine created this morning.

Intensity-wise for the next 24 hours...I weaken Nadine faster than the 11 PM EDT NHC forecast shows...as I expect her currently quick eastward pace will move her into unfavorable upper convergence and zonal shearing upper westerly flow on the SW quad of the paragraph P5 deep-layer vortex. The slow down in track between 24 and 48 hours should allow the less shearing (more favorable) NW Atlantic upper ridge wave ahead of what is now the paragraph P2 1008 mb low to catch up to and overspread Nadine...so I flatten the weakening rate during this time. I accelerate the weakening rate by 72 to 120 hrs as Nadine's forecast erratic track straddles the marginally favorable 26 deg C sea-surface temp isotherm...and experiences unfavorable SW vertical shear delivered by shortwave upper trough associated with the aforementioned 1008 mb low.

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius...which I lean rightward with respect to the storm track by 72 to 120 hrs to account for a forecasted increase in SW vertical shear for that timeframe. The swath is also shrunken to represent a weakening Nadine in the latter part of the forecast.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 92-L...
The tropical wave approaching the Lesser Antilles and Caribbean Sea has developed a 1010 mb low pressure spin and become more vigorous in the last 24 hours due to split flow upper divergence on the SW quad of paragraph P7 central Atlantic upper ridge cell. Computer models insist that the eastern Caribbean upper trough (paragraph P4 and P7) would continue to retrograde westward as to allow the SW quad of the central Atlantic upper ridge cell to split off into a favorable shear-reducing and outflow-enhancing upper anticyclone directly over this tropical wave. Moreover...model runs suggest this favorable upper anticyclone becoming enhanced by warm air advection ahead of what is now the paragraph P1 frontal system as this tropical wave crosses the Caribbean Sea. Therefore...interests in the Caribbean should monitor this tropical wave over the next days. However...development could be obstructed if the tropical wave gets too close to the less favorable retrograding east Caribbean upper trough...or later on gets too close to what is forecast by models to be a highly-amplified upper trough associated with the paragraph P1 frontal system.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough and surface frontal system in mid-latitude westerlies is entering the upper-left corner of the above birdseye charts from the NW US and western Canada.

P2...Cut-off upper vortex over the SW US has weakened into an upper trough...but its eastern divergence still supports a 1017 mb frontal low currently over northern Mississippi. Shortwave upper trough has moved from central to eastern Canada whose eastern divergence supports a 1008 mb frontal low moving into SE Canada and whose western convergence supports a 1024 mb low-level ridge across the eastern US. Warm air advection ahead of the 1008 mb low supports a NW Atlantic upper ridge wave...and this 1008 mb system is absorbing the western Atlantic low pressure system in paragraph P3. Shortwave upper trough across Greenland is sliding eastward toward Europe...with its western convergence supporting a 1026 mb low-level ridge moving into the Atlantic high seas from Atlantic Canada.

P3...Cut-off upper vortex over the W Atlantic has weakened into an upper trough whose eastern divergence supports a broad area of low surface pressures that has consolidated into a 1007 mb frontal low. This 1007 mb system however will soon merge with the 1008 mb system in paragraph P2. Cut-off upper vortex in the Gulf of Mexico has weakened into an upper trough.

P4...Cut-off upper vorticity in the open Atlantic and eastern Caribbean has been moved into paragraph P7 of the tropical belt discussion.

P5...Deep-layered vortex persists south of the Azores...with the shortwave upper trough and associated surface low (now 1007 mb) that entered the west half of this system yesterday becoming the dominant feature of this system. Long-lasting remnant low of Isaac has been absorbed by the east half of the 1007 mb low in the last 24 hours.

P6...Eastern Atlantic surface ridge...currently with a 1018 mb center... persists with support from eastern convergence of the paragraph P7 central Atlantic upper ridge cell.

...TROPICAL BELT DISCUSSION...
P7...Upper ridging across the tropical Atlantic persists. The Caribbean to central Atlantic upper ridge cell has split into two cells separated by an eastern Caribbean upper trough (paragraph P4). The strength of the central Atlantic upper ridge cell...in part due to the latent heat release of Hurricane Nadine...has caused a portion of the paragraph P5 deep-layer vortex to fracture into a separate upper vortex WNW of the Cape Verde Islands. Remainder of the upper ridging is located toward west Africa in relatively higher pressures SE of this new upper vortex.

P8...The tropical wave approaching the Lesser Antilles in the previous discussion has been upgraded to disturbance Invest 92-L...and has been moved into the Invest 92-L special feature section above.

P9...The suspect tropical wave south of the Cape Verde Islands in the previous discussion has been added into the NHC TAFB maps during the last 24 hours.

2012 Atlantic Hurricane Season Birdseye Discussion #107

By: NCHurricane2009, 9:14 AM GMT on September 15, 2012

...SEPTEMBER 15 2012...5:15 AM EDT...
Nadine has strengthened into a hurricane in the last 24 hours. The tropical cyclone could threaten the Azores just beyond 5 days out...so interests in the Azores should monitor the progress of this system. See Nadine special feature section below for further details.

The tropical wave in paragraph P7 of the tropical belt discussion (approaching the Lesser Antilles) has been introduced into the National Hurricane Center Tropical Weather Outlook during the last 24 hours...but is currently in unfavorable upper winds. See paragraph P7 of the tropical belt discussion for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0124Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE NADINE...
Nadine turned northward in the last 24 hrs as expected...but then has begun accelerating eastward ahead of schedule when looking at previous forecasts. In fact...the current eastward pace of Nadine on satellite imagery appears ahead of the more recent 11 PM EDT NHC forecast...but at least Nadine is turning eastward along the suggested latitude of that forecast. Therefore my forecast in Figure 1 shows an eastward motion along the 11 PM EDT NHC track for the shorter-term...but I suggest a faster forward speed than the 11 PM EDT forecast. I believe Nadine strengthened into a hurricane as her more eastward than expected current position has distanced herself from the unfavorable western upper outflow blockage that the paragraph P3 upper vorticity previously applied.

My track forecast for the next 24 hours in Figure 1 is a big rightward shift from my previous philosophies for this timeframe. I was thinking that Nadine would begin stalling or slowing under conflicting steering influences during this timeframe...with the paragraph P4 deep-layered vortex grabbing her and pulling her eastward later on. However...the statements in the previous paragraph make it apparent that her attraction toward this vortex is occuring ahead of schedule. I support an increasingly slower eastward progression after 48 hours due to resistance from a NE Atlantic low-level ridge...which is currently the 1028 mb Atlantic Canada ridge in paragraph P1 expected to zoom eastward to that location. This slow down allows the paragraph P4 deep-layered vortex to slip away from Nadine...leaving Nadine trapped in a narrowing gap between the paragraph P5 low-level ridge to the south and NE Atlantic low-level ridge. As the two low-level ridges bridge to the east of Nadine...I support a northward turn between 72 and 96 hrs. In between 96 and 108 hrs...I believe Nadine will wiggle left toward a low-level ridge weakness associated with what is now the 1013 mb low in paragraph P1. As the weakness passes by to the north between 108 and 120 hrs...I believe Nadine will then wiggle back rightward toward the western Azores as it tries to move toward that weakness.


Figure 1: My forecast for Hurricane Nadine created this morning.

Intensity-wise...I agree with the NHC on the potential for Nadine to strengthen further to 80 mph max winds. I then weaken Nadine sooner than the NHC forecast suggests...as my more easterly position of Nadine by 48 hours (with repsect to NHC's track forecast) places her in unfavorable upper convergence and zonal shearing upper westerly flow on the SW quad of the paragraph P4 deep-layer vortex. The slow down in track between 48 and 72 hours should allow the less shearing (more favorable) upper ridge wave ahead of what is now the paragraph P1 1013 mb low to catch up to and overspread Nadine...so I flatten the weakening rate during this time. I accelerate the weakening rate by 96 and 120 hrs as Nadine crosses the 26 deg C isotherm into cooler waters...and experiences unfavorable SW vertical shear delivered by shortwave upper trough associated with the aforementioned 1013 mb low.

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius...which I lean rightward with respect to the storm track by 96 and 120 hrs to account for a forecasted increase in SW vertical shear for that timeframe. The swath is also shrunken to represent a weakening Nadine in the latter part of the forecast.

...MID-LATITUDES DISCUSSION...
P1...Upper trough and surface frontal system in mid-latitude westerlies has broken into a few features strung SW-NE in the last 24 hours. A cut-off upper vortex persists over the SW US which supports a 1018 mb frontal low over the TX/AK/OK/LA area. A shortwave upper trough is over central Canada and the Great Lakes whose eastern divergence supports a 1013 mb frontal low over the NY/Canada border and whose western convergence supports a 1027 to 1028 mb low-level ridge across central Canada and the central US. A shortwave upper trough and surface frontal cyclone continue ejecting NE across Greenland and will soon be exiting the scope of the above atmo birdseye chart...but the western convergence of this exiting upper trough supports a 1028 mb low-level ridge over Atlantic Canada.

P2...North Atlantic upper trough associated with ex-Leslie has exited the picture while heading toward north Europe...but has left behind several features strung SW-NE. A shortwave fragment of this upper trough is merging with the west half of the paragraph P4 deep-layered vortex (see paragraph P4 for details). Cut-off upper vortex over the W Atlantic persists...supporting a broad area of low surface pressures with its eastern peripherial divergence. Cut-off upper vortex persists in the Gulf of Mexico.

P3...Cut-off upper vorticity remains in the open Atlantic and eastern Caribbean...established as a south-north upper trough squeezed between the Caribbean-to-C Atlc upper ridge cell and upper ridge cell over Nadine and toward Africa (both cells mentioned in paragraph P7).

P4...Deep-layered vortex associated with the long-lasting remnants of Isaac persists south of the Azores...and is receiving additional support as a shortwave upper trough from the paragraph P2 system digs into the west half of this system. Eastern divergence from the intruding shortwave supports a new 1010 mb surface low. Models have been suggesting this 1010 mb surface low will grow into the dominant surface feature in place of ex-Isaac....but will watch to see which of the two surface lows actually dominates.

P5...Eastern Atlantic surface ridge...currently with a 1020 mb center... persists with support from eastern convergence of paragraph P6 upper ridge cell located over Nadine.

...TROPICAL BELT DISCUSSION...
P6...Upper ridging across the tropical Atlantic persists. Previously...warm air advection ahead of the paragraph P2 system extended the Caribbean to Central Atlantic upper ridge cell into the NE Atlantic toward Europe...but now this NE extension has split off. There is a new northward extension of this upper ridge cell into the NW Atlantic supported by warm air advection ahead of the paragraph P1 system. Remainder of the upper ridging is located toward the west coast of Africa and above Hurricane Nadine.

P7...The tropical wave midway between the Lesser Antilles and Cape Verde Islands in the previous discussion is pushing toward the Lesser Antilles. Its elevated but still somewhat meager t-storm activity has persisted in the last 24 hrs...but so far is embedded in straight and shearing upper easterly flow on the south side of paragraph P6 upper ridging. Until the t-storm latent heat release increases to support an embedded warm core upper outflow structure in the upper ridge...not considering this an area for tropical development.

P8...Animation of satellite imagery suggests a tropical wave has emerged from the west coast of Africa and into the waters south of the Cape Verde Islands in the last 48 hours. The t-storm activity remains unimpressive at this hour....and so far this wave has not been added to NHC TAFB maps.

2012 Atlantic Hurricane Season Birdseye Discussion #106

By: NCHurricane2009, 6:41 AM GMT on September 14, 2012

...SEPTEMBER 14 2012...2:42 AM EDT...
Tropical Storm Nadine has stopped strengthening in the last 24 hours...therefore avoiding hurricane strength. The tropical cyclone could threaten the Azores beyond 5 days out...so interests in the Azores should monitor the progress of this system. See Nadine special feature section below for further details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0126Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
Track-wise...once again as in the previous two discussions...the short-term NHC recorded storm track has a slight leftward angle with respect to the NHC track forecast. The NHC short-term track forecast has continued to require ever-so-slight leftward shifts in the last 24 hrs...and therefore I am predicting another such slight leftward shift as shown by my forecast track in Figure 1 below. This means I am currently slightly left of NHC for the short-term track forecast.

With that said...in order for my and the NHC's short-term track forecast to verify...Nadine would have to turn straight north in the next 24 hours. Nadine is probably strong/tall enough in structure to "feel" steering influence from the paragraph P4 upper vorticity to the SW and paragraph P2 W Atlantic cut-off upper vortex to the NW. Along ex-Leslie's long front...a broad western Atlantic surface low is forming with the support of paragraph P2 W Atlantic cut-off upper vortex peripherial divergence. For the next 24 hrs...00Z GFS still develops a new low-level ridge west of Nadine which I surmise is from upper convergence as Nadine's upper outflow clashes with paragraph P7 Caribbean-to-C Atlc upper ridge. This low-level ridge prevents Nadine from progressing further westward...so Nadine has would have no choice but to go straight north while attracted toward the western Atlantic low to the NW. All of the facts in this paragraph support Nadine tracking straight north by 24 hours from now.

The steering picture still gets more complicated for the timeframe that is now between 24 and 48 hrs...with Nadine between the western Atlantic low to her west...paragraph P2 W Atlantic low-level ridge arriving to her north...the paragraph P6 low-level ridge to her southeast...and the ex-Isaac deep-layered vortex (paragraph P5) to her northeast. With all this conflicting steering...I honestly prefer to keep Nadine stationary between 24 and 48 hrs. But because the models and NHC forecast still want to move Nadine faster to the NE than what I am thinking for that timeframe...I still show some NE progress (but still slower than the NHC forecast).

After 48 hrs...00Z GFS shows a shortwave fragment of paragraph P2 upper trough re-enforcing the ex-Isaac deep-layered vortex...which helps drag Nadine eastward towards it. But what really inclines me to agree with an eastward motion by that time is that the low-level ridge to her north getting knocked out by the paragraph P1 upper trough. Coupled with paragraph P6 low-level ridge to her south and re-enforced ex-Isaac deep-layered vortex to her east...it makes sense to show eastward motion after 48 hrs. I support an increasingly slowing eastward track between 48 and 120 hrs (which makes me even further behind the NHC track forecast)...due to Nadine catching up to what is now the paragraph P2 W Atlantic low-level ridge which will be in the NE Atlantic by this timeframe. In fact...I would prefer to stall out the track towards 120 hrs with the ex-Isaac vortex diminishing...leaving Nadine trapped in a narrowing gap between the paragraph P6 low-level ridge to the south and NE Atlantic low-level ridge. Interestingly...this morning's 00Z GFS prefers to reverse the track WNW away from the Azores after 120 hrs (not shown in Figure 1). I suppose 00Z GFS is giving more credence to the NE Atlantic low-level ridge which should be stronger than the paragraph P6 low-level ridge by that time.

Due to uncertainty beyond 120 hrs and Nadine being closer to the Azores by that time...interests in the Azores should still watch Nadine even though there are emerging scenarios as posed above that suggest the Azores would have no impact.


Figure 1: My forecast for Tropical Storm Nadine created this morning.

Intensity-wise...Nadine has stopped strengthening in the last 24 hrs...maintaining a flat intensity of 70 mph max sustained winds (just below hurricane force). This is contrary to earlier intensity forecasts which made Nadine a hurricane by now. In hindsight...it appears all prior forecasts ignored the western upper outflow blockage that Nadine is currently experiencing from paragraph P4 upper vorticity. At this time...I no longer expect Nadine to ever become a hurricane...but the NHC suggests Nadine will have a fast enough eastward track such that it is buried below a more favorable upper outflow environment beneath an upper ridge wave just ahead of the paragraph P1 upper trough. In fact...the NHC suggests Nadine becoming a hurricane later on by day 5. Because of my Nadine track forecast continuing to show a slower eastward progression than NHC's...I still prefer being below the NHC intensity guidance with the assumption that Nadine will be closer to the westerly shear of the paragraph P1 upper trough. I slightly weaken Nadine to 60 mph max winds by 11 PM Saturday as the westerly shear initially hits Nadine...then I keep Nadine modeled as a steady-state tropical storm balanced by unfavorable paragraph P1 upper trough westerly shear over her west half and favorable upper ridge wave outflow over her east half. This is contrary to my earlier handling of Nadine...which showed continuous long-term weakening...but now I do not want to stray too far away from the newly-elevated NHC intensity guidance for day 5.

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius. The impact swath remains generally the same size thru the forecast period due to the flat intensity forecast. Note the rightward bias in the initial impact swath (with respect to forecast track)...due to the upper outflow blockage caused by paragraph P4 upper vorticity limiting the activity in the west half of Nadine. The impact swath is drawn with the assumption that Nadine remains lopsided like this thru the forecast period...especially with potential westerly vertical shear from the incoming paragraph P1 upper trough later on.

...MID-LATITUDES DISCUSSION...
P1...Upper trough and surface frontal system in mid-latitude westerlies has entering the upper-left corner of above atmo birdseye chart. The surface front is advancing into the eastern US...which curls into a frontal cyclone that has ejected northeastward from Hudson Bay and toward Greenland in last 24 hrs. Warm air advection ahead of the front supports upper ridge wave over the NE US and Atlantic Canada. Upper convergence of the upper trough supports a 1030 mb low-level ridge building over the central US. Finally...a fragment of this upper trough has dropped off a cut-off upper vortex over the SW US.

P2...Upper trough has advanced from W Atlantic and into the north Atlantic in last 24 hrs...and its eastern divergence supports the still-strong remnant low of Leslie which has zipped NE toward northern Europe and out of the scope of the above atmo birdseye chart. Even though Ex-Leslie has exited the picture...it should be noted she will continue to bring elevated winds and surf to the Atlantic high seas shipping lanes as she heads toward northern Europe. The associated surface frontal zone tailing from ex-Leslie is stretched across the central Atlantic and the Bahamas. Elsewhere...the western upper convergence of the upper trough used to support large area of dry air and a 1029 mb surface ridge over the W Atlantic...but now is supported by SE convergence of upper ridge mentioned in paragraph P1. The upper trough has left behind a new cut-off upper vortex over the W Atlantic in last 24 hrs. Another old fragment of this upper trough remains cut-off...now located over the SE US/Gulf of Mexico area.

P3...To the southeast of ex-Leslie...the remnant of Michael has continued NE along the cold front extending from ex-Leslie...also out of the scope of the above birdseye charts like ex-Leslie. Ex-Michael appeared intermittently in HPC analyses to the east of S Greenland...but now has also left the scope of the HPC analyses. I am currently assuming that ex-Michael has lost its identity within the larger low pressure field of ex-Leslie.

P4...Cut-off upper vorticity remains in the open Atlantic...still established as SW-NE upper trough squeezed between the Caribbean-to-C Atlc upper ridge cell and upper ridge cell toward Africa (both cells mentioned in paragraph P7). An embedded upper vortex is currently just N of Puerto Rico.

P5...While currently positioned just south of the Azores...remnant deep-layered low of Isaac is turning slowly northwestward toward break in paragraph P6 and P7 lower and upper ridges...the break generated by paragraph P2 mid-latitude system.

P6...Atlantic surface ridge has been eroded out of the western Atlantic as ex-Leslie's Atlantic cold front continues advances in. Its remaining eastern Atlantic portion is supported by the SE convergence of lengthy paragraph P7 Caribbean-to-C Atlc upper ridge cell. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P7...Upper ridging across the tropical Atlantic persists. The Caribbean to Central Atlantic upper ridge cell has been stretched into the NE Atlantic toward Europe thanks to low-level warm air advection ahead of the ex-Leslie (paragraph P2) system. Like a mid-latitude upper ridge wave...southeastern convergence of this Caribbean-to-C Atlc upper ridge supports pockets of dry air in the Caribbean and east of the Lesser Antilles. Remainder of the upper ridging is located toward the west coast of Africa and above Tropical Storm Nadine.

P8...The tropical wave SW of the Cape Verde Islands in the previous discussion is now midway between the Lesser Antilles and Cape Verde Islands. Its t-storm cluster has slightly increased in the last 24 hrs...but so far is embedded in straight upper easterly flow on the south side of paragraph P7 upper ridging. Until the t-storm latent heat release increases to support an embedded warm core upper outflow structure in the upper ridge...not considering this an area for tropical development. It is also headed toward paragraph P7 dry air and less favorable paragraph P4 upper vorticity.

P9...Satellite imagery suggests a tropical wave has emerged from the west coast of Africa and into the waters SE of the Cape Verde Islands. The t-storm activity is not as impressive as it was earlier when it was along the African coast.

2012 Atlantic Hurricane Season Birdseye Discussion #105

By: NCHurricane2009, 8:09 AM GMT on September 13, 2012

...SEPTEMBER 13 2012...4:15 AM EDT...
Tropical Storm Nadine strengthening a little faster than anticipated...and should become a hurricane later this morning. Nadine is then expected to recurve northward then eventually eastward...hence keeping the storm over open waters. The tropical cyclone could threaten the Azores beyond 5 days out...so interests in the Azores should monitor the progress of this system. See Nadine special feature section below for further details.

In the last 24 hours...the National Hurricane Center has introduced in their tropical weather outlook an area of loosely-clustered t-storms along a cold front over the western Bahamas. Currently...the outlook probability is at a low percent...and I do not see enough signs of development to consider this a special feature on this blog. See paragraph P2 in mid-latitudes discussion for details on this area.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1928Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
In the last 24 hours....Nadine has strengthened faster than expected. My forecast on Nadine versus the NHC's is shown in Figure 1.

Track-wise...Nadine at 11 PM EDT is just north of 50W-20N rather than over that location...which means Nadine has been following the NHC's track better than my previous (which had an initial southward bias). The call on the initial southward bias was made based on projecting the NHC recorded storm track...and once again if the recorded storm track in Figure 1 is extrapolated...one would see an initial southward (or leftward) bias. However...I do not want to repeat another bias with the NHC forecasting doing well in the last 24 hrs...and moreover there has been a slight left shift in the NHC track in the last 24 hrs that I think is enough compensation for the fact that the current NHC recorded storm track seems to have a slight left angle with respect to the NHC forecast track. Therefore I agree with the NHC short-term forecast track.

With that said...in order for the short-term NHC track to verify...Nadine would soon have to turn NW. Nadine is probably strong/tall enough in structure to "feel" steering influence from the paragraph P4 upper vorticity...and computer models continue to support the gradual formation of a broad western Atlantic surface low along ex-Leslie's cold front (supported by cut-off upper vortex along paragraph P2 upper trough) in the next 24 hrs. Both of these facts suggest that a NW turn for Nadine should soon occur. Between 24 and 48 hrs...00Z GFS still develops a new low-level ridge west of Nadine which I surmise is from upper convergence as Nadine's upper outflow clashes with paragraph P7 Caribbean-to-C Atlc upper ridge. This low-level ridge prevents Nadine from progressing further westward between 24 and 48 hrs...so Nadine has no choice but to go straight north while attracted toward the western Atlantic low to the NW.

The steering picture still gets more complicated between 48 and 72 hrs...with Nadine between the western Atlantic low to her west...paragraph P2 east US low-level ridge arriving to her north...the paragraph P6 low-level ridge to her southeast...and the ex-Isaac deep-layered vortex (paragraph P5) to her northeast. With all this conflicting steering...I honestly prefer to keep Nadine stationary between 48 and 72 hrs. But because the models and NHC forecast still want to move Nadine faster to the NE than what I am thinking for that timeframe...I have decided to conform a bit more and move Nadine more NE than I had done before.

After 72 hrs...00Z GFS shows a shortwave fragment of paragraph P2 upper trough re-enforcing the ex-Isaac deep-layered vortex...which helps drag Nadine eastward towards it. But what really inclines me to agree with an eastward motion by that time is that the low-level ridge to her north getting knocked out by the paragraph P1 upper trough. Coupled with paragraph P6 low-level ridge to her south and re-enforced ex-Isaac deep-layered vortex to her east...it makes sense to show eastward motion after 72 hrs. After 120 hrs (not shown in Figure 1)...models show Nadine catching up to what is now the paragraph P1 eastern US low-level ridge cell which would be NE of Nadine by that time. This catch-up to the ridge could deflect Nadine northward toward the Azores region later on.


Figure 1: My forecast for Tropical Storm Nadine created this morning.

Intensity-wise...Nadine has strengthened more than expected in the last 24 hours with a 30 mph max wind increase during that time. If this continues...Nadine should become a hurricane later this morning. Despite the faster than expected intensification...the NHC still forecasts a peak of 85 mph max winds as they did 24 hrs ago...but I prefer to make Nadine have a higher peak than previously given the higher initial intensity. However...I do not prefer extrapolating another 30 mph max wind rise for the next 24 hrs...nor significantly overshooting the NHC intensity forecast...because Nadine's western upper outflow looks to be becoming increasingly choked by the paragraph P4 upper vorticity. Because of my westward bias in position from 48 hrs and onwards...Nadine would be closer to westerly shear generated by what will be an incoming paragraph P1 upper trough...so I weaken Nadine faster than the NHC shows during that time with the assumption that Nadine follows my forecast track. However...my weakening rate is slower than shown before because my forecast track positions during that time are now closer to the NHC track positions.

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius. The impact swath is gradually grown in size until peak intensity...and then shrunken in accordance with weakening late in the forecast period. Note the rightward bias in the impact swath (with respect to my forecast track) beginning Friday...associated with westerly vertical shear that should be starting by that time.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough and surface frontal system in mid-latitude westerlies is entering the upper-left corner of above atmo birdseye chart. The surface front has advanced eastward into the central US...which curls into a 983 mb frontal cyclone over Hudson Bay in Canada. Warm air advection ahead of the front supports upper ridge wave over the NE US and E Canada. Upper convergence of the upper trough supports a low-level ridge building over Montana.

P2...Upper trough has advanced into the western Atlantic...and its eastern divergence supports the still-strong remnant low of Leslie which has zipped NE by the south tip of Greenland. Even though Ex-Leslie is exiting the picture...it should be noted she will continue to bring elevated winds and surf to the Atlantic high seas shipping lanes as she heads toward northern Europe. The associated surface frontal zone tailing from ex-Leslie is stretched across the central Atlantic and Gulf of Mexico. A portion of the frontal zone (over the W Bahamas) is beneath a favorable upper anticyclonic center of paragraph P7 Caribbean upper ridging...an area the National Hurricane Center has highlighted in their tropical weather outlook in the last 24 hrs. So far there are no signs of tropical development on satellite imagery...and there is no computer model support for this area. Albeit the closest support is from an outlier NOGAPS solution...which is the only model that retrogrades a fragment of the western Atlantic surface low (mentioned in the Nadine special feature section) westward to the north of the Bahamas...but the NOGAPS positioning places it north of the favorable upper anticyclonic center such that it would get ripped by unfavorable westerly vertical shear. Elsewhere...the western upper convergence of the upper trough supports a large area of dry air and a 1029 mb surface ridge over the eastern US and W Atlantic. A fragment of this upper trough remains cut-off...now located over Arkansas/Louisiana/Gulf of Mexico area.

P3...To the southeast of ex-Leslie...the remnant of Michael has continued NNE and merged with the cold front extending from ex-Leslie. Evidence of this is found by archived 0000Z and 1200Z September 12 infrared satellite images found on UNISYS (http://weather.unisys.com/archive/sat_ir/1209/). Ex-Michael was positioned ESE of the south tip of Greenland as of 1928Z HPC analysis...but was then dropped in further HPC analyses as if ex-Michael has become indistinct along the front.

P4...Cut-off upper vorticity remains in the open Atlantic...now established as SW-NE upper trough squeezed between the Caribbean-to-C Atlc upper ridge cell and upper ridge cell toward Africa (both cells mentioned in paragraph P7). An embedded upper vortex is currently over the northern Lesser Antilles.

P5...Remnant deep-layered low of Isaac is retrograding westward to the south of the Azores...steered by south side of P6 surface ridge supported by eastern convergence of amplified Caribbean-to-C Atlc upper ridge cell mentioned in paragraph P7.

P6...Atlantic surface ridge has been eroded out of the western Atlantic as ex-Leslie's Atlantic cold front continues advances in. Its remaining eastern Atlantic portion is supported by the SE convergence of lengthy paragraph P7 Caribbean-to-C Atlc upper ridge cell. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P7...Upper ridging across the tropical Atlantic persists. The Caribbean to Central Atlantic upper ridge cell has been stretched into the NE Atlantic toward Europe thanks to low-level warm air advection ahead of the ex-Leslie (paragraph P2) system. Like a mid-latitude upper ridge wave...southeastern convergence of this Caribbean-to-C Atlc upper ridge supports a large area of dry air across the Caribbean and east of the Lesser Antilles. Remainder of the upper ridging is located toward the west coast of Africa and above Tropical Storm Nadine.

P8...The tropical wave south of the Cape Verde Islands in the previous discussion is now SW of the islands. Its t-storm cluster remains weak...and using the above thermo birdseye chart...this appears to be due to paragraph P6 dry air.

2012 Atlantic Hurricane Season Birdseye Discussion #104

By: NCHurricane2009, 6:52 AM GMT on September 12, 2012

...SEPTEMBER 12 2012...2:55 AM EDT...
See paragraphs P2 and P3 in the mid-latitudes discussion for details on the remnants of Leslie and Michael entering the north Atlantic high seas.

Eastern Atlantic tropical wave Invest 91-L has finally organized into a tropical cyclone. It has strengthened into tropical depression fourteen then Tropical Storm Nadine in the last 24 hours. Nadine is expected to strengthen into a hurricane while also recurving northward then eventually eastward...hence keeping the storm over open waters. See Nadine special feature section below for further details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1924Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM NADINE...
Strong tropical wave Invest 91-L with t-storms and low pressure spin...now midway between the Cape Verde Islands and Lesser Antilles...has strengthened into tropical depression fourteen...then into Tropical Storm Nadine...in the last 24 hours. My forecast on this new tropical storm versus the NHC's is shown in Figure 1.

Track-wise...Nadine was in its formative stages earlier...featuring a tighter swirl center wobbling inside of a larger outer circulation. The wobbling motion of the swirl center created an unsteady initial motion as shown by the NHC recorded storm track in Figure 1. When smoothing out the wobble (by picking the first and last points on the NHC recorded storm track)...Nadine should continue WNW to 50W-20N by 24 hrs...and the 00Z GFS also supports this idea. Therefore my initial WNW track in Figure 1 has a hair of a southward-bias from the get-go...and therefore much of my forecast in Figure 1 has some form of a leftward bias (with respect to NHC) thru the forecast period.

The initial WNW track is supported by paragraph P6 low-level ridge. Between 24 and 48 hrs...the paragraph P2 upper trough develops a cut-off upper low whose eastern peripherial divergence supports the gradual formation of a broad western Atlantic surface low along ex-Leslie's cold front. Based on the 00Z GFS low-level wind field of this surface low...it is reasonable to show a NW track between 24 and 48 hrs. Interestingly between 48 and 72 hrs...00Z GFS develops a new low-level ridge west of Nadine which I surmise is from upper convergence as Nadine's upper outflow clashes with paragraph P7 Caribbean-to-C Atlc upper ridge. This low-level ridge prevents Nadine from progressing further westward between 48 and 72 hrs...so Nadine has no choice but to go straight north while attracted toward the western Atlantic low to the NW.

The steering picture gets even more complicated between 72 and 96 hrs...with Nadine between the western Atlantic low to her west...paragraph P2 east US low-level ridge arriving to her north...the paragraph P6 low-level ridge to her southeast...and the ex-Isaac deep-layered vortex (paragraph P5) to her northeast. With all this conflicting steering...I honestly prefer to keep Nadine stationary between 72 and 96 hrs...but out of respect for the NHC track and models I move Nadine slowly NE.

Between 96 and 120 hrs...00Z GFS shows a shortwave fragment of paragraph P2 upper trough re-enforcing the ex-Isaac deep-layered vortex...which helps drag Nadine eastward towards it. But what really inclines me to agree with an eastward motion by that time is that the low-level ridge to her north getting knocked out by the paragraph P1 upper trough. Coupled with paragraph P6 low-level ridge to her south and re-enforced ex-Isaac deep-layered vortex to her east...it makes sense to show eastward motion by 120 hrs.


Figure 1: My forecast for Tropical Storm Nadine created this morning.


Figure 2: Morning (0415Z) Infrared Satellite Image of Tropical Storm Nadine

Intensity-wise...Nadine's t-storms are quiet healthy and symmetric about the center judging by infrared satellite image in Figure 2...with the storm under low shear and enhanced upper outflow as she has developed an embedded warm core upper anticyclone within the upper ridge cell located toward Africa (paragraph P7). This low shear condition should last thru the next 48 hrs...making it difficult for paragraph P6 and P7 dry air to penetrate. I like and currently agree with the NHC intensification forecast shown at 11 PM EDT...which first shows a gradually intensifying system whose broad core has not yet consolidated...then a more rapidly intensifying system who reaches hurricane strength (75+ mph max winds)...and then a system who stops intensifying on Friday as it encounters upper outflow blockage from the west courtesy of incoming paragraph P2 cut-off upper vortex (which currently does not exist but should by 48 to 72 hrs)...and paragraph P1 upper trough right behind that. Because of my westward bias in track from 72 thru 120 hrs (in part because of my suggested slow track between 72 and 96 hrs)...Nadine would be closer to westerly shear generated by incoming paragraph P1 upper trough...so I weaken Nadine faster than the NHC shows with the assumption that Nadine follows my forecast track.

The impact swath in Figure 1 is initialized based on trying to identify circular core of t-storms seperate from the outer spiral bands. This circular core I identified is highlighted in Figure 2. I think this core represents the area in which the tropical storm and hurricane force wind field will gradually develop in...but I do mention in impact statement (b) the rain squalls outside of the core due to the large and sprawling nature of Nadine. The impact swath is gradually grown in size until peak intensity...and then shrunken in accordance with weakening late in the foreast period. Note the rightward bias in the impact swath (with repsect to my forecast track) beginning Friday...associated with westerly vertical shear that should be starting by that time.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough and surface frontal system in mid-latitude westerlies is entering the upper-left corner of above atmo birdseye chart...from western Canada and the NW US. Warm air advection ahead of the front supports upper ridge wave over central US.

P2...Eastern divergence of upper trough in mid-latitude westerlies once supported a strong surface frontal cyclone between Canada and Greenland 24 hrs ago...but that cyclone has whirled beneath the less-convergent upper trough axis and has now weakened from 976 to 986 mb in last 24 hrs. The associated surface frontal zone is stretched across the west Atlantic and Gulf of Mexico...where it has overspread Tropical Storm Leslie racing NNE from Newfoundland and hence transitioned her to a non-tropical frontal cyclone. In fact...ex-Leslie is now the dominant frontal cyclone of this system...getting the supportive eastern divergence of the upper trough. Hence...ex-Leslie remains a high wind...high surf threat for the Atlantic high seas shipping lanes as she races NE to the south of Greenland and towards north Europe in the next days. Western upper convergence of the upper trough supports a large area of dry air and a 1028 mb surface ridge over the eastern US. A fragment of this upper trough has cut-off over TX/LA/NW Gulf of Mexico...to the south of the central US upper ridge wave mentioned in paragraph P1.

P3...To the south-southeast of Leslie...Michael has dissipated quickly into a remnant low in the last 24 hrs while losing its t-storms in dry air created by western upper convergence of the upper trough in paragraph P5. Warm air advection ahead of Leslie created an upper ridge wave west of Michael that initiated Michael's demise 24 hrs ago as that upper ridge wave applied northerly shear to Michael. That upper ridge wave is now over ex-Michael (which has reduced the shear)...and ex-Michael may soon intersect the eastern divergence of the ex-Leslie upper trough (paragraph P2) to survive as a non-tropical entity. However...the aforementioned dry air has prevented any t-storms from refiring...so ex-Michael may very well dissipate before it gets the chance to survive as a non-tropical frontal low. Because it is much weaker than ex-Leslie...ex-Michael should not contribute to the high wind...high surf that ex-Leslie will bring to the Atlantic high seas shipping lanes.

P4...Cut-off upper vorticity remains in the open Atlantic...now established as an east-west upper trough squeezed between the Caribbean-to-C Atlc upper ridge cell and upper ridge cell toward Africa (both cells mentioned in paragraph P7)...with leftover upper vortex east of the Lesser Antilles.

P5...Remnant surface low of Isaac is diving southward to the east of the Azores...steered by east side of intensifying paragraph P6 surface ridge supported by eastern convergence of amplifying Caribbean-to-C Atlc upper ridge cell mentioned in paragraph P7. This Caribbean-to-C Atlc upper ridge cell is amplifying thanks to warm air advection ahead of the ex-Leslie system (paragraph P2)...and as a result the shortwave upper trough that has been supporting ex-Isaac over the last several days has equally amplified into a cut-off upper vortex superimposed over ex-Isaac (hence ex-Isaac is now a deep-layered vortex).

P6...Atlantic surface ridge has been eroded out of the northern Caribbean Islands as ex-Leslie's western Atlantic cold front advances in. However...it is supported by the SE convergence of lengthy paragraph P7 Caribbean-to-C Atlc upper ridge cell. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P7...Upper ridging across the tropical Atlantic persists. The Caribbean to Central Atlantic upper ridge cell has been stretched into the north Atlantic by low-level warm air advection ahead of the ex-Leslie (paragraph P2) system. Like a mid-latitude upper ridge wave...southeastern convergence of this Caribbean-to-C Atlc upper ridge supports a large area of dry air across the Caribbean and east of the Lesser Antilles. Remainder of the upper ridging is located toward the west coast of Africa.

P8...I added a tropical wave in the lower-right corner of birdseye atmo chart 24 hrs ago...surmising a tropical wave had emerged from Africa to the SE of the Cape Verde Islands. This was based on a westward-moving cluster of t-storms seen on satellite. The NHC has added this tropical wave to their TAFB maps...located just south of the Cape Verde Islands. Associated t-storm cluster in last 24 hrs has significantly weakened...and using the above thermo birdseye chart...this appears to be due to paragraph P6 dry air.

2012 Atlantic Hurricane Season Birdseye Discussion #103

By: NCHurricane2009, 8:13 AM GMT on September 11, 2012

...SEPTEMBER 11 2012...4:15 AM EDT...
Tropical Storm Leslie bearing down on Newfoundland while just under hurricane force. Weather across much of Newfoundland should soon detereorate through the morning hours...then rapdily improve during the afternoon and evening. Tropical storm and hurricane advisories are in effect for a large part of Newfoundland...see www.nhc.noaa.gov for latest watches and warnings. As a large non-tropical (extratropical) gale...it will then affect the Atlantic high seas shipping lanes after striking Newfoundland. See Leslie special feature section for further details.

Michael continues to weaken. It is expected to stay over open waters and potentially lose its identity along the cold front of non-tropical (extratropical) Leslie in the next 48 hours. See Michael special feature section for details on this hurricane.

I expect Leslie and Michael to no longer be tropical entities by the next full discussion 24 hours from now. If so...there will no longer be special feature sections on either storm on this blog...but this does not mitigate the high winds and surf that both storms will contribute to shipping lanes as they race into the Atlantic high seas.

Eastern Atlantic tropical wave Invest 91-L has continued to become better organized...and is about to become the next Atlantic tropical cyclone. See third special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0123Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM LESLIE...
Leslie continues to accelerate north-northeastward ahead of the next frontal system in the mid-latitude westerlies (paragraph P1). She has so far been perfectly on track with the previous NHC forecast...although the NHC in the last 24 hrs has adjusted the track a bit more leftwards for Leslie's non-tropical phase across the Atlantic high seas later on. This adjustment does not affect anything about the short-term track across Newfoundland...so the 11 AM Tue forecast point in Figure 1 is unchanged from the previous. I still believe Leslie will be non-tropical by 11 AM Tue (11 AM this morning)...while the NHC still forecasts transition to non-tropical for 11 PM Tue.


Figure 1: My forecast for Tropical Storm Leslie created this morning.

Intensity-wise...Leslie was previously forecast by NHC to become a hurricane (75 mph max winds). The NHC argued that the rapid acceleration to the northeast would increase the winds in the east half of the cyclonic circulation (FYI...this same effect decreases the winds in the west half of the cyclonic circulation). This indeed has occurred...with max winds now at 70 mph...but she did not quiet become a hurricane. I forecasted previously for Leslie to stay a tropical storm...and even though I was correct...my intensity error at this point it time is bigger than NHc's because I predicted 60 mph max winds when Leslie is now at 70 mph. The NHC predicts Leslie to stay at 70 mph max winds thru transition to non-tropical...and I agree as Leslie should continue to get supportive divergence from the east side of the paragraph P1 upper trough.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I do not shrink nor expand with the forecast intensity remaining flat. I previously leaned the impact swath to the right-of-track...but I no longer do this as Leslie has amazed me with her wind field symmetry even though she is moving rapidly NE. However...the NHC does note that within this current wind field...the winds are in fact stronger in the east half and diminished in the west half thanks to the rapid NE motion. Without my impact swath leaning to the right as I showed before...Newfoundland is now much more covered by the swath than previously...but the swath remains within the tropical storm warnings and hurricane watches that have been in effect for Newfoundland.

...SPECIAL FEATURE...HURRICANE MICHAEL...
Updated NHC track forecast is in Figure 2 below. Michael in the last 24 hours has tracked a little to the left of the previous NHC track forecast...but is now finally turning northward around the west side of the paragraph P3 low-level ridge. The NHC forecast track is now a hair to the left and faster than the previous...I suppose the faster aspect also making sense since Michael's more westerly position puts him a bit closer to the faster SW flow in advance of Leslie. The rightward bend in forecast track later on remains as Michael should now experience SW flow on the SE half of Leslie instead of getting absorbed by the E side of Leslie as we thought days ago. With the NHC foreast track making sense...I see no reason to disagree with it...especially as infrared satellite imagery shows Michael's surface swirl tracking straight north as the NHC track forecast shows Michael should be. I also continue to agree with the NHC solution of making Michael non-tropical by 24 hrs...coincident with when Michael should merge with the cold front extending from what should be a non-tropical Leslie.


Figure 2: My forecast for Hurricane Michael created this morning.

Intensity-wise...Michael continues to weaken...and is probably no longer a hurricane (75+ mph max winds) based on his disheveled satellite appearance. I had previously predicted that Michael would avoid northerly shear as Leslie's upper outflow would get sheared eastward to a position north or over Michael. Indeed...this upper outflow shows itself as a distinct upper anticyclone due north of Michael (see blue H north of Michael in above atmo chart). However...there is an upper ridge wave due west of Michael (blue zig-zag line in above atmo chart...which is a NE extension of C Atlc-to-Caribbean upper ridge supported by warm air advection ahead of paragraph P1 frontal system). This upper ridge wave is now applying northerly shear across Michael as evidenced by the infrared satellite showing a meager cluster of t-storms to the south and an exposed swirl center to the north. Based on this satellite appearance...and the fact that Michael should be crossing over cooler waters soon...I prefer to weaken Michael faster than I previously showed and faster than what the 11 PM EDT NHC advisory showed for the next 12 hours. Between 12 and 24 hours...I think Michael will be getting supportive eastern upper divergence from the paragraph P1 upper trough as he becomes non-tropical...so I weaken him slower...but my suggested intensity for 24 hours is still lower than what the 11 PM EDT NHC advisory showed.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 11 PM EDT...which I slightly shrink in size as Michael weakens. Only a slight shrink is needed because Michael is already a tropical cyclone with a compact wind field that cannot get much more compact from the current.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 91-L...
Strong tropical wave with t-storms persists west of the Cape Verde Islands. It features a 1008 mb low pressure center whose t-storms have continued to become better organized...and this system is now close to becoming the next Atlantic tropical cyclone. The t-storms are still ever-so-slightly biased to the west side...indicative of easterly vertical shear. I diagnose that the shear is due to the tropical wave's t-storm latent heat release being mostly on the south side of the paragraph P5 upper ridge...which enhances the easterly flow on the south side of the upper ridge. Continued latent heat release should allow a relaxation in the shear when an embedded warm core upper anticyclone forms directly over the tropical wave...and in fact we are beginning to see this. The easterly shear did let in some of the paragraph P4 dry air located to the east...which has delayed the tropical cyclone formation of this system.

Computer model runs suggest that a tropical cyclone that forms from this wave would first track westward to WNW about the south side of paragraph P4 ridge...followed by a northward recurvature into the open Atlantic beginning in 48 hrs. A strong eastern US low-level ridge (paragraph P1) should soon enter the Atlantic. However...what prevents the models steering this tropical wave west under the strong ridge is a cut-off upper vortex (and associated surface low supported by peripheral divergence of that vortex) to be delivered by non-tropical Leslie's upper trough. The models agree that this cut-off low pressure system will curve this system northward after 48 hrs. Statistically by this point in the season...a recurvature to the north for an eastern Atlantic tropical cyclone is more likely anyway...as mid-latitude systems such as this forecast cut-off low pressure system are more prominent as the jet stream sags southward beginning in mid-September and onwards.

...MID-LATITUDES DISCUSSION...
P1...Eastern divergence of upper trough in mid-latitude westerlies continues to support a strong surface frontal cyclone (currently 976 mb) as the cyclone pushes from the east coast of Canada and into Greenland. The associated surface frontal zone is stretched across the west Atlantic and southern Gulf of Mexico. Western upper convergence of the upper trough supports a large area of dry air and a 1022 mb surface ridge over the eastern US. Relatively warmer air south of the surface front supports northern Gulf of Mexico upper ridge...which in the last 24 hours has merged with the Caribbean-to-C Atlc upper ridge cell mentioned in paragraph P5. The surface ridge once supported by this north Gulf upper ridge has now joined the Atlantic surface ridge mentioned in paragraph P4.

P2...Cut-off upper vorticity remains south of Tropical Cyclone Michael in the open Atlantic...and has elongated into a north-south upper trough squeezed between the Caribbean-to-C Atlc upper ridge cell and upper ridge cell toward Africa (both cells mentioned in paragraph P5). This upper trough features one upper vortex just south of Michael...and another upper vortex just east of the Lesser Antilles. Elsewhere...cut-off upper trough over the Canary Islands has been absorbed by paragraph P3 upper trough.

P3...Remnant surface low of Isaac is moving eastward across the Atlantic high seas and is currently positioned NE of the Azores. Eastern divergence of its shortwave upper trough continues supporting the system. This shortwave upper trough's western convergence supports a greater-than-1020 mb ridge SSE of Greenland. The paragraph P5 C Atlc-to-Caribbean upper ridge should soon gain great amplitude (thanks to strong low-level warm air advection ahead of what should be non-tropical Leslie). As this occurs...the ex-Isaac shortwave upper trough will become a cut-off upper vortex that dives southward around the east side of the amplified upper ridge...which means ex-Isaac itself will soon dive southward to the east of the Azores.

P4...Atlantic surface ridge has been extended into the northern Caribbean Islands as it merges with what was a Gulf low-level ridge in paragraph P1. The entire surface ridge is now supported by SE convergence of lengthy paragraph P5 Caribbean-to-C Atlc upper ridge cell. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across the tropical Atlantic persists. Northern Gulf of Mexico upper ridge (paragraph P1) has merged with the Caribbean to Central Atlantic upper ridge cell in last 24 hrs. Embedded upper vorticity in relatively lower pressures persists in the western Caribbean. The Caribbean to Central Atlantic upper ridge has been stretched into the north Atlantic by low-level warm air advection ahead of the paragraph P1 system. Like a mid-latitude upper ridge wave...southeastern convergence of this Caribbean-to-C Atlc upper ridge supports a large area of dry air across the Caribbean and east of the Lesser Antilles. Remainder of the upper ridging is located toward the west coast of Africa.

P6...Surface trough west of the Cape Verde Islands in the previous discussion (now midway between the Cape Verdes and Lesser Antilles) is still suprisingly a separate feature as it becomes absorbed by the west side of tropical wave Invest 91-L. At any time now...I expect this surface trough to be removed from the NHC TAFB maps.

P7...Due to a westward-moving cluster of t-storms...satellite imagery suggests the next tropical wave has just rolled off of Africa (see lower-right corner of above atmo chart).

2012 Atlantic Hurricane Season Birdseye Discussion #102

By: NCHurricane2009, 8:33 AM GMT on September 10, 2012

...SEPTEMBER 10 2012...4:30 AM EDT...
Tropical Storm Leslie still showing no signs of regaining hurricane strength...although the National Hurricane Center forecasts it will before it strikes eastern Newfoundland early Tuesday. Tropical storm and hurricane watches have been raised for eastern Newfoundland...see www.nhc.noaa.gov for latest watches and warnings. As a large non-tropical (extratropical) gale...it will affect the Atlantic high seas shipping lanes after striking Newfoundland. See Leslie special feature section for further details.

Michael has started weakening again in the last 24 hours. It is expected to stay over open waters and potentially lose its identity along the cold front of non-tropical (extratropical) Leslie in the next 48 to 72 hours. See Michael special feature section for details on this hurricane.

Eastern Atlantic tropical wave Invest 91-L has continued to become better organized...and is about to become the next Atlantic tropical cyclone. See third special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1929Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM LESLIE...
Leslie is beginning to accelerate north-northeastward ahead of the next frontal system in the mid-latitude westerlies (paragraph P1). For the short-term...Leslie in the last 24 hrs has been better-following the NHC forecast (as opposed to mine) while bending rightward (I had predicted a straight north track for the previous 24 hrs). In the longer-term...I previously speculated that Leslie should aid in amplifying the paragraph P1 upper trough while advecting the cool air associated with the upper trough southward. As the upper trough amplifies...the steering flow ahead of it should become more southerly (as opposed to southwesterly)...causing Leslie to hook northward. This effect caused me to have a leftwared bias to NHC's forecast late in the period...but now this is no longer the case as the NHC forecast has aligned with that solution in the last 24 hrs. Therefore...there is now no disagreement between my and the NHC's track forecast as shown in Figure 1. The NHC forecast continues to quicken the pace at which Leslie accelerates...which now means that Leslie should arrive to eastern Newfoundland early Tuesday instead of late Tuesday as previously mentioned.


Figure 1: My forecast for Tropical Storm Leslie created this morning.

Intensity-wise...Leslie seems to have mixed out the dry air mentioned in the previous two discussions....but their remains a large ragged eye signatory of the earlier dry air ingestion. However...she has weakened further from 65 to 60 mph max winds in last 24 hrs. The NHC forecasts Leslie to regain hurricane strength (75 mph max winds) before it strikes eastern Newfoundland...because of the rapid northeastward acceleration is predicted to increase the winds in the east half of the cyclonic circulation (FYI...this same effect decreases the winds in the west half of the cyclonic circulation). I prefer to keep Leslie at the same intensity she is now thru the forecast period...as a balance between the aforementioned favorable acceleration effect...the unfavorable shearing southwesterly upper jet in advance of the paragraph P1 frontal system's upper trough...favorable upper divergence from the east side of the paragraph P1 upper trough....and unfavorable cooler waters she is about to enter. Because the updated NHC forecast track is even faster...she should hit cooler waters even sooner...so I prefer to roll back the time of transition to non-tropical by 12 hours from the previous...but the NHC keeps the transition time the same from the previous.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I do not shrink nor expand with the forecast intensity remaining flat. I progressively lean the impact swath towards the right of the storm track to represent SW shear Leslie is expected to soon encounter. Since she should be moving rapidly northeast by the end of the forecast...winds will also tend to be stronger in the east half and diminished in the west half...another reason to lean the impact swath to the right.

...SPECIAL FEATURE...HURRICANE MICHAEL...
Updated NHC track forecast is in Figure 2 below. Michael in the last 24 hours has very-well followed the previous NHC forecast track as he turned westward (and even a bit south of west) while following the flow around the south side of the paragraph P3 low-level ridge...and the NHC has not made any adjustments to their track forecast for the next 24 hrs. I agree with the NHC's solution for the next 24 hrs...with that solution matching up pretty well with the forecast flow around this ridge in the 00Z GFS model. This means not long after 24 hrs...Michael should hook and accelerate northward about the west side of the ridge. After the northward hook...it is the longer-term NHC solution that has changed from 24 hrs ago...because Leslie is forecast to track faster to the NE than previously to the degree that Michael should now experience SW flow on the SE half of Leslie instead of getting absorbed by the E side of Leslie. This SW flow has forced to make the longer-term NHC solution more to the right than the previous...and I agree with this thinking. I also agree with the NHC solution of making Michael non-tropical by 48 hrs...coincident with when Michael should merge with the cold front extending from what should be a non-tropical Leslie.


Figure 2: My forecast for Hurricane Michael created this morning.

Intensity-wise...Michael has resumed weakening...now at a rate of 15 mph max winds every 24 hrs. The NHC has diagnosed the weakening due to northerly shear from Leslie's upper outflow. I disagree because once again Michael appears shallower than a typical tropical cyclone (hence less sensitive to vertical shear)...with its upper anticyclonic outflow disappearing from the 200 mb wind barbs in the above atmo chart...and becoming replaced by nearby upper vortex racing northward (vortex mentioned in paragraph P2). A typical tall tropical cyclone would have lost its t-storms as an upper vortex moves over...but Michael right now has a good ring of t-storms around its eye as if its whole vertical structure (surface spin and supportive upper outflow) is tucked shallow below that upper vortex. Even if I am wrong and Michael is tall enough to be sensitive to the northerly vertical shear...I argue that soon...Leslie's upper outflow will get sheared eastward by the paragraph P1 mid-latitude system...which displaces it over and north of Michael such that northerly shear is eliminated. In fact...with the outflow being over Michael...this may actual ventilate and help him between 24 and 48 hrs! But because I cannot deny that Michael has weakened...my intensity forecast for the next 24 hrs is based on extrapolating his current weakening rate...and my intensity forecast between 24 and 48 hrs I think is a good compromise between aforementioned favorable upper outflow and unfavorable cool waters. My intensity forecast in Figure 2 is now in 100% agreement with the NHC's 11 PM EDT forecast...so there is no difference between my and NHC's intensity forecast at this time.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 11 PM EDT...which I only slightly shrink in size on the presumption Michael remains a compact tropical cyclone that slowly weakens.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 91-L...
Strong tropical wave with t-storms passing over the Cape Verde Islands in the previous discussion is now west of the islands. It still features a 1009 mb low pressure center whose t-storms have continued to become better organized...and this system is now close to becoming the next Atlantic tropical cyclone. The t-storms are biased to the west side...indicative of easterly vertical shear. I diagnose that the shear is due to the tropical wave's t-storm latent heat release being mostly on the south side of the paragraph P5 upper ridge...which enhances the easterly flow on the south side of the upper ridge. Continued latent heat release should allow a relaxation in the shear when an embedded warm core upper anticyclone forms directly over the tropical wave.

Computer model runs suggest that a tropical cyclone that forms from this wave would first track westward to WNW about the south side of paragraph P4 ridge...followed by a northward recurvature into the open Atlantic beginning in 72 hrs for complex reasons. In my previous statement on this tropical wave...I stated that Leslie being forecast to move faster north than thought before would decrease the chances of this system recurving northward into non-tropical Leslie's associated ridge weakness...especially when considering what is currently a strong central US ridge (paragraph P1) building in right behind non-tropical Leslie. But what prevents the models steering this tropical wave west under that strong ridge is a cut-off upper vortex (and associated surface low supported by peripheral divergence of that vortex) to be delivered by non-tropical Leslie's upper trough. The models agree that this cut-off low pressure system will curve this system northward after 72 hrs. Statistically by this point in the season...a recurvature to the north for an eastern Atlantic tropical cyclone is more likely anyway...as mid-latitude systems such as this forecast cut-off low pressure system are more prominent as the jet stream sags southward beginning in mid-September and onwards.

...MID-LATITUDES DISCUSSION...
P1...Upper trough in mid-latitude westerlies has intensified a strong surface frontal cyclone from 986 to 977 mb in last 24 hrs as the frontal cyclone pushes onto the east coast of Canada. The associated surface frontal zone is stretched across the west Atlantic just offshore of the US...northern Gulf of Mexico...and Texas-Mexico border. Western upper convergence of the upper trough supports a large area of dry air and a building 1024 mb surface ridge over the central US. Relatively warmer air south of the surface front supports the southern US upper ridge...which has been pushed into the northern Gulf of Mexico in the last 24 hours as the front sags south. Upper convergence on the SE half of this upper ridge supports a southern Gulf of Mexico surface ridge extending eastward into the tropical Atlantic...and narrowly has joined the surface ridge in paragraph P4.

P2...Upper trough approaching western Europe in the previous discussion has exited the picture while moving into Europe. The cut-off upper vortex it left behind south of Hurricane Michael days ago is surging northward while trying to link with the paragraph P3 upper trough. It also leaves behind a cut-off upper trough over the Canary Islands. Associated surface cold front was strewn across the NE Atlantic as of 1800Z TAFB last night.

P3...Remnant surface low of Isaac is moving eastward across the Atlantic high seas and passing north of the Azores. Eastern divergence of its shortwave upper trough continues supporting the system. This shortwave upper trough's western convergence supports a greater-than-1020 mb ridge that has moved from SE of Atlantic Canada to now east of Atlantic Canada. The paragraph P5 C Atlc-to-Caribbean upper ridge should soon gain great amplitude (thanks to strong low-level warm air advection ahead of what should be non-tropical Leslie). As this occurs...the ex-Isaac shortwave upper trough will become a cut-off upper vortex that dives southward around the east side of the amplified upper ridge...which means ex-Isaac itself will dive southward to the east of the Azores.

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to the ex-Isaac cyclone (paragraph P3) and NE Atlantic cold front (paragraph P2). What is left is in the eastern Atlantic at 1023 mb...a portion of which is supported by western convergence of paragraph P2 Canary Islands upper trough. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across the tropical Atlantic persists. Southern US upper ridge has been pushed into the northern Gulf of Mexico (paragraph P1). Embedded upper vorticity in relatively lower pressures south of this upper ridge persists...located in the western Caribbean. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched into the north Atlantic by low-level warm air advection ahead of the paragraph P1 system. Like a mid-latitude upper ridge wave...eastern convergence of this Caribbean-to-C Atlc upper ridge supports a large area of dry air southwest of Tropical Storm Leslie. Remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures south of the Canary Islands upper trough mentioned in paragraph P2.

P6...Surface trough west of the Cape Verde Islands in the previous discussion is no longer a separate feature as it becomes absorbed by the west side of tropical wave Invest 91-L.

2012 Atlantic Hurricane Season Birdseye Discussion #101

By: NCHurricane2009, 8:26 AM GMT on September 09, 2012

...SEPTEMBER 9 2012...4:30 AM EDT...
Tropical Storm Leslie shows no signs of regaining hurricane strength. Leslie should pass east of Bermuda in the next 24 hours...but the wind field of the storm has grown large enough in the last 24 hours such that tropical storm conditions should soon clip Bermuda. A tropical storm warning is currently in effect for Bermuda. In addition...Leslie is bringing surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...the east US shore...soon to spread into Atlantic Canada (Nova Scotia and Newfoundland). Direct impacts (high winds and rain) to Atlantic Canada appear most likely in eastern Newfoundland beginning late Tuesday. See Leslie special feature section for further details.

Michael has stopped weakening in the last 24 hours. It is expected to stay over open waters and potentially get absorbed by the east side of Leslie in the next 72 to 96 hours. See Michael special feature section for details on this strong hurricane.

The vigorous tropical wave that emerged from Africa in the previous discussion has become better organized...and has been upgraded to disturbance Invest 91-L. See third special feature section below for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0117Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM LESLIE...
Leslie is beginning to accelerate northward as the next frontal system in the mid-latitude westerlies (paragraph P1) knocks out the blocking 1021 mb low-level ridge (paragraph P3) to the north. Leslie has accelerated a bit faster to the north than I previously predicted...and moreover the NHC has accelerated the predicted pace at which Leslie advances northward. Therefore...my new forecast track in Figure 1 is at (instead of behind) the NHC forecast track pace. My initial leftward bias with respect to NHC is because I still see enough paragraph P3 low-level ridging to the east of Leslie (in the 00Z GFS) for a straight northward track in the next 24 hrs. For the late part of the track forecast...I speculate that Leslie should aid in amplifying the paragraph P1 upper trough while advecting the cool air associated with the upper trough southward. As the upper trough amplifies...the steering flow ahead of it should become more southerly (as opposed to southwesterly)...causing Leslie to hook northward. However...because Leslie is now forecast to be further north when she meets the upper trough...the cool air will not be advected as far south...and hence the upper trough will not be as amplified. Therefore...my track and the NHC's has shifted to the right for the end of the forecast...but my forecast grows a leftward bias by that time as I think the NHC's forecast track underplays the northward hook.


Figure 1: My forecast for Tropical Storm Leslie created this morning.

Intensity-wise...the dry air ingested into the south half of the storm in the previous discussion has rotated into the east half and eroded the core...creating a large ragged eye feature. What's left of the t-storms is largely in the west half...so perhaps the western rain bands currently over Bermuda will soon diminish as the dry air continues to rotate around (although there is a new t-storm bubble that has just formed north of the center that could delay this). We saw with Isaac in discussion #90 how dry air eroded the core...and without a core the t-storm latent heat release is not focused at the center. Rather the t-storm latent heat release inflates the upper outflow over a large area...and hence surface pressure falls occur over a large area that has expanded the wind field size of Leslie in the last 24 hours. Because Isaac did not strengthen that much under such circumstances...and since the 00Z GFS still shows a strong southwesterly upper jet soon to develop in advance of the paragraph P1 frontal system's upper trough at a location just north Bermuda...my updated intensity forecast in Figure 1 no longer re-strengthens Leslie into a hurricane (75+ mph max winds) under the assumption that SW shear from the jet will soon start to affect Leslie. Even though she should be under SW shear and cross the 26 deg C isotherm (into cooler waters) late in the forecast...I show no weakening either...thanks to expected upper divergence from the paragraph P1 upper trough supporting Leslie (much like how a non-tropical system is supported). I agree with NHC on moving up the timeframe to non-tropical transition to 24 hrs earlier...because the now-faster northward track in Figure 1 takes Leslie to cooler waters sooner.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I do not shrink nor expand with the forecast intensity remaining flat. Previous impact statements in Figure 1 said Bermuda would get clipped by tropical storm conditions if the track shifted left. We can now expect Bermuda to get clipped by tropical storm conditions...not because of a shift in track...but because the wind field size expanded as explained in the previous paragraph. I progressively lean the impact swath towards the right of the storm track to represent SW shear Leslie is expected to soon encounter. Since she should be moving rapidly northeast by the end of the forecast...winds will also tend to be stronger in the east half and diminished in the west half...another reason to lean the impact swath to the right. There are several hazards Leslie will bring in the next 5 days. Read all impacts statements in Figure 1 for headlines on all these hazards.

...SPECIAL FEATURE...HURRICANE MICHAEL...
Updated NHC track forecast is in Figure 2 below. Michael has deviated from the previous NHC track forecast by hooking more northward toward ex-Isaac. Intuitively this would seem to necessitate a rightward shift of the whole forecast...but the NHC has maintained their previous track from 24 hrs ago. A look at the 00Z GFS shows a quick strengthening of the paragraph P3 1021 mb low-level ridge to the NW of Michael...driven by the western upper convergence of the ex-Isaac upper shortwave...and driven by eastern upper convergence of the paragraph P5 C Atlc-to-Caribbean upper ridge (as that upper ridge itself gains great amplitude thanks to strong low-level warm air advection ahead of what should be non-tropical Leslie). With the GFS showing the 1021 mb ridge NW of Michael strengthening quickly...and with the NHC forecast in Figure 2 matching up pretty well with the forecast flow around this ridge...I currently agree with the NHC track forecast. Therefore...Michael should soon turn wesward around the south side of the ridge...then later hook and accelerate northward about the west side of the ridge and into the east side of Leslie (where he gets absorbed). Compared to my previous forecast...I gave Michael 12 more hours till absorption time (this is also 12 hours later than the 11 PM EDT NHC forecast in Figure 2)...because Michael is currently maintaining his strength longer than previously forecast.


Figure 2: My forecast for Hurricane Michael created this morning.

Intensity-wise...Michael has essentially stopped weakening...becoming a steady-state category 2 hurricane of 100 mph max winds. With the above forecast track...for the next 48 hours he will be over the same water temps he is currently over...so I maintain his current strength thru 48 hrs. 200 mb wind barbs in the above atmo chart suggest that Michael may have finally become tall like a typical tropical cyclone...with 200 mb upper anticyclonic flow present over and east of the storm. So perhaps he is now more sensitive to any northerly shear that Leslie's upper outflow may deliver. Whether he is or is not sensitive to this northerly shear...I argue that soon...Leslie's upper outflow will get sheared eastward by the paragraph P1 mid-latitude system...which displaces it over and north of Michael such that northerly shear is avoided. In fact...with the outflow being over Michael...this may actual ventilate and help him after 48 hrs hrs! However...he will have made a rapid northward acceleration into cool waters. So after 48 hrs...I show a weakening rate that I think is a good compromise between unfavorable cool waters and favorable upper outflow.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 11 PM EDT...which I only slightly shrink in size on the presumption Michael remains a compact tropical cyclone that slowly weakens.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 91-L...
A strong tropical wave with t-storms has emerged from Africa in the previous discussion...currently passing over the Cape Verde Islands with a 1009 mb low pressure center whose t-storms have become better organized. The t-storms are biased to the west side...indicative of easterly vertical shear. I diagnose that the shear is due to the tropical wave's t-storm latent heat release being mostly on the south side of the paragraph P5 upper ridge...which enhances the easterly flow on the south side of the upper ridge. Continued latent heat release should allow a relaxation in the shear when an embedded warm core upper anticyclone forms directly over the tropical wave.

Early computer model runs suggest that a tropical cyclone that forms from this wave would first track westward about the south side of paragraph P4 ridge...followed by a northward recurvature into the open Atlantic while drawn into the large-scale ridge weakness induced by what should be the large and powerful extratropical (non-tropical) remnants of Leslie and its cold front. Statistically by this point in the season...a recurvature to the north for an eastern Atlantic tropical cyclone is more likely anyway...as mid-latitude systems get more backing from jet stream upper troughs that get more intense from mid-September and forward. However...I will concede that because Leslie will not amplify the paragraph P1 upper trough as much as previously thought (see Leslie special feature section)...the chance that this system does not recurve northward into the associated ridge weakness has increased.

...MID-LATITUDES DISCUSSION...
P1...Upper trough in mid-latitude westerlies has intensified a strong surface frontal cyclone from 996 to 986 mb in last 24 hrs as the frontal cyclone pushes east from S Hudson Bay and into eastern Canada. The associated surface frontal zone is stretched across the eastern coast of the US...northern Gulf of Mexico...and US-Mexico border. Western upper convergence of the upper trough supports building 1024 to 1022 mb surface ridge over the western US. Relatively warmer air south of the surface front supports the southern US upper ridge. Upper convergence on the SE half of this upper ridge supports a Gulf of Mexico surface ridge extending to a 1015 mb center over the Bahamas...albeit the surface ridge is collapsing due to the incoming frontal zone.

P2...Upper trough SE of Greenland in the previous discussion is now approaching western Europe...and in the last 72 hours has left behind an upper vortex currently just south of Hurricane Michael. It has re-absorbed the Azores upper vortex it left behind in the previous discussion. The upper trough's eastern divergence supports a surface frontal cyclone now pushing into Europe from the British Isles. Upper convergence behind this upper trough used to support a low-level ridge SE of Atlantic Canada...which is now becoming supported by upper convergence behind ex-Isaac's shortwave upper trough (paragraph P3).

P3...Remnant surface low of Isaac is moving eastward across the Atlantic high seas. Eastern divergence of its shortwave upper trough continues supporting the system. This shortwave upper trough's western convergence supports a 1021 mb ridge currently SE of Atlantic Canada (this ridge formerly supported by convergent west side of paragraph P2 upper trough). Remnant surface trough of disturbance Invest 90-L in the northern Gulf of Mexico should become absorbed by cold front in paragraph P1 in next 24 hrs.

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to surface cyclone mentioned in paragraph P2. What is left is in the eastern Atlantic at 1023 to 1024 mb...a portion of which is supported by western convergence of paragraph P2 upper trough. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...supported by warm air ahead of cold front in paragraph P1. Embedded upper vorticity in relatively lower pressures south of this upper ridge persists...located offshore of the SE US and the western Caribbean. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched into the north Atlantic by low-level warm air advection ahead of the remnant of Isaac (paragraph P3). Embedded Canary Islands upper trough has been absorbed by paragraph P2 upper trough. Remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures south of the paragraph P2 upper trough.

P6...Surface 1012 mb low west of the Cape Verde Islands in the previous discussion has weakened to a surface trough to the west of tropical wave Invest 91-L in the last 48 hours. It has some small t-storm bursts...but is currently under southerly shear as it gradually approaches upper vortex south of Michael mentioned in paragraph P2.

2012 Atlantic Hurricane Season Birdseye Discussion #100

By: NCHurricane2009, 8:39 AM GMT on September 08, 2012

...SEPTEMBER 8 2012...4:45 AM EDT...
Hurricane Leslie has weakened to a tropical storm the last 24 hours while still lurking north of the Lesser Antilles and south of Bermuda. It is forecast to regain hurricane strength and gradually accelerate northward across the west Atlantic. Leslie should pass east of Bermuda. However...a tropical storm watch is in effect for Bermuda and should be acted upon. In addition...Leslie is bringing surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...the east US shore...expected to eventually spread into Atlantic Canada (Nova Scotia and Newfoundland). Direct impacts (high winds and rain) to Atlantic Canada appear most likely in eastern Newfoundland on Wednesday. See Leslie special feature section for further details.

Michael is a slowly-weakening hurricane...his max winds now weakening at the rate of about 5 mph every 24 hours. It is expected to stay over open waters and potentially get absorbed by the east side of Leslie in the next 96 hours. See Michael special feature section for details on this strong hurricane.

A tropical disturbance southeast of Hurricane Michael (in the eastern tropical Atlantic) has been dropped as a special feature on this blog. See paragraph P7 in tropical belt discussion for details on this disturbance.

A vigorous tropical wave has emerged from Africa...newly-introduced into the National Hurricane Center tropical weather outlook. Computer model support has been persistent with this wave...and coupled with the fact it is entering favorable upper winds...it is now introduced as a special feature on this blog. See 3rd special feature section below for details.

I still do not expect development from tropical disturbance Invest 90-L in the northern Gulf of Mexico...and it is receiving a reduced probability in the latest NHC tropical weather outlook. This system is currently mentioned in paragraph P3 in the mid-latitudes discussion below.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0122Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM LESLIE...
Leslie remains quasi-stationary over the western tropical Atlantic...but is soon expected to accelerate generally to the north as the next frontal system in the mid-latitude westerlies (paragraph P1) knocks out the blocking 1017 mb low-level ridge (paragraph P3) to the north. Both my and the NHC's updated forecast track in Figure 1 represent a continuation of the previous.

I am slower than the NHC from get-go and throughout...because Leslie has so far followed my previous forecast quiet well which had a slower progression to the north. My leftward bias toward the latter part of the forecast is because I still see enough paragraph P2 and P3 low-level ridging to the east of Leslie (in the 00Z GFS) for a straight northward track between 11 PM Sat (24 hrs) and 11 PM Sun (48 hrs). I speculate that Leslie should aid in amplifying the paragraph P1 upper trough while advecting the cool air associated with the upper trough southward. As the upper trough amplifies...the steering flow ahead of it should become more southerly (as opposed to southwesterly)...causing Leslie to hook northward. The 00Z GFS also supports this idea...and so I show such a hook toward the end of my track forecast. My 120 hr position is a bit north of the GFS...which shows an interesting blocking low-level ridge to the north of Leslie that causes GFS to slow her northward progress. From studying the upper wind forecast from GFS...it appears this blocking low-level ridge is the result of upper southerly flow of the paragraph P1 upper trough (amplified by Leslie) converging heavily with an upper westerly jet stream to the northeast.


Figure 1: My forecast for Tropical Storm Leslie created 2 AM EDT this morning.

Intensity-wise...in the last 24 hrs Leslie has weakened from a 75 mph max wind hurricane to a 65 mph wind tropical storm. With grand anticyclonic outflow over Leslie...coupled outflow enhancement into the upper vortex east of Florida (paragraph P5) and outflow enhancement into an upper vortex dropped off by paragraph P2 upper trough to her east...there is a recipe for strengthening...so what caused the weakening? The above thermo chart suggests that upper convergence on the SE half of paragraph P5 C Atlc-to-Caribbean upper ridge has intensified some of the paragraph P4 dry air...and in turn Leslie has ingested this dry air into her south half. Moreover...the NHC cites other resources at their disposal which finds that her quasi-stationary motion has upwelled cooler waters...with temps as low as 26 deg C right below the center. Note that this 26 deg C minimum is not shown in the above thermo chart..because the chart uses a different resource than what the NHC is citing. Since we expect her to accelerate northward away from the dry air and any cooler waters she has upwelled....I re- strengthen her to 75 mph max winds in the next 24 hrs. Beyond 24 hrs...GFS still shows a strong southwesterly upper jet developing in advance of the paragraph P1 frontal system's upper trough at a location just north Bermuda. So I am forced to not strengthen Leslie further as she approaches Bermuda's latitude....because of anticipated SW shear from the jet. Even though she should be under SW shear and cross the 26 deg C isotherm (into cooler waters) by day 5...I show no weakening below 75 mph max winds...thanks to expected upper divergence from the paragraph P1 upper trough supporting Leslie. I agree with NHC on a transition to non-tropical on day 5...as she becomes entirely supported by the eastern divergence of the upper trough.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I no longer inflate in size due to the now-expected meager strengthening. Even though Leslie weakens later in the forecast...I keep the swath the same size as I anticipate her interacting with divergence of the paragraph P1 upper trough. This interaction will cause pressure falls over a wide area that should still keep the wind field size dilated. I lean the impact swath towards the right of the storm track by day 5 to represent SW shear Leslie is expected to encounter by that time. Since she should be moving rapidly northeast by that time...winds will also tend to be stronger in the east half and diminished in the west half...another reason to lean the impact swath to the right. There are several hazards Leslie will bring in the next 5 days. Read all impacts statements in Figure 1 for headlines on all these hazards.

...SPECIAL FEATURE...HURRICANE MICHAEL...
Updated NHC track forecast is in Figure 2 below. Michael has finally turned NW while becoming steered by the paragraph P2 deep-layered ridge coming in from the NW as the NHC predicted. Since he has almost exactly followed the previous NHC track forecast...and the current NHC track forecast is a continuation of the previous...I see no reason to disagree with that track forecast. Later in the forecast track...he should accelerate northward about the west side of the deep-layered ridge and into the east side Of Leslie (where I agree with NHC that he will get absorbed by 96 hours).


Figure 2: My forecast for Hurricane Michael created at 2 AM EDT this morning.

Intensity-wise...his weakening rate has flattened to only 5 mph every 24 hours (during my previous discussion it was 5 mph every 6 hours). With the above forecast track...he will be south of the 26 deg C sea-surface-temp isotherm thru 72 hrs...so I weaken him on the current observed rate thru 72 hrs with the assumption that the waters are warm enough to support at least a hurricane (75+ mph) in the next 72 hrs. Michael has been behaving as a shallower-than-usual tropical cyclone...so I also assume Michael is not tall enough to be sensitive to northerly shear from Leslie's upper outflow. Even if he was...I argue that soon...Leslie's upper outflow gets sheared eastward by the paragraph P1 mid-latitude system...which displaces it over and north of Michael such that northerly shear is avoided. In fact...with the outflow being over Michael...this may actual ventilate and help him between 72 and 96 hrs hrs! However...he will have made a rapid northward acceleration into cool waters. So between 72 and 96 hrs...I show the same weakening rate as I did in my previous forecast for that timeframe...which I think is a good compromise between unfavorable cool waters and favorable upper outflow. Michael should be absorbed by Leslie by 96 hrs.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 11 PM EDT...which I only slightly shrink in size on the presumption Michael remains a compact tropical cyclone that weakens.

...SPECIAL FEATURE...TROPICAL WAVE FROM AFRICA...
A strong tropical wave with t-storms has emerged from Africa...currently located SE of the Cape Verde Islands. It has had computer model support over the last days...is entering a moist air environment in the wake of the disturbance in paragraph P7...and should leverage favorable upper outflow induced by upper ridge portion located toward Africa (paragraph P5). Therefore...I do not currently see a reason why this will not develop into the next Atlantic tropical cyclone. Early model runs suggest a tropical cyclone first tracking westward about the south side of paragraph P4 ridge...followed by a northward recurvature into the open Atlantic while drawn into the large-scale ridge weakness induced by what should be the large and powerful extratropical (non-tropical) remnants of Leslie and its cold front. Statistically by this point in the season...a recurvature to the north for an eastern Atlantic tropical cyclone is more likely anyway...as mid-latitude systems get more backing from jet stream upper troughs that get more intense from mid-September and forward. Generally...mid-latitude systems create the ridge weaknesses for the storms to recurve northward.

...MID-LATITUDES DISCUSSION...
P1...Upper trough in mid-latitude westerlies remains concentrated into an upper vortex stacked above a strong 996 mb frontal cyclone currently over southern Hudson Bay...due to locally strong cool air advection on the back side of the cyclone. The associated surface frontal zone is stretched across the eastern half of the United States and curls into this frontal cyclone. Western upper convergence of the upper trough supports building 1028 to 1018 mb surface ridge over the western US. Warm air advection ahead of this front supports the southern US upper ridge. Upper convergence on the SE half of this upper ridge supports a Gulf of Mexico surface ridge extending to a pair of 1016 mb centers offshore of the SE US.

P2...Upper trough south of Greenland in the previous discussion is now SE of Greenland...and in the last 48 hours has deposited a new upper vortex over and just south of Hurricane Michael....and another upper vortex over the Azores. The upper trough's eastern divergence supports a surface frontal cyclone now located SE of Greenland and evaluated at less-than-1000 mb. Upper convergence behind this upper trough used to support a low-level ridge south of Atlantic Canada...which is now tucked under upper ridging (created by warm air advection ahead of paragraph P1 system)...resulting in a 1018 mb deep-layered ridge currently SE of Atlantic Canada. Upper convergence behind this upper trough also supports a 1015 mb low-level ridge midway between Canada and Greenland.

P3...Remnant surface low of Isaac is entering the Atlantic high seas from Atlantic Canada. Ex-Isaac is getting support from eastern divergence of the paragraph P1 upper trough...and its local cool air advection has created a new supporting shortwave upper trough. This new shortwave upper trough's western convergence supports a 1017 mb ridge offshore of the NE US. Split flow divergence between the west side of upper vortex east of Florida (paragraph P5) and southern US upper ridge (paragraph P1) supports a 1009 mb low (tropical disturbance Invest 90-L). All of its t-storms have dissipated in what is dry air induced by upper convergence on the SE half of southern US upper ridge.

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to surface cyclone mentioned in paragraph P2. What is left is in the eastern Atlantic supported by western convergence of Canary Islands upper trough mentioned in paragraph P5. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...supported by warm air advection ahead of cold front in paragraph P1. Embedded upper vortex in relatively lower pressures south of this upper ridge persists...located east of Florida. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched across the NE Atlantic and into west Europe by low-level warm air advection ahead of the paragraph P2 mid-latitude system. Embedded Canary Islands upper vortex has weakened into an upper trough in last 24 hrs. Remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of the Canary Islands upper upper trough.

P6...1012 mb low SE of Tropical Storm Leslie and ENE of the Lesser Antilles in the previous discussion has been absorbed by the outer low pressure field of Leslie.

P7...A surface 1012 mb low west of the Cape Verde Islands was a special feature on this blog in previous discussion #99 (see Eastern Atlantic Tropical Disturbance special feature section in that post). It is no longer a special feature on this blog as its t-storms have decreased...but not sure why (the above thermo birdseye chart shows the system is embedded in a moist air mass...the above atmo chart shows it a surface trough below a favorable upper ridge portion located toward Africa...this upper ridge mentioned in paragraph P5).

2012 Atlantic Hurricane Season Birdseye Discussion #99

By: NCHurricane2009, 8:59 AM GMT on September 07, 2012

...SEPTEMBER 7 2012...5:00 AM EDT...
Hurricane Leslie refuses to strengthen during the last 24 hours while still lurking north of the Lesser Antilles and south of Bermuda...forecast to gradually accelerate northward across the west Atlantic. There has been a rightward shift in the forecast track...and therefore the threat to Bermuda has been reduced. However...a tropical storm watch is in effect for Bermuda and should be acted upon. In addition...Leslie is bringing surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...the east US shore...expected to eventually spread into Atlantic Canada (Nova Scotia and Newfoundland). Due to the rightward shift in forecast track...direct impacts (high winds and rain) to Atlantic Canada appear most likely in eastern Newfoundland sometime after Tuesday. See Leslie special feature section for further details.

In the last 24 hours...Michael exploded into the season's first major hurricane with 115 mph max sustained winds. His max winds are now weakening at the rate of about 5 mph every 6 hours. It is expected to stay over open waters and potentially gets absorbed by the east side of Leslie just after 5 days. See Michael special feature section for details on this strong hurricane.

A tropical disturbance southeast of Hurricane Michael (in the eastern tropical Atlantic) is currently disorganized and beginning to track northwest into less favorable upper winds. The t-storm activity associated with the disturbance is less intense. If these trends continue...this will be the last blog post in which I make this system a special feature. See third special feature section below for details on this disturbance.

I still do not expect development from tropical disturbance Invest 90-L in the northern Gulf of Mexico...even though it is still mentioned in this morning's NHC tropical weather outlook. Therefore I still do not have a special feature section on 90-L. This system is currently mentioned in paragraph P3 in the mid-latitudes discussion below.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0120Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE LESLIE...
Leslie remains quasi-stationary over the western tropical Atlantic...but is soon expected to accelerate generally to the north as the next frontal system in the mid-latitude westerlies (paragraph P1) knocks out the blocking 1020 mb deep-layered ridge (paragraph P2) to the north. My and the NHC's updated forecast track in Figure 1 represents quiet a rightward shift from the previous. A search through NHC advisories in last 24 hours reveals the computer models shifted to the right...and pulled Leslie more slowly northward...but why? The only thing the NHC advisories mentioned that changed in the models was the amplitude of the paragraph P1 upper trough. It is claimed this upper trough has less amplitude than what the models showed before...probably due to better sampling of the upper trough by weather stations over Canada.

As paragraph P3 (mid-latitudes discussion) mentions...ex-Isaac (remnant of Isaac) has also gone thru some changes in last 24 hours. Its shortwave upper trough is dissipating...but it is explained that it is expected to develop another shortwave upper trough. That means we are still expecting western upper convergence from some sort of ex-Isaac upper shortwave...which means we still expect a transient low-level ridge (supported by that upper convergence) to build north of Leslie.

Previously...we said the paragraph P1 frontal system would come roaring in...causing Leslie to accelerate northward...with a wiggle to the left caused by the transient low-level ridge. With the paragraph P1 system now less amplified than thought before...it will take longer for this system to knock out the transient low-level ridge. Rather...the transient low-level ridge is going to be stronger...persist longer...hence acting as a blocking feature that now elongates the amount of time Leslie remains stalled. Based on the new impressiveness of the transient low-level ridge in 00Z GFS...I hardly move Leslie northward in the next 24 hrs (just as it has done in the past 24 hrs)...causing me to be slower than the NHC's track from the get-go and throughout. 00Z GFS shows some of this low-level ridge remaining by 48 hrs...just enough for me to wiggle the track ever-so-slightly left by 11 PM Sat. Note that Leslie accelerates northward after 24 hrs...as the paragraph P1 frontal system finally comes roaring in. My leftward bias toward the latter part of the forecast is because I still see enough low-level ridging to the east of Leslie (in the 00Z GFS) for a straight northward track between 11 PM Sat (48 hrs) and 11 PM Sun (72 hrs).

Just beyond 120 hrs (not shown in Figure 1)...we previosuly speculated Leslie should aid in amplifying the paragraph P1 upper trough while advecting the cool air associated with the upper trough southward. As the upper trough amplifies...the steering flow ahead of it should become more southerly (as opposed to southwesterly)...causing Leslie to hook northward. This northward hook is not likely to be as pronounced as we thought before...because the paragraph P1 upper trough is not as amplified to begin with as we previously thought.


Figure 1: My forecast for Hurricane Leslie created 2 AM EDT this morning.

Intensity-wise...in the last 24 hrs Leslie has not strengthened at all (maintaining 75 mph max winds). With grand anticyclonic outflow over Leslie...coupled outflow enhancement into the south Florida upper vortex to her west (paragraph P5) and outflow enhancement into an upper vortex dropped off by paragraph P2 upper trough to her east...I see a recipe for strengthening right now. However...the NHC cites other resources at their disposal which finds that her quasi-stationary motion has upwelled cooler waters...with temps as low as 26 deg C right below the center. Note that this 26 deg C minimum is not shown in the above thermo chart..because the chart uses a different resource than what the NHC is citing. Since we expect her to accelerate northward away from any cooler waters she has upwelled....I strengthen her to 85 mph max winds by Saturday (the time she begins accelerating northward). By day 2 and beyond...GFS still shows a strong southwesterly upper jet developing in advance of the paragraph P1 frontal system's upper trough at a location just north Bermuda. So I am forced to not strengthen Leslie further as she approaches Bermuda's latitude....because of anticipated SW shear from the jet. Even though she should be under SW shear and cross the 26 deg C isotherm (into cooler waters) by day 5...I show no weakening below 75 mph max winds...thanks to expected upper divergence from the paragraph P1 upper trough supporting Leslie. It is conceivable that just after day 5...she could easily become a strong non-tropical low of category 1 hurricane force...entirely supported by divergence of the upper trough.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I no longer inflate in size due to the now-expected meager strengthening. Even though Leslie weakens later in the forecast...I keep the swath the same size as I anticipate her interacting with divergence of the paragraph P1 upper trough. This interaction will cause pressure falls over a wide area that should still keep the wind field size dilated. I lean the impact swath towards the right of the storm track by day 5 to represent SW shear Leslie is expected to encounter by that time. Since she should be moving rapidly northeast by that time...winds will also tend to be stronger in the east half and diminished in the west half...another reason to lean the impact swath to the right. There are several hazards Leslie will bring in the next 5 days. Read all impacts statements in Figure 1 for headlines on all these hazards. These impact statements have been modified due to the changes in forecast in last 24 hours.

...SPECIAL FEATURE...HURRICANE MICHAEL...
My updated track forecast is in Figure 2 below. Michael has been dragged NE by the frontal cyclone in paragraph P2...which has caused the NHC to slightly adjust their forecast even more NE in the last 24 hrs. He is also quasi-stationary with a wobbling eye based on infrared satellite this morning...which means the immediate NW track shown in the NHC forecast has not yet begun. Because of his more NE than expected track...I think he is going to be more trapped in the armpit between the paragraph P2 deep-layered ridge coming in from the NW and paragraph P4 low-level ridge to the east. Furthermore...I cannot imagine a NW track beginning with the quick passage of ex-Isaac to the north thru the next 48 hrs...which would briefly weaken the paragraph P2 deep-layered ridge that would push him NW. Therefore...I show Michael drifting northward thru 48 hrs...ultimately forcing me to be slower and to the right-of-NHC thru the forecast period. After 48 hrs...00Z GFS shows the paragraph P2 and P4 low-level ridges melding together to the ENE of Michael...which would cause him to accelerate northward about the west side of the melded ridge and into the east side Of Leslie (where he will get absorbed just after 120 hrs...or just after 11 PM Tue).


Figure 2: My forecast for Hurricane Michael created at 2 AM EDT this morning.

Intensity-wise...I am assuming that Micheal's vertical warm core structure (surface spin and upper outflow) should have gained some height during his rapid intensification episode...though I don't see evidence of this in the above atmo chart 200 mb wind barbs...which shows an upper vortex over and south of Michael instead of upper anticyclonic outflow over Michael (source of this upper vortex mentioned in paragraph P2). With my assumption (again which I have no evidence of)...this means he is more sensitive to any northerly shear that would be delivered by Leslie's upper outflow.

Since the rapid intensification...Michael has weakened by 5 mph for every 6 hours. The eye is shrinking on infrared. As we saw in compact Hurricane Kirk recently...the eye quickly disappeared just after shrinkage (as if it underwent an eye wall replacement cycle). Kirk also weakened quickly after the eye disappeared. Assuming Michael is like Kirk at this phase...I would at least prefer to maintain the observed weakening rate thru 24 hrs...which makes me below the 11 PM EDT NHC intensity guidance from the get-go. I am also assuming that a quasi-stationary Michael should soon upwell cooler waters like we are now seeing with Leslie...another reason to weaken Michael.

The weakening rate is flattened after 24 hrs...my assumption being that...

(1) He is east enough of Leslie's outflow to dodge northerly shear
(2) Whatever cooler waters he upwells is still warm enough for a minimal (75 mph) hurricane.

I weaken Michael a touch more by 11 PM Mon...when I think he gets a taste of Leslie's northerly shear. Quickly after 11 PM Mon (after 96 hrs)...Leslie's upper outflow gets sheared eastward by the paragraph P1 mid-latitude system...which displaces it over and north of Michael such that northerly shear is gone between 96 and 120 hrs. In fact...with the outflow being over Michael...this may actual ventilate and help him between 96 and 120 hrs! However...he will have made a rapid northward acceleration into cool waters. So between 96 and 120 hrs...I show slow weakening as a compromise between unfavorable cool waters and favorable upper outflow. Not long after 120 hrs...Micheal should be absorbed by Leslie.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 5 PM EDT yesterday afternoon...which I only slightly shrink in size on the presumption Michael remains a compact tropical cyclone that weakens.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL DISTURBANCE...
T-storms persist beneath upper outflow of the upper ridge located toward Africa (mentioned in paragraph P5). Within these t-storms...a surface 1012 mb low persists west of the Cape Verde Islands and tracking slowly NW in the last 24 hours (NW track caused by weakening of paragraph P4 surface ridge thanks to paragraph P2 mid-latitude system and Hurricane Michael). It has been a while since a tropical wave emerged from Africa...so this new surface low could be the next tropical wave that has emerged from Africa...a feature left behind by the tropical wave mentioned in paragraph P6...or a surface low that has spun up along the ITCZ due to pressure falls generated by the upper outflow.

The t-storms have decreased in last 24 hours...but not sure why (the above thermo birdseye chart shows the system is embedded in a moist air mass). If the NW track of the surface low continues...it will soon leave the favorable upper ridge and into a less favorable environment of westerly vertical shear. Therefore...this maybe the last time I give this disturbance a special feature section on this blog.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough in mid-latitude westerlies has concentrated into an upper vortex stacked above a strong frontal cyclone currently over southern Hudson Bay...due to locally strong cool air advection on the back side of the cyclone. The associated surface frontal zone is stretched across the northern United States and curls into this frontal cyclone. Western upper convergence of the upper trough supports building 1022 to 1015 mb surface ridge over the western US. Warm air advection ahead of this front supports the southern US upper ridge. Upper convergence on the SE half of this upper ridge supports a Gulf of Mexico surface ridge.

P2...Upper trough in the NW Atlantic has merged with southern Greenland shortwave upper trough mentioned in the previous discussion. The upper trough has deposited a new upper vortex over and just south of Hurricane Michael. The merged upper trough's eastern divergence supports a surface frontal cyclone now located SE of Greenland and evaluated at less-than-1004 mb. Upper convergence behind this merged upper trough used to support a 1020 mb low-level ridge south of Atlantic Canada...which is now tucked under an upper ridge center (created by warm air advection ahead of paragraph P1 system)...resulting in a 1020 mb deep-layered ridge center. Upper convergence behind this upper trough also supports a 1021 mb low-level ridge midway between Canada and Greenland.

P3...Remnant surface low of Isaac is centered over Atlantic Canada. Its new supporting shortwave upper trough has been pushed SE and is dissipating...thanks to a building upper ridge caused by warm air advection in advance of the paragraph P1 system. With the shortwave upper trough dissipating...ex-Isaac is now getting support from eastern divergence of the paragraph P1 upper trough...and its local cool air advection is expected by models to break off a chunk of this upper trough into its own supporting shortwave. Meanwhile...upper convergence behind the dissipating shortwave used to support a surface ridge over the eastern Great Lakes that is now over the SE US (at 1014 to 1016 mb). Split flow divergence between the west side of Florida upper vortex (paragraph P5) and southern US upper ridge (paragraph P1) supports a 1010 mb low (tropical disturbance Invest 90-L) and associated t-storms shifting southward into the northern Gulf of Mexico. The system is under vertical shear as evidenced by the surface low being centered near Louisiana while the t-storms are blown southwestward by northerly flow west of the Florida upper vortex. The system will remain generally stationary while trapped between the new western US surface ridge and Gulf surface ridge (both ridges mentioned in paragraph P1)...and has 48 hours to develop before the surface front in paragraph P1 sags south and absorbs it. It is conceivable that the shear relaxes before that time if the Florida upper vortex weakens (as cut-off upper vortices typically do over time)...but until that occurs...I am not considering this a special feature on this blog...especially as the t-storms are not that impressive.

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to NW Atlantic surface cyclone mentioned in paragraph P2. What is left is in the eastern Atlantic centered below the NE lobe of the paragraph P5 upper ridging to create a deep-layered 1021 mb ridge north of the Azores. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...supported by warm air advection ahead of cold front in paragraph P1. Embedded upper vortex in relatively lower pressures south of this upper ridge persists...located near the east coast of Florida. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched into the NE Atlantic by low-level warm air advection ahead of the paragraph P2 mid-latitude system. What's left of upper vorticity that was near Michael is concentrated into a Canary Islands upper vortex...whose NE split flow divergence supports nearby 1014 mb surface low. Remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of the Canary Islands upper vortex.

P6...A 1012 mb low has been added SE of Hurricane Leslie and ENE of the Lesser Antilles in the tropical Atlantic. This surface low may be the tropical wave that was cancelled in yesterday's NHC TAFB maps (see eastern tropical Atlantic disturbance special feature section of previous discussion #98). The surface low is embedded in paragraph P4 dry air...and may turn NW and get absorbed into quasi-stationary Hurricane Leslie.

2012 Atlantic Hurricane Season Birdseye Discussion #98A (Special Update)

By: NCHurricane2009, 11:27 PM GMT on September 06, 2012

...SEPTEMBER 6 2012...7:30 PM EDT...
As of 5 AM EDT this morning...Michael became a category 3 hurricane of 115 mph max sustained winds. More recently as of 5 PM EDT...he has weakened to a top end category 2 of 110 mph max sustained winds. This makes Michael the strongest hurricane so far in the 2012 Atlantic Hurricane Season...and the first major hurricane (115 mph+ max winds) of this season. Currently Michael is a marine threat out over open waters...and not expected to affect any land areas in its lifetime.

Unfortunately for me...I had issued a forecast in this morning's full discussion #98 predicting Michael to reach such an intensity...but had technical issues (related to my poor laptop computer) releasing the full discussion in time before the actual intensification occurred. In effect...my Michael forecast was invalidated because it was released during or after the actual intensification episode. This underscores how quickly things can change in the tropics...and why its good to be able to have a quick blog update cycle. For me...my blog update cycle includes updating the birdseye view charts...updating the mid-latitude and tropical belt sections...updating any special feature sections...and creating forecasts and associated graphics for any active tropical cyclones. Practice makes the blog update cycle go faster...and also I think it is time for me to replace my well-used laptop computer.

Assuming I experience no technical difficulties...my next full update should be released early tomorrow morning and hopefully by my intended release time. Return to full discussion #98 for an assessment on the rest of the Atlantic tropics.

2012 Atlantic Hurricane Season Birdseye Discussion #98

By: NCHurricane2009, 11:30 AM GMT on September 06, 2012

...SEPTEMBER 6 2012...4:00 AM EDT...
This discussion was intended for release at 4 AM EDT as titled here...but actually became released three-and-a-half hours later (7:30 AM EDT) due to technical difficulties I experienced. Also...the past couple of released discussions had the incorrect date listed in their title...but have now been corrected.

Tropical Storm Leslie has intensified into a hurricane during the last 24 hours while still lurking north of the Lesser Antilles and south of Bermuda...forecast to gradually accelerate northwestward than northward across the west Atlantic and pass very close or over Bermuda over the next days. She could intensify into a strong hurricane...and hurricane watches may go up for Bermuda around Friday morning (based on the possible arrival time of hurricane force conditions and the standard practice for issuing watches 48 hours in advance of those conditions). Tropical storm conditions could reach Bermuda as early as Saturday evening. In addition...Leslie is bringing surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...the east US shore...expected to eventually spread into Atlantic Canada. Direct impacts (high winds and rain) to Atlantic Canada can also be expected by late Monday and Tuesday. See Leslie special feature section for further details.

In the last 24 hours...Tropical Storm Michael has briskly intensified into a hurricane at a location well east-northeast of Leslie. It is expected to stay over open waters and potentialy gets absorbed by the east side of Leslie beyond 5 days. See Michael special feature section for details on this new hurricane.

A tropical disturbance southeast of Hurricane Michael (in the eastern tropical Atlantic) is currently disorganized but still in favorable upper winds...and therefore I still have a special feature section for this system. See third special feature section below for details on this tropical wave.

A new tropical disturbance has emerged in the northern Gulf of Mexico as it sags southward from the Mississippi-Alabama area. The disturbance has been upgraded to Invest 90-L and continues to be mentioned in this morning's NHC tropical weather outlook. I am not yet convinced that tropical cyclone formation will occur with this system...and therefore still do not have a special feature section on 90-L. This system is currently mentioned in paragraph P3 in the mid-latitudes discussion below.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1402Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE LESLIE...
Note that my updated forecast in Figure 1 below was done on the 5 PM EDT NHC forecast track graphic from last afternoon. The 11 PM EDT NHC track forecast did not change by much since that time.

Leslie remains quasi-stationary over the western tropical Atlantic...but is soon expected to accelerate generally to the north as the next frontal system in the mid-latitude westerlies (paragraph P1) knocks out the blocking Atlantic Canada low-level ridge (paragraph P2) to the north. My updated forecast in Figure 1 is a continuation of my previous...as I see no reason at all to change the forecast points based on the output of last afternoon's 18Z GFS model. This means right now I am a bit left of the NHC track...which has shifted a bit to the right in the last 24 hours. I continue to agree that a wiggle to the left will occur when she first accelerates northward...caused by a shortwave upper trough ejecting eastward well in advance of the main bulk of this frontal system (the western convergence of that shortwave supporting a trasient low-level ridge to the north of Leslie that causes her to wiggle left). This shortwave system was first mentioned in my previous Leslie discussion...but now I realize this shortwave is associated with the remnant of Isaac and mentioned in paragraph P3! It is the presentation of the transient low-level ridge's expanse (in 18Z GFS) that causes me to maintain my previous forecast track (even though as said above this is now left of NHC and even left of 18Z GFS itself).

Longer term track bends to the right as the southwesterly flow ahead of the paragraph P1 frontal system finally hits Leslie. I am still to the left of the NHC during that time...as my starting track is left of NHC. This means Leslie should pass near but offshore of Nova Scotia. Just beyond 120 hrs (not shown in Figure 1)...Leslie should aid in amplifying the paragraph P1 upper trough while advecting the cool air associated with the upper trough southward. As the upper trough amplifies...the steering flow ahead of it should become more southerly (as opposed to southwesterly)...causing Leslie to hook northward into all of Newfoundland and then the east coast of mainland Canada.


Figure 1: My forecast for Hurricane Leslie created 11 PM EDT September 5

Leslie has strengthened into a category 1 hurricane of 75 mph max winds in the last 24 hours. Her t-storm latent heat release appears to have punched out the unfavorable upper vortex over her west half mentioned in the previous discussion. Her quasi-stationary motion so far has not upwelled cooler waters...with healthy 30 deg C waters still in her area when looking at the above thermo birdseye chart. Since we expect her to accelerate northward away from any cooler waters she may soon upwell....gradual strengthening still appears likely. With the 00Z GFS developing grand anticyclonic outflow over Leslie...coupled outflow enhancement into the south Florida upper vortex to her west (paragraph P5) and outflow enhancement into an upper vortex dropped off by paragraph P2 upper trough to her east...I see a recipe for a major hurricane (115 mph+ max winds) by day 3...a little more bullish than the 11 PM EDT NHC forecast. Note that I adopted my strengthening rate from my previous forecast since it has done generally well recently. By day 3 and beyond...GFS still shows a strong southwesterly upper jet developing in advance of the paragrah P1 frontal system's upper trough at a location just north Bermuda. That is why I weaken Leslie as she approaches Bermuda's latitude...due to anticipated shear from that jet. Even though she should be under southwesterly shear and cross the 26 deg C isotherm (into cooler waters) by day 5...I only keep the weakening rate gradual as she interacts with upper divergence from the paragrah P1 upper trough. It is conceivable that just after day 5...she could easliy become a strong non-tropical low of category 1 hurricane force...supported by divergence of the upper trough.

Impact swath in Figure 1 is initialized on the 5 PM EDT NHC tropical storm wind radius from last afternoon...which I ever-so-slightly inflate in size based on the anticipated strengthening. Even though Leslie weakens later in the forecast...I keep the swath the same size as I anticipate her interacting with divergence of the paragraph P1 upper trough. This interaction will cause pressure falls over a wide area that should still keep the wind field size dilated. I lean the impact swath towards the right of the storm track by day 5 to represent possible southwesterly shear Leslie is expected to encounter by that time. Since she should be moving rapidly northeast by that time...winds will also tend to be stronger in the east half and diminished in the west half...another reason to lean the impact swath to the right. There are several hazards Leslie will bring in the next 5 days. Read all impacts statements in Figure 1 for headlines on all these hazards.

...SPECIAL FEATURE...HURRICANE MICHAEL...
Tropical Storm Michael has strengthened briskly into a hurricane in last 24 hrs. As I anticipated in my previous discussion...Michael is under a 200 mb divergent westerly jet between the paragraph P2 upper trough and paragraph P5 C Atlc upper ridge...and his intensification is due to taking advantage of the divergent upper westerly jet as a shallow tropical cyclone (rather than getting ripped apart by the jet as a tall tropical cyclone would). This is simlar to the recent analogues of Chris in June this year...and Gordon in the latter part of his life in August this year. Chris and Gordon also strengthened in what should have been a shearing environment due to their shallower-than-usual structure for a full-fledged tropical cyclone.

Note that my updated forecast in Figure 2 below was done on the 5 PM EDT NHC forecast track graphic from last afternoon. The 11 PM EDT NHC track forecast did not change by much since that time. Michael strengthened from a 70 mph max wind tropical storm to a 75 mph wind hurricane between 5 and 11 PM EDT...as indicated in Figure 2.


Figure 2: My forecast for Hurricane Michael created at 11 PM EDT September 5.

Track-wise...we last left Michael 24 hrs ago when he was stalling...which told me at the time that he was getting jammed between the paragraph P4 low-level ridge to the east and the Atlantic Canada low-level ridge (paragraph P2) coming in from the NW. Therefore I previously downplayed the interaction with the paragraph P2 frontal cyclone's cold front...so I had a track forecast that was biased SW of the NHC's. Of course...this was wrong in hindsight as Michael has turned NE while dragged by the front...which has caused the NHC to adjust their forecast track NE in the last 24 hrs. His developing eye has been following the current NHC track forecast...so I am simply going to agree with the NHC track forecast for the time being. That track forecast assumes the paragraph P2 cyclone's cold front should soon leave behind Michael...which will cause him to northwest while steered by the Atlantic Canada low-level ridge (paragraph P2) coming in from the NW. The NW track is shown to be slow...as he will be trapped in some conflicting steering in the armpit between the paragraph P4 low-level ridge to the east and paragraph P2 Atlantic Canada low-level ridge.

I currently wonder if the slow NW track forecast is too fast. What if Michael becomes nearly stationary in conflicting steering between the two low-level ridges? This is idea is further supported by the quick passage of ex-Isaac to the north during the forecast period...which would briefly weaken the Atlantic Canada low-level ridge that is supposed to steer him NW in the first place. Beyond day 5 (not shown in Figure 2)...if the slow NW track forecast verifies...Micheal would eventually curve around the west side of the Atlantic Canada low-level...taking him northward into the east side of Leslie...where in this case he would meet his demise while getting absorbed my much larger Leslie.

Intensity-wise...as a shallower than usual tropical cyclone seemingly invincible against the effects of vertical shear...Michael has briskly strengthened under a 200 mb divergent westerly jet between the paragraph P2 upper trough and paragraph P5 C Atlc upper ridge. Normally a jet like this would shear apart a taller tropical cyclone. If I look at the 5 PM EDT intensity of 70 mph...I can say that Michael has strengthened by 20 mph in the past 24 hrs...which is what my new intensity forecast in Figure 2 is based on. Realistically...he has strengthened by 25 mph in 24 hrs (considering the 11 PM EDT intensity of 75 mph)...so my intensity forecast in Figure 2 could be conservative. I saturate him as a category 3 hurricane of 115 mph max winds out of conservativeness in case I am wrong. His more NE than expected track makes me think he will now stay north a cut-off upper vortex to be dropped off by the paragraph P2 upper trough...dodging a potential upper convergent source that would cap his intensification...another reason I show continuous strengthening. One could argue that Leslie's outflow could induce weakening thru northerly shear late in the forecast period...but again he appears shallower than usual and invincible to vertical shear at the moment...and his more NE than expected track increases the chances of him staying away from this shear source anyway. The above forecast track keeps Michael over 28 to 27 deg C waters thru the forecast...and with him currently strengthening over 28 deg C water...I have no choice but to keep him at 115 mph thru day 5. I think what will ultimately destroy Michael beyond day 5 is absorption into Leslie's east side...or acceleration northward over cooler waters below 26 deg C.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 5 PM EDT yesterday afternoon...which I only slightly inflate in size on the presumption Michael remains a compact tropical cyclone (but the forecast intensification is what forces me to inflate the size slightly). In Figure 2...I added a statement about surf in the vicinity of the storm. Previously I thought he would be a small & weakish tropical cyclone not capable of stirring much water. Now...he looks like he could become a strong small cyclone that stirs water in the vicinity.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL DISTURBANCE...
Tropical wave SW of the Cape Verde Islands in previous discussion has been cancelled from NHC TAFB maps recently...due to the NHC's struggle in finding evidence of its existence. However...plenty of t-storms persist east of dissipated tropical wave and beneath this upper outflow of the upper ridge located toward Africa (mentioned in paragraph P5). Wihtin these t-storms...a surface 1012 mb low was removed and then re-added west of the Cape Verde Islands in the last 24 hours. It has been a while since a tropical wave emerged from Africa...so this new surface low could be the next tropical wave that has emerged from Africa...a feature left behind by the above-mentioned dissipated tropical wave...or a surface low that has spun up along the ITCZ due to pressure falls generated by the upper outflow. Because of the favorable upper outflow environment and persistence of this surface low...I still am considering this a special feature on this blog.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough in mid-latitude westerlies has entered the upper-left corner of above charts...with its surface frontal zone across south-central Canada and north-central US. Western upper convergence of the upper trough supports buidling 1020 mb surface ridge over the western US. Warm air advection ahead of this front supports the southern US upper ridge. Upper convergence on the SE half of this upper ridge supports a Gulf of Mexico surface ridge.

P2...Cut-off upper vortex in the NW Atlantic has de-amplified into an upper trough...and its eastern divergence continues to support a surface frontal cyclone located SE of Newfoundland and evaluated at less-than-1012 mb. To the north of that frontal cyclone...a shortwave upper trough and associated second surface frontal cyclone has moved across southern Greenland. Upper convergence behind this shortwave upper trough and the NW Atlantic upper trough supports a 1020 mb low-level ridge just south of Atlantic Canada....and another 1023 mb low-level ridge on the east coast of Canada.

P3...Remnant surface low of Isaac is centered over the NE US. Its local cool air advection has carved out a new supporting shortwave upper trough in advance of the paragraph P1 upper trough. Upper convergence behind this new shortwave supports 1014 mb ridge over the eastern Great Lakes. Meanwhile...the cut-off upper vortex over the western Carolinas mentioned in the previous discussion has merged with the upper vortex near south Florida mentioned in paragraph P5. Now...split flow divergence between the west side of south Florida upper vortex and southern US upper ridge (paragraph P1) supports a 1010 mb low (newly upgraded to tropical disturbance Invest 90-L) and associated t-storms shifting southward into the northern Gulf of Mexico. The system is under vertical shear as evidenced by the surface low being centered near the Florida panhandle while the t-storms are blown southwestward by northerly flow west of the south Florida upper vortex. The system will remain generally stationary while trapped between the new western US surface ridge and Gulf surface ridge (both ridges mentioned in paragraph P1)...and has 72 hours to develop before the surface front in paragraph P1 sags south and absorbs it. It is conceivable that the shear relaxes before that time if the south Florida upper vortex weakens (as cut-off upper vortices typically do over time)...but until that occurs...I am not considering this a special feature on this blog...especially as some of the t-storms have died off during these overnight hours.

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to NW Atlantic surface cyclone mentioned in paragraph P2. What is left is in the eastern Atlantic centered below the NE lobe of the paragraph P5 upper ridging to create a deep-layered ridge center north of the Azores. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...supported by warm air advection ahead of cold front in paragraph P1. Embedded upper vortex in relatively lower pressures south of this upper ridge persists...located near south Florida. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched into the NE Atlantic by low-level warm air advection ahead of the paragraph P2 mid-latitude system. Large and elongated upper vortex above Hurricane Michael has broken into a few upper troughs now located southeast of Leslie and Michael...while the remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of these upper troughs.

2012 Atlantic Hurricane Season Birdseye Discussion #97

By: NCHurricane2009, 8:32 AM GMT on September 05, 2012

...SEPTEMBER 5 2012...4:35 AM EDT...
This is one of the most important birdseye discussions I have released all season...due to the ominous threat that Leslie brings to Bermuda. Tropical Storm Leslie is still lurking north of the Lesser Antilles and south of Bermuda...forecast to gradually accelerate northwestward than northward across the west Atlantic and pass very close or over Bermuda over the next days. She could intensify into a strong hurricane...and hurricane watches may go up for Bermuda around Friday morning (based on the possible arrival time of hurricane force conditions and the standard practice for issuing watches 48 hours in advance of those conditions). Tropical storm conditions could reach Bermuda as early as Saturday evening. In addition...Leslie is bringing surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...the east US shore...expected to eventually spread into Atlantic Canada. See Leslie special feature section for further details.

In the last 24 hours...tropical depression thirteen has become Tropical Storm Michael at a location well to the east of Leslie. This is the third earliest thirteenth tropical storm in the Atlantic on record...behind Maria of 2005 and Maria of 2011. Despite the hyperactive pace of this season...no storms yet have become a major hurricane (115 mph+ max sustained winds). See Michael special feature section for details on this storm.

A tropical disturbance southeast of Tropical Storm Michael (in the eastern tropical Atlantic) is currently disorganized but still in favorable upper winds...and therefore I still have a special feature section for this system. See third special feature section below for details on this tropical wave.

A new tropical disturbance could emerge in the northern Gulf of Mexico as it sags southward from the Mississippi-Alabama area. This system has already been introduced into this morning's NHC tropical weather outlook...but I am waiting for this system to emerge over the northern Gulf before making it yet another special feature section on this blog. This system is currently mentioned in paragraph P3 in the mid-latitudes discussion below.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1929Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM LESLIE...
Leslie remains quasi-stationary over the western tropical Atlantic...with the storm trapped between the Atlantic Canada low-level ridge to her north (paragraph P2)...Gulf of Mexico low-level ridge to her west (paragraph P1)...and Atlantic low-level ridge to her east (paragraph P4). In its trapped state...the tropical storm has advanced northward in the last 24 hours. Based on her northward progression in last 24 hours and the blocking low-level ridge pattern persisting thru 48 hrs (thru 11 PM Thu) in this morning's 00Z GFS...I agree with the NHC track thru that time.

Longer-term...models show the next frontal system in the mid-latitude westerlies (paragraph P1) knocking out the blocking Atlantic Canada low-level ridge to the north (paragraph P2)....which will cause Leslie to generally accelerate faster to the north. I also agree with the longer-term forecast shown by NHC this morning based on how the 00Z GFS animates the low-level winds. This means I am showing a westward shift from my previous philosophy...as I return a wiggle to the left. My previous forecast had cancelled the wiggle to the left as I thought Leslie would be too far south to feel the initial break in the blocking low-level ridging from the incoming frontal system. With Leslie advancing northward in the last 24 hours (and the expectation that she continues to do so)...I now think she has re-earned her potential to feel this initial break. Details that were not previously their about the next frontal system are emerging in the models that also support this westward wiggle. For instance...00Z GFS shows a small frontal cyclone and shortwave upper trough ejecting eastward well in advance of the main bulk of this frontal system. Upper convergence on the west side of this passing shortwave begins building a short-lived surface ridge to the north of Leslie beginning in 72 hrs (11 PM Fri)...the surface ridge helping her westward wiggle. The forecast track in Figure 1 is more threatening to Bermuda than ever...with tropical storm conditions expected to reach the island by late Saturday...and severe hurricane force conditions reaching the island by Sunday morning...if the current forecast verifies.

Beyond 120 hrs (not shown in Figure 1)...Leslie looks more threatening to Atlantic Canada (Nova Scotia and Newfoundland) than previously thought with the westward wiggle in track expected. Keep in mind she will be accelerating very rapidly northeastward ahead of the frontal system...and could be closing in on the Atlantic Canada area as early as 144 hrs.


Figure 1: My forecast for Tropical Storm Leslie this morning

Leslie has not strengthened in the last 24 hrs. It looks as though her circulation has advected in cooler upper air at the base of the paragraph P2 upper trough into her west half...resulting in an upper vortex above her west half. This convergent upper vortex is a source of sinking dry air. This is why the west half of Leslie has no t-storms this morning. Her quasi-stationary motion so far has not upwelled cooler waters...with healthy 30 deg C waters still in her area when looking at the above thermo birdseye chart. I expect her t-storm latent heat release to punch out the upper vortex...and we expect her to accelerate northward away from any cooler waters she may soon upwell. Therefore...gradual strengthening still appears likely. With the 00Z GFS developing grand anticyclonic outflow over Leslie...coupled outflow enhancement into the south Florida upper vortex to her west (paragraph P5) and outflow enhancement into an upper vortex dropped off by paragraph P2 upper trough to her east...I see a recipe for a major hurricane (115 mph+ max winds) by day 4...a little more bullish than the 11 PM EDT NHC forecast. Note that I adopted my strengthening rate from my previous forecast. By days 4 and 5...00Z GFS shows a strong southwesterly upper jet developing in advance of the frontal system's upper trough at a location just north of Bermuda. Therefore I show Leslie weakening to a category 2 hurricane (from southwesterly shear) as she approaches and passes Bermuda's latitude on day 5. Increasingly cooler waters along the storm track by day 5 also support weakening Leslie by that time.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I ever-so-slightly inflate in size based on the anticipated strengthening. I lean the impact swath towards the right of the storm track by day 5 to represent possible southwesterly shear Leslie is expected to encounter by that time.

...SPECIAL FEATURE...TROPICAL STORM MICHAEL...
Tropical depression thirteen has been upgraded to Tropical Storm Michael in last 24 hrs. Despite being below a cold core upper vortex...it has continued to develop...so I imagine this is a system whose entire vertical warm core circulation (surface spin and upper outflow) is tucked below the 200 mb upper wind layer. Most recent analogues of this are tropical cyclone Chris in June this year...and Gordon in the latter part of his life in August this year. My forecast versus the NHC's is shown in Figure 2 below.


Figure 2: My forecast for Tropical Storm Michael this morning

Track-wise...it is apparent that 24 hrs ago I had mis-judged TD 13 (now Michael's) track...probably due to its poor definition of the center on nighttime infrared. I had suggested it was bending more west in track when in fact it has been moving NW since birth. It did hook northward as expected...as it got pulled up by cold front of NW Atlantic frontal cyclone mentioned in paragraph P2. However...Michael has stalled earlier than expected...which underscores how narrow the front's ridge weakness is. Even now...infrared satellite animation suggests Michael is pinwheeling stationary. Therefore...I assess Michael is already wedged between paragraph P4 low-level ridge to the east and the Atlantic Canada low-level ridge (paragraph P2) coming in from the NW. 00Z GFS seems still melds together the two low-level ridges to the north of Michael...which should cause Michael to turn more westward. I prefer to have a southwest biased track as shown in Figure 2...as I believe Micheal's earlier than expected stall suggests less influence from the paragraph P2 frontal cyclone passing to the north. I believe this makes him more susceptible to a westward acceleration toward Leslie's low-level circulation later in the forecast period...which is why I move Michael faster than the NHC does by day 5. Beyond day 5 (not shown in Figure 2)...Michael should turn more northward and accelerate in southerly flow to the east of Leslie's circulation and to the west of the low-level ridging. It is possible for the small circulation of Michael to lose his identity in the east side of Leslie's large circulation at some point past day 5.

I agree with the NHC in strengthening Michael to 60 mph winds in 24 hrs. I argue that this is a shallower tropical cyclone than normal tucked below the 200 mb upper wind layer...so this system could end up being less sensitive to vertical shear than normal (the analogues of Chris and Gordon actually strengthened under what should have been a shearing environment). By 24 hrs...00Z GFS shoves off the upper vortex currently over Michael...and replaces it with a 200 mb divergent westerly jet between the paragraph P2 upper trough and paragraph P5 C Atlc upper ridge...so I surmise Michael strengthening while taking advantage of the divergent upper westerly jet as a shallow tropical cyclone (rather than getting ripped apart by the jet as a tall tropical cyclone would). After 24 hrs...the paragraph P2 upper trough drops off a new cut-off upper vortex over Michael. Like the current upper vortex...I think this new one won't be cold enough to de-stabilize things to Michael's advantage. So like the current upper vortex...I view this one as a source of upper-convergence (i.e. a cap) that prevents Micheal from strengthening further. This new upper vortex is pushed south of Michael (thanks to Leslie's outflow) by 72 to 96 hrs (11 PM Fri to 11 PM Sat)...so that is when I strengthen Michael into an 80 mph max wind category 1 hurricane...a little more aggressive than the NHC's 11 PM EDT suggestion given that Michael is a small system (and small systems strengthen more quickly than large ones). I think some northerly shear from Leslie's outflow will overtake Michael by 120 hrs...so I weaken him a bit to 75 mph by that time. I don't think the northerly shear will destroy Michael...again because he maybe a shallower-type tropical cyclone less sensitive to vertical shear. Therefore...I think what will ultimately destroy Michael beyond day 5 is absoprtion into Leslie's east side...or acceleration northward over cooler waters.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 11 PM EDT...which I maintain on the presumption Michael remains a compact tropical cyclone. I have a southward bias in the swath (with respect to storm track) by day 5 to reflect possible northerly shear by that time.

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL DISTURBANCE...
Tropical wave SW of the Cape Verde Islands in previous discussion is now midway between the Cape Verdes and Lesser Antilles. On its westward track...it is exiting the favorable outflow beneath the upper ridge portion toward Africa (mentioned in paragraph P5). However...plenty of t-storms persist east of this tropical wave and beneath this upper outflow. Wihtin these t-storms...a surface 1011 mb low was added by NHC TAFB SW of the Cape Verde Islands this evening. It has been a while since a tropical wave emerged from Africa...so this new surface low could be the next tropical wave that has emerged from Africa...a feature left behind by the above-mentioned tropical wave...or a surface low that has spun up along the ITCZ due to pressure falls generated by the upper outflow. Because of the favorable upper outflow environment and presence of this new surface low...I still am considering this a special feature on this blog.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough in mid-latitude westerlies is entering the upper-left corner of above charts...with its surface frontal zone across south-central Canada and north-central US. Warm air advection ahead of this front now supports all of the southern US upper ridge. Upper convergence on the SE half of this upper ridge supports a 1018 mb Gulf of Mexico surface ridge.

P2...Cut-off upper vortex in the NW Atlantic persists...whose eastern divergence continues to support a surface frontal cyclone now located SE of Newfoundland and still evaluated at less-than-1008 mb. To the north of that frontal cyclone...a shortwave upper trough and associated second surface frontal cyclone has moved across north Canada and entered the top-center of the above atmo chart...hence moving into southern Greenland. Upper convergence behind the shortwave upper trough and NW Atlantic upper vortex supports a 1020 mb to 1021 mb low-level ridge just south of Atlantic Canada.

P3...Remnant surface low of Isaac is centered over the SE Michigan/NW Ohio border as of 1929Z HPC analysis. Due to blocking effect of Atlantic Canada low-level ridge (paragraph P2)...its slow eastward speed has caused it decouple from the nearby shortwave upper trough...which is now a cut-off upper vortex over the western Carolinas. Meanwhile...split flow divergence between the west side of this upper vortex and the southern US upper ridge (paragraph P1) has caused a new 1011 mb low and associated t-storms to form over the Mississippi-Alabama area. This surface low could drift southward into the northern Gulf of Mexico...where tropical cyclone development is possible. Waiting to see how much exposure this surface low actually gets before considering it a special feature on this blog.

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to NW Atlantic surface cyclone mentioned in paragraph P2. Easterly flow on the south side of this surface ridge (in conjunction with easterly flow on south side of paragraph P5 upper ridge) is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics. This surface ridge is supported by convergence ahead of the central Atlantic-to-Caribbean upper anticyclonic cell mentioned in paragraph P5.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...now supported by warm air advection ahead of cold front in paragraph P1. Embedded upper vortex in relatively lower pressures south of this upper ridge persists...located near south Florida. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched into the NE Atlantic by low-level warm air advection ahead of the paragraph P2 mid-latitude system. The NE lobe has aligned with 1024 mb surface center of paragraph P4 ridge to make a deep-layered ridge center north of the Azores. Large and elongated upper vortex above Tropical Storm Michael persists in relatively lower pressures east of this central Atlantic upper anticyclone...while the remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of that upper vortex.

2012 Atlantic Hurricane Season Birdseye Discussion #96

By: NCHurricane2009, 7:33 AM GMT on September 04, 2012

...SEPTEMBER 4 2012...3:32 AM EDT...
See paragraph P3 of mid-latitudes discussion for updated assessment of Isaac remnant on this blog.

Tropical Storm Leslie lurking north of the Lesser Antilles and south of Bermuda...forecast to move slowly over western Atlantic waters for next few days. As she does so...she could creep toward Bermuda and eventually Atlantic Canada...so interests in these areas should monitor the progress of Leslie over the next days. In addition...Leslie could bring surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...the east US shore...and eventually Atlantic Canada. See Leslie special feature section for further details.

Subtropical disturbance Invest 99-L becomes tropical depression thirteen at a location well east-northeast of Tropical Storm Leslie. See TD thirteen special feature section for details.

A tropical wave southeast of tropical depression thirteen is becoming better organized in favorable upper winds...and therefore has a special feature section. See third special feature section below for details on this tropical wave.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1933Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM LESLIE...
As expected...Leslie has become stalled with the storm trapped between the Atlantic Canada low-level ridge to her north (paragraph P2)...Gulf of Mexico low-level ridge to her west (paragraph P1)...and Atlantic low-level ridge to her east (paragraph P4). Prior to becoming stalled...the surface center underwent a large westward wobble in the last 24 hrs that has caused her to be more south and west of where all forecasts predicted her to be at this point. Leslie is a tropical cyclone under northerly shear with a tremendous t-storm mass biased to the southeast of the surface center. The t-storm mass is where the greatest amount of latent-heat-release-driven upper outflow is occurring...so there maybe a mid-level center more embedded in the t-storm mass supported by the pressure falls of the upper outflow. Such a mid-level center would be southeast of the current surface center position...so maybe the mid-level center has dragged the surface center westward...perhaps explaining the unexpected westward wobble. Notice my new forecast track in Figure 1 from the get-go has a rightward bias to NHC's...as I think the surface center could easily regenerate southeastward or eastward into the pressure falls generated by the upper outflow of the t-storm mass...particularly as Leslie is currently quasi-stationary with no other net forces to push her one way or the other at this time.

Longer-term...models show the next frontal system in the mid-latitude westerlies (paragraph P1) knocking out the blocking Atlantic Canada low-level ridge to the north (paragraph P2)....so my forecast in Figure 1 shows a northward acceleration ahead of this frontal system based on how the morning's 00Z GFS model animates the low-level flow. I show no eastward bend thru 11 PM Sat (120 hrs) as their should be enough paragraph P4 surface ridging to her east. My earlier forecasts showed a wiggle to the left as Leslie became attracted toward the initial break in the blocking low-level ridging from the incoming frontal system...but now I think this will not occur since Leslie is further south than thought earlier...so its possible for her to not feel this initial break. Without a leftward wiggle...this means my new forecast in the longer-term is to the right of my previous...passing Leslie (and most of her impacts) east of Bermuda. However...Bermuda should not take their eye off of Leslie because of my opinion. My track forecast is also slower to the north than the 00Z GFS and the NHC's (unlike my previous)...to account for the fact that Leslie could regenerate toward her sheared-off t-storm mass early in the forecast period...and because I see no incentive to move Leslie at all thru 11 PM Wed (thru 48 hrs) based on the way 00Z GFS shows the blocking low-level ridge pattern persisting.

Beyond 120 hrs (not shown in Figure 1)...Leslie should eventual curve eastward depending on how much southwesterly flow she experiences ahead of the incoming frontal system. Because of her more southward-than-expected initial position...she is more likely to link with southwesterly flow on the SE side of the system (rather than southerly flow E of the system)...so at this time I would lean toward a more rightward track toward Newfoundland or offshore of Atlantic Canada...as opposed to a more Nova Scotia solution.


Figure 1: My forecast for Tropical Storm Leslie this morning

Despite being under northerly shear from flow on the back side of the paragraph P2 upper trough...Leslie has gradually strengthened from 60 to 65 mph max winds in last 24 hrs. I think she is experiencing less northerly shear than expected because of her more southward-than-expected position which keeps her distant from the paragraph P2 upper trough. Because she is already strengthening under the northerly shear...and because the northerly shear is going to relax after 11 PM Tue (after 48 hrs) according to the 00Z GFS model 200 mb upper winds...the NHC strengthens Leslie thru the forecast period...and I also agree with that philosophy. Despite the shear relaxing after 11 PM Tue...I still show a slow strengthening rate thru 11 PM Wed as I think a quasi-stationary Leslie would have upwelled cooler waters beneath the sea surface with her winds. I accelerate the strengthening rate as she then moves northward into "untouched" 29 to 30 deg C waters (more 29 deg C toward Bermuda). With the 00Z GFS suggesting grand anticyclonic outflow...coupled outflow enhancement into the south Florida upper vortex to her west (paragraph P5) and outflow enhancement into an upper vortex dropped off by paragraph P2 upper trough to her east...I see a recipe for a major hurricane (115 mph+ max winds) by day 5. The 11 PM EDT NHC forecast shows 100 mph max winds by day 5. I do not strengthen Leslie past 115 mph winds out of conservativeness...as for some reason it seems no Atlantic tropical cyclone this season has wanted to take the title of a major hurricane even when conditions appear ripe to do so.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I slightly inflate in size based on the anticipated strengthening.

...SPECIAL FEATURE...TROPICAL DEPRESSION THIRTEEN...
Subtropical disturbance Invest 99-L has been upgraded to tropical depression thirteen in last 24 hrs. Despite forming below a cold core upper vortex...it is considered fully tropical...so I imagine this is a system whose entire vertical warm core circulation (surface spin and upper outflow) is tucked below the 200 mb upper wind layer. Most recent analogues of this are tropical cyclone Chris in June this year...and Gordon in the latter part of his life in August this year. My forecast versus the NHC's is shown in Figure 2 below.


Figure 2: My forecast for Tropical Depression Thirteen this morning

Track-wise...I am not sure why TD 13 began on a NW heading...but the recent satellite animation and very recent segment of NHC recorded storm track suggest a more westward heading. An initial west track makes sense to me...with steering from the deep-layered ridge north of the Azores mentioned in paragraph P5. Albeit this westward track is slowed by the 1021 mb center seen west of TD 13 in the above atmo chart (1021 mb center part of paragraph P4 surface ridge). My observation of an initial westward track makes me to the south of the NHC forecast from the get-go. I agree with the NHC on a northward hook beginning in 24 hrs...as the system gets pulled up by cold front of NW Atlantic frontal cyclone mentioned in paragraph P2. But the ridge weakness of the front is quiet narrow as to leave behind this system by day 5 (11 PM Sat)...so I show TD 13 becoming stalled while it gets wedged between paragraph P4 low-level ridge to the east and what is now the Atlantic Canada low-level ridge (paragraph P2) coming in from the NW. After day 5...the 00Z GFS seems to dissipate TD 13 as it melds together the two low-level ridges.

To me...the intensity forecast is uncertain. This is a fragile and small tropical cyclone at the moment with very little t-storm activity...with paragraph P4 dry air lurking to the south. I think the t-storms are limited by the dry air...coupled with the temps of the cold core upper vortex not being cold enough. Therefore this upper vortex is then acting as a cap whose upper convergence is countering the upper outflow of TD 13...so I choose to keep TD 13 steady-state thru next 24 hrs while the NHC makes it a minimal tropical storm. By 48 hrs..the NHC argues that a blast of NW shear from the paragraph P2 upper trough will destroy this fragile tropical cyclone into a remnant low.

On the other hand...48 hrs is when I prefer to make TD 13 into a tropical storm...but why? I argue that this is a shallower tropical cyclone than normal tucked below the 200 mb upper wind layer...so this system could end up being less sensitive to vertical shear than the NHC is thinking (the analogues of Chris and Gordon actually strengthened under what should have been a shearing environment). Moreover...48 hrs is when 00Z GFS shoves off the capping upper vortex currently over TD 13...and replaces it with a 200 mb divergent westerly jet between the paragraph P2 upper trough and paragraph P5 C Atlc upper ridge...so I surmise TD 13 strengthening while taking advantage of the divergent upper westerly jet as a shallow tropical cyclone (rather than getting ripped apart by the jet as a tall tropical cyclone would). After 48 hrs...the paragraph P2 upper trough drops off a new cut-off upper vortex over TD 13. Like the current upper vortex...I think this new one won't be cold enough to de-stabilize things to TD 13's advantage. So like the current upper vortex...I view this one as a cap that prevents TD 13 from strengthening further. This new upper vortex is pushed south of TD 13 (thanks to Leslie's outflow) by 120 hrs...but with TD 13 being highly fragile right now...I don't want to strengthen TD 13 by that time as I could be wrong (i.e. TD 13 dissipates like the NHC says).

...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE...
Tropical wave SW of the Cape Verde Islands has finally produced t-storms and become better organized under favorable outflow beneath the upper ridge portion toward Africa (mentioned in paragraph P5). As such...due to the recent string of tropical waves that have developed into tropical cyclones...I have now considered this a special feature on this blog. Have yet to do my own analysis of where this system would head toward if it became a tropical cyclone...due to the activity with Tropical Depression Thirteen and Tropical storm Leslie.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough in mid-latitude westerlies is entering the upper-left corner of above charts...with its surface frontal zone across south-central Canada and north-central US. Warm air advection ahead of this front now supports all of the southern US upper ridge once supported by the ex-Isaac's (paragraph P3) warm air advection. Upper convergence on the SE half of this upper ridge supports Gulf of Mexico surface ridge with 1019 to 1020 mb centers.

P2...Main portion of upper trough over NW Atlantic and the Atlantic highs seas has ejected northeastward out of the picture to the east of Greenland. Correspondingly...its main 993 mb frontal cyclone from the previous discussion has also exited the picture. The upper trough leaves behind a cut-off upper vortex in the NW Atlantic whose eastern divergence has intensified a new 1015 mb frontal cyclone NE of Bermuda into less-than-1008 mb in last 24 hrs. To the north of that frontal cyclone...a shortwave upper trough and associated surface frontal cyclone moving across north Canada is entering the top-center of the above atmo chart. Upper convergence behind the north Canada upper trough and NW Atlantic upper vortex supports a 1023 mb low-level ridge over Atlantic Canada.

P3...Remnant surface low of Isaac is centered over southern Indiana as of this evening and early morning. Due to blocking effect of Atlantic Canada low-level ridge (paragraph P2)...its slow eastward speed has caused it to fall behind the nearby Shortwave upper trough...but perhaps eastern divergence of the upper shortwave will continue to cause surface pressure falls to continue attracting the remnant low eastward. Some scattered severe weather across the SE US again popped up today. I suppose the severe threat emerged from instability of daytime heating coupled with directional vertical wind shear in the east half of Isaac (low-level southerlies in Isaac's east half coupled with upper westerlies ahead of upper shortwave). Therefore...watch out for possible severe threat in Isaac's east half thru the next few days. Low-level warm air advection ahead of Isaac's circulation no longer supports the southern US low-level ridge...but instead is now supported by warm air advection ahead of the frontal system NW of Isaac (paragraph P1).

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to NW Atlantic surface cyclone mentioned in paragraph P2. Easterly flow on the south side of this surface ridge (in conjunction with easterly flow on south side of paragraph P5 upper ridge) is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics. This surface ridge is supported by convergence ahead of the central Atlantic-to-Caribbean upper anticyclonic cell mentioned in paragraph P5.

...TROPICAL BELT DISCUSSION...
P5...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...now supported by warm air advection ahead of cold front in paragraph P1. Embedded upper vortex diving SW into the Bahamas in previous discussion is now moving westward toward south Florida while steered about southern US upper ridge. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched into the NE Atlantic by low-level warm air advection ahead of the paragraph P2 mid-latitude system. The NE lobe has aligned with 1028 mb surface center of paragraph P4 ridge to make a deep-layered ridge center north of the Azores. Large and elongated upper vortex above tropical depression thirteen persists in relatively lower pressures east of this central Atlantic upper anitcyclone...while the remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of that upper vortex.

P6...Tropical wave SW of the Cape Verde Islands in the previous discussion has been moved to above 3rd special feature section as it becomes better organized. See 3rd special feature section for details on this tropical wave.

2012 Atlantic Hurricane Season Birdseye Discussion #95A (Special Update)

By: NCHurricane2009, 10:22 PM GMT on September 03, 2012

...SEPTEMBER 3 2012...6:15 PM EDT...
This special update serves to address changes in Tropical Storm Leslie and subtropical disturbance Invest 99-L that have occurred since the release of full discussion #95.

Tropical Storm Leslie has taken longer to advance to the north...and instead has taken quiet a westward wobble over the last several hours. Will re-assess in my next full blog update sometime tonight or early tomorrow morning how this westward wobble changes the threat to Bermuda...the US east coast...and Atlantic Canada in the long range. Will also try to assess why the unexpected westward wobble occurred.

This afternoon...subtropical disturbance Invest 99-L has become tropical depression thirteen well to the east-northeast of Tropical Storm Leslie. I had previously assessed that the t-storm activity was too little for tropical cyclogenesis...but it now appears 99-L is a very small circulation that does not require a signficant areal extent of t-storms. In fact...99-L has been firing a small but strong enough area of t-storms in the last 12 hours to be considered a small tropical cyclone. I will be doing a forecast on the new tropical cyclone during my next full discussion sometime tonight or early tomorrow morning.

Return to full discussion #95 for my most recent assessment on the rest of the Atlantic tropics.

2012 Atlantic Hurricane Season Birdseye Discussion #95

By: NCHurricane2009, 6:58 AM GMT on September 03, 2012

...SEPTEMBER 3 2012...2:59 AM EDT...
See paragraph P3 of mid-latitudes discussion for updated assessment of Isaac remnant on this blog.

Kirk quickly becomes non-tropical along a frontal boundary in the north Atlantic...soon to be an indistinct feature. See paragraph P2 in mid-latitudes discussion for details.

Tropical Storm Leslie passing NE of the Lesser Antilles and forecast to stall over open western Atlantic waters for next few days. As she does so...she could creep toward Bermuda...so interests in Bermuda should monitor the progress of Leslie over the next days. In addition...Leslie could bring surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...and the east US shore. See Leslie special feature section for further details.

Relatively new subtropical disturbance Invest 99-L persists northeast of Tropical Storm Leslie...but I do not expect subtropical or tropical cyclone formation from this system. Origin of 99-L is discussed in special update #94A. See paragraph P5 in mid-latitudes discussion for details on 99-L.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1933Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM LESLIE...
As expected previously...Leslie has turned sharply northward toward low-level ridge ridge weakness associated with developing NW Atlantic frontal cyclone (frontal cyclone is marked with a 1015 mb center in above atmo chart and as mentioned in paragraph P2). Better yet...the northward turn occurred along the longitude I expected previously...which caused me to previously be a bit to the right of the NHC forecast. Albeit the northward turn began earlier than I forecasted...so my new Leslie forecast in Figure 1 below has its points further north than my previous. For some reason though...the recent NHC recorded storm track shows a wobble to the west...which will cause Leslie to depart from the longitude I previously showed her continuing northward along. Therefore my new forecast points in Figure 1 are also more left than my previous...although I stll develop a bit of a rightward bias (with respect to NHC) because I think Leslie will be attracted to the NW Atlantic frontal cyclone as it zips away to the northeast. From 11 AM Mon to 11 PM Tue...it seems imminent Leslie will make hardly any northward progress...with the storm trapped between the Atlantic Canada low-level ridge to her north (paragraph P2)...Gulf of Mexico low-level ridge to her west (paragraph P3)...and Atlantic low-level ridge to her east (paragraph P6).

Longer term...models show the remnants of Isaac (paragraph P3) and the next frontal system in the mid-latitude westerlies (paragraph P1) knocking out the paragraph P2 low-level ridge to the north. 36 hrs ago...12Z GFS showed Leslie remaining stalled and not getting swept up ahead of this frontal system...but at the time I was skeptical. Now...this morning's 00Z GFS...as well as the CMC...GFS...Euro (ECMWF)...and NOGAPS all agree Leslie will get swept up ahead of this frontal system. The less reliable NOGAPS is an eastern outlier showing all of Leslie passing east of Bermuda...and the less reliable CMC is the western outlier as it shows a low-level ridge from the Great Lakes (behind the frontal system) bending Leslie NW toward the mid-Atlantic area (Virginia/Delmarva Peninsula area). I prefer to agree with the more reliable GFS and Euro...and I show my own version of Leslie curving ahead of the frontal system based on how the 00Z GFS evolves the low-level winds thru 120 hrs. I am more north than 00Z GFS by 11 PM Fri (120 hrs)...perhaps I am underplaying the resistance from the Gulf of Mexico low-level ridge to the west (paragraph P3).


Figure 1: My forecast for Tropical Storm Leslie this morning

My previous intensity forecast 36 hrs ago showed Leslie weakening from 70 to 65 mph...then recovering back to 70 mph for reasons I cited in that discussion. Leslie first followed that forecast perfectly...but then began weakening to 60 mph...hence falling behind the previous intensity forecast. The weakening is due to northerly shear from flow on the back side of the paragraph P2 upper trough passing to the north. Cool air advection on the back side of 1015 mb frontal cyclone (also mentioned in paragraph P2) should amplify the upper trough...which will increase the northerly shear across Leslie thru 11 PM Tue (thru 48 hrs) based on the 00Z GFS 200 mb (upper wind) forecast. So despite the mighty impressive t-storm cluster currently in the south half of Leslie...I choose to weaken Leslie thru that time (unlike the 11 PM EDT NHC forecast which keeps Leslie at 60 mph). After that time...the upper winds become quiet favorable over Leslie...and coupled with sea-surface temps of 29 to 30 deg C (more 29 deg C toward Bermuda)...I strengthen Leslie to 90 mph by 11 PM Fri (a bit more aggressive than the 11 PM EDT NHC forecast).

Impact swath in Figure 1 is based on 11 PM EDT NHC tropical storm wind radius...which I change its size based on my forecasted weakening then subsequent strengthening.

...MID-LATITUDES DISCUSSION...
P1...Next upper trough in mid-latitude westerlies is entering the upper-left corner of above charts from the NW US and western Canada with the surface cold front it supports. Warm air advection ahead of this front is helping to pump up a portion of the southern US upper ridge at a location behind the remnants of Isaac.

P2...Upper trough over eastern Canada in the previous discussion is now pushing into the NW Atlantic and the Atlantic highs seas south of Greenland ...intensifying its main 996 mb frontal cyclone into 993 mb as the frontal cyclone rapidly ejected NE from the coast of Canada and past the south tip of Greenland all in the last 36 hrs. It also supports a new 1015 mb frontal cyclone NE of Bermuda. Cool air advection behind both frontal cyclones have amplified the upper trough. Cold front of the 993 mb cyclone has absorbed the circulation of Tropical Storm Kirk...making him extratropical (non-tropical) as he accelerates rapidly NE into the far north Atlantic...albeit he became extratropical 12 hours later than I thought he would in my previous forecasts. The final NHC advisory on Kirk suggests he will become indistinct along frontal zone in next 24 hrs...and he is quickly exiting the picture of the above birdseye charts...so this will be my last statement on Kirk in this blog. Other news with this upper trough...a portion of it has become a cut-off upper vortex heading southwestward toward the Bahamas. Since this upper vortex is embedded in tropical Atlantic upper ridging...it will be moved to paragraph P7 in the tropical belt discussion. Finally...upper convergent west side of this upper trough is currently supporting a 1021 to 1023 mb low-level ridge over eastern Canada that will soon be marching into the Atlantic Canada area.

P3...Remnant surface low of Isaac is centered over Illinois as of this evening and early morning. Shortwave upper trough over and just west of Isaac continues to support Isaac itself with its eastern divergence. Because the shortwave upper trough is coupled with the mid-latitude upper westerlies...the shortwave upper trough will move east...and so will Isaac now that he is supported by this shortwave. This means he will move into the Ohio Valley...although his eastward progress is slowed by the E Canada low-level ridge mentioned in paragraph P2. Some scattered severe t-storms and tornado warning across the SE US popped up this evening. I suppose the severe threat emerged from instability of daytime heating coupled with directional vertical wind shear in the east half of Isaac (low-level southerlies in Isaac's east half coupled with upper westerlies ahead of upper shortwave). Therefore...watch out for possible severe threat in Isaac's east half thru the next few days. Low-level warm air advection ahead of Isaac's circulation is supporting the southern US upper ridge. Upper convergence on the SE half of this upper ridge supports Gulf of Mexico surface ridge with 1018 to 1020 mb centers.

P4...Upper trough SE of Greenland in the previous discussion has quickly exited the picture from the upper right corner of the above charts in the last 36 hrs.

P5...Weather is quiet active to the northeast of Leslie. Large cut-off upper vortex near the Azores (paragraph P4 of discussion #94) has been diving quickly southward into the open eastern Atlantic...steered by the central Atlantic upper anticyclonic cell mentioned in paragraph P7. What I assessed to be a dissipating surface trough below this upper vortex has persisted as a 1015 mb low with a few t-storms supported by instability of the cold air of the upper vortex...although this t-storm activity is not immense enough to support subtropical cyclone formation even though this was upgraded to Invest 99-L in last 24 hrs. To the west of 99-L...a large cluster of t-storms is supported by outflow of paragraph P7 central Atlantic upper anticyclonic cell...but this t-storm activity is occurring directly above 1021 mb center of paragraph P6 surface ridge. Therefore...surface pressures are too high for this t-storm cluster to evolve into a tropical disturbance.

P6...Atlantic surface ridge has been eroded out of the western Atlantic thanks to cold front pushing in from the NW...associated with paragraph P2 and P4 systems. Easterly flow on the south side of this surface ridge (in conjunction with easterly flow on south side of paragraph P7 upper ridge) is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics. This surface ridge is supported by convergence ahead of the central Atlantic upper anticyclonic cell mentioned in paragraph P7.

...TROPICAL BELT DISCUSSION...
P7...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...now supported by warm air advection ahead of Isaac's remnant and ahead of cold front as mentioned in paragraphs P1 and P3. A portion of paragraph P2 upper trough has cut-off into an embedded upper vortex diving SW into the Bahamas while steered about southern US upper ridge. Anticyclonic upper ridge center in the central Atlantic (pumped up by the outflow of Leslie and once pumped up by Kirk) has been stretched into the NE Atlantic by low-level warm air advection ahead of paragraph P2 cold front. The NE lobe has aligned with 1027 mb surface center of paragraph P6 ridge to make a deep-layered ridge center north of the Azores. Large upper vortex of Invest 99-L (paragraph P5) is in relatively lower pressures east of this central Atlantic upper anitcyclone...while the remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of that upper vortex.

P8...Tropical wave SSW of the Cape Verde Islands in the previous discussion is now SW of the islands. Although it is under favorable outflow beneath the upper ridge portion toward Africa (mentioned in paragraph P7)...it is showing no signs of significant t-storm activity...perhaps due to dry air mentioned in paragraph P6. This tropical wave's westward progress has been slow...I think due to the unusual southward extent of the paragraph P6 Atlantic surface ridge to its NW. As stated in paragraph P6...this surface ridge is supported by convergence ahead of the central Atlantic upper anticyclonic cell mentioned in paragraph P7...a portion of which is enhanced by Leslie's upper outflow. Therefore I think it is Leslie's upper outflow that is contributing to the unusually far south extent of the paragraph P6 Atlantic surface ridge.

2012 Atlantic Hurricane Season Birdseye Discussion #94A (Special Update)

By: NCHurricane2009, 7:23 AM GMT on September 02, 2012

...SEPTEMBER 2 2012...3:30 AM EDT...
Subtropical disturbance Invest 99-L forms well to the northeast of Tropical Storm Leslie in the open eastern Atlantic. While this may seem a feature that has popped up from nowhere...here are a few prior statements on this blog tracking the disturbance's origin:

Paragraph P4...mid-latitudes discussion...discussion #94...statement on cut-off upper vortex near the Azores and surface trough diving southward

Paragraph P4...mid-latitudes discussion...discussion #93...statement on cut-off upper vortex near the Azores and surface 1021 mb frontal low.

Paragraph P3...mid-latitudes discussion...discussion #92...statement on cut-off upper vortex forming near the Azores from Atlantic high seas upper trough...and statement on surface 1017 mb frontal low moving into the Azores.

Paragraph P2...mid-latitudes
discussion...discussion #91...statement on 1010 mb surface frontal low heading toward Azores suppored by Atlantic high seas upper trough.


Paragraph P2...mid-latitudes discussion...discussion #90...statement on 1018 mb surface frontal low in north-central Atlantic supported by NW Atlantic shortwave upper trough merging with parent high seas upper trough.

Paragraph P2...mid-latitudes discussion...discussion #89...statement on 1020 mb surface frontal low north of Bermuda supported by NW Atlantic shorwave upper trough

Paragraph P2...mid-latitudes discussion...discussion #88...statement on new 1019 mb surface frontal low north of Bermuda supported by NW Atlantic shortwave upper trough originating from parent upper trough moving into Atlantic high seas.

My most recent prognosis on this disturbance in paragraph P4 of discussion #94 was that the surface trough was dissipating...and the upper vortex was diving southward about the upper ridge cell inflated by Kirk and Leslie's outflow. Moreover...the upper vortex was merging with an inverted upper trough near the Cape Verde Islands mentioned in paragraph P6 of that discussion. As such...I had seen this disturbance as something insignificant...and planned in future discussions to move it into the tropical belt section as merely an upper vortex embedded in the upper ridge described in paragraph P6.

Now with this disturbance being upgraded to Invest 99-L and given a 20% chance of subtropical or tropical cyclone formation on the 2 AM EDT NHC outlook...I am re-assessing the situation. For my next full discussion...I am deciding whether to keep this upper vortex and surface trough in its own mid-latitudes paragraph...or better yet provide its own special feature section. Satellite imagery suggests the t-storms of the surface trough are aided by the intrinsic cold temps of the upper vortex. As the surface trough and upper vortex continue southward in tandem...the system is moving over increasingly warm waters that creates an increased surface to upper air temp contrast with respect to the upper vortex's cold air...hence increasing the system's instability necessary for subtropical or tropical cyclone formation. My action on this system in my next full discussion will be dependent on if the t-storm activity continues to improve on satellite displays.

Return to full discussion #94 for my latest assessment on the rest of the Atlantic tropics.

2012 Atlantic Hurricane Season Birdseye Discussion #94

By: NCHurricane2009, 7:20 PM GMT on September 01, 2012

...SEPTEMBER 1 2012...3:25 PM EDT...
See paragraph P2 of mid-latitudes discussion for updated assessment of Isaac remnant on this blog.

Kirk weakens to from a category 2 hurricane to tropical storm in last 36 hours...and remains a marine threat as he curves northeastward into the open north Atlantic. See Kirk special feature section for details.

Tropical Storm Leslie is taking longer to become a hurricane than previously thought. Although currently expected to pass NE of the Lesser Antilles and stay over open water for the next 5 days...she could spread surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...and the east US shore. See Leslie special feature section for further details.

Leslie makes this the second earliest formation of an Atlantic season's twelvth tropical storm...behind Luis of 1995. This now means Lee of 2005 is thrid place for this record. Despite the hyperactive pace reminiscent of the 1995 and 2005 seasons...none of this seasons's storm have yet to become a major hurricane (115 mph+ max winds)...especially after Kirk failed to do so yesterday.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0729Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...

This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...TROPICAL STORM KIRK...
Concerning Kirk...I previously had a slight leftward bias tracked in my previous discussion...for a few reasons posted in that discussion. He bent to the NE earlier than thought...which made my previous track forecast too far left initially. However...the angle at which he has taken his NE acceleration has allowed him to catch up to my previous track forecast...and moreover the NHC in last 24 hrs has adjusted their track forecast leftwards toward my previous one. The recent NHC recorded storm track in Figure 1 is technically angled ever-so-slightly leftwards of my previous forecast track and the NHC's current. Right now I choose to go with the current NHC (my previous) track in Figure 1 below...because this ever-so-slight angular difference is not big enough to make me confident in adjusting my track more left from my previous. As I showed previously...I still think Kirk will be transitioning to an extratropical (non-tropical) remnant low by 5 AM Sunday as he merges with cold front currently mentioned in paragraph P1....or alternatively the cold front just to his NW mentioned in paragraph P3. While my 5 AM Sunday position is on the same track I showed previously (what the NHC shows currently)...it is further NE to account for the NHC curently showing a faster track forecast.


Figure 1: My forecast for Tropical Storm Kirk this afternoon

Kirk's small pinhole eye when I did my previous forecast 36 hrs ago disappeared not long after (as if there was an eye replacement cycle)...and so he weakened earlier than I anticipated. So for my 5 AM Sunday forecast point...my intensity of 60 mph max winds is 10 mph less than what I showed previously..and this intensity point is in general agreement with the 11 AM NHC forecast (which predicts 65 mph for 11 PM tonight...then 60 mph for 11 AM Sunday).

Impact swath in Figure 1 is initialized based on a the very small tropical storm wind radius shown at 11 AM NHC advisory and the presumption that Kirk should be a compact system thru the forecast period. Although he should get southwesterly shear anytime soon...the NE track by that time and beyond still allows the impact swath to be symmetric about the storm track.

...SPECIAL FEATURE...TROPICAL STORM LESLIE...
Since birth...Leslie's westerly track is best described as a stair-step...tracking more westward initially...stair stepping a bit more north while feeling the low-level ridge weakness to the north (located between paragraph P5 Atlantic surface ridge to the NE and paragraph P2 east US surface ridge to the NW)...then returning to a more westerly track this morning. I speculated previously that as Kirk lifted northward...the low-level ridge weakness would fade...creating a more westerly track...and that is what I believe we saw this morning. Her stair-step in track made her follow the more north NHC forecast track at first...then this morning's more westerly heading is pointing her toward my previous forecast which predicted her to pass over 60W-20N. Therefore my new forecast track in Figure 2 below in the short-term is similar to my previous short-term...but adjusted to be a tad north of 60W-20N...because I am impressed with a new paragraph P1 frontal feature that the GFS model did not previously show...a feature which I think will curve her sharply northward as described in the next paragraph.

Longer term...towards 24 hrs and beyond...models show another low-level ridge weakness developing thanks to paragraph P1 front. What is interesting is today's 12Z GFS (unlike 00Z GFS from 36 hrs ago) shows an upper-level shortwave impulse developing in the back side of the paragraph P1 upper trough...whose eastern divergence causes a strong frontal cyclone to develop in the NW Atlantic...and in turn cool air advection from the new frontal cyclone amplifies the shortwave impulse into the dominant feature of the paragraph P1 upper trough. Therefore between 24 and 48 hrs...I sharply curve Leslie northward toward this frontal cyclone. From 48 to 96 hrs...I show Leslie making hardly any progress (slowing down and becoming stationary)...as the 12Z GFS shows Leslie trapped between the Atlantic Canada low-level ridge to her north (paragraph P1)...eastern US low-level ridge to her west (paragraph P2)...and Atlantic low-level ridge to her east (paragraph P5). Between 96 and 120 hrs...12Z GFS shows the next frontal system in the mid-latitude westerlies (which contains the remnants of Isaac...or at least its moisture) knocking out the paragraph P1 low-level ridge to the north...so I think Leslie would begin sliding NW toward the frontal system at 120 hrs. Beyond 120 hrs (not shown in Figure 2)...my intuition tells me Leslie would curve and accelerate northward then northeastward in southwesterly flow ahead of that frontal system. Interestingly though...the GFS prefers to keep Leslie stalled for much longer than 120 hrs out! I suppose the GFS sees the conflicting steering of the paragraph P2 east US low-level ridge to the west and and paragraph P5 Atlantic ridge to the east keeping Leslie trapped.


Figure 2: My forecast for Tropical Storm Leslie this afternoon

My new intensity forecast in Figure 2 is much more toned-down from my previous. In the short-term...Leslie has seen unexpected northwesterly shear...or at least resistance to its northwestern outflow. I attribute this to not previously considering a pesky upper trough just NW of Leslie (mentioned in paragraph P6)...and also a small upper ridge NW of Leslie located in relatively higher pressures between this upper trough and upper vortex headed toward Bahamas (upper vortex mentioned in paragraph P1). She looks a little disheveled from this northwesterly shear...so I weaken her a bit by 12 hours. However...latest satellite-derived 200 mb wind barbs show her upper outflow beating out the pesky upper trough...and merging with the adjacent upper ridge to the NW...so I re-strengthen her a bit by 24 hrs.

Longer term...I did previously mention some northwesterly shear developing due to northerly flow on back side of paragraph P1 upper trough passing by to the north. With GFS now showing the development of an impressive shortwave impulse (and associated surface frontal cyclone in NW Atlantic) in the back side of this upper trough...I think this northerly shear will be more impressive than previously thought...so like the NHC did at 11 AM...I am reluctant to show strengthening beyond 70 mph max winds for quiet some time. I take longer than the 11 AM NHC forecast to suggest hurricane strength (75 mph +)...barely showing her becoming a hurricane by 120 hrs. This is because I think the stalled motion between 48 and 120 hrs could allow her winds to upwell cooler waters below the ocean surface. Ultimately what pushes me to suggest hurricane strength at 120 hrs is that the northerly shear should abate by that time...based on today's 12Z GFS upper wind forecast.

Impact swath in Figure 2 is based on 5 AM NHC tropical storm wind radius...which I maintain the same size due to uncertainty in Leslie strengthening.

...MID-LATITUDES DISCUSSION...
P1...Shortwave upper trough with low amplitude...and surface frontal system with 996 mb surface low...continues to rapidly eject eastward into eastern Canada...and will soon be crossing the north Atlantic. As it does so...it is expected to transition Kirk to an extratropical system. A portion of this upper trough's vorticity (and upper vorticity from paragraph P3 upper trough) is in the process of evolving into a cut-off upper vortex heading southwestward toward the Bahamas. Its upper convergent west side is currently building a 1022 mb low-level ridge over central Canada that will soon be marching into the Atlantic Canada area. Low-level Warm air advection ahead of this system once supported the upper ridge over the southern US...but now this is supported by warm air advection ahead of Isaac's remnant (see paragraph P2).

P2...Remnant surface low of Isaac is centered over Missouri as of this morning. See paragraph P2 of discussion #93 for how an upper vortex above Isaac originated. This upper vortex is now just west of Isaac as a shortwave upper trough whose eastern divergence supports Isaac itself...meaning Isaac is more structured as an extratropical (non-tropical) low. Because the shortwave upper trough is coupled with the mid-latitude upper westerlies...the shortwave upper trough will move east...and so will Isaac now that he is supported by this shortwave. This means he will move more eastward into the paragraph P1 low-level rather than around the low-level ridge's west side...taking him into the Ohio valley (GFS and final NHC solution) rather than the southern Great Lakes region like I showed in discussion #92. Low-level warm air advection ahead of Isaac's circulation is supporting the southern US upper ridge formerly supported by warm air advection ahead of the front in paragraph P1. Upper convergence on the SE half of this upper ridge supports eastern US surface ridge with 1022 mb centers...formerly supported by the back side of upper trough in paragraph P3.

P3...Upper trough in the mid-latitude westerlies entering the NW Atlantic in the previous discussion is already SE of Greenland. Upper divergence east of this upper trough supports a surface front that has moved into the north-central Atlantic to the north and west of Kirk...the front curling into a vigorous cyclone that has exited the picture to the east of Greenland. Upper convergence behind this upper trough formerly supported a surface ridge centered over the eastern US....which has now been moved to paragraph P2 because it is supported by different means.

P4...Upper trough moving into Europe has exited the picture...but still leaves behind a large cut-off upper vortex near the Azores. The 1021 mb surface low near the Azores that was weakening in the non-divergent environment of this upper vortex is now a dissipating surface trough. Upper vortex has begun diving southward about the Leslie-Kirk upper anticyclonic cell mentioned in paragraph P6...and has merged with the inverted upper trough near the Cape Verde Islands also mentioned in paragraph P6. Therefore this upper vortex will now be moved to paragraph P6.

P5...Atlantic surface ridge has been eroded out of the western Atlantic thanks to cold front pushing in from the NW...associated with paragraph P3 system. Easterly flow on the south side of this surface ridge (in conjunction with easterly flow on south side of paragraph P6 upper ridge) is helping to waft Africa desert dry air westward across the Atlantic tropics. This surface ridge was once supported by convergence on back side of upper trough mentioned in paragraph P4...but now supported by convergence ahead of the Leslie-Kirk upper anticyclonic cell mentioned in paragraph P6.

...TROPICAL BELT DISCUSSION...
P6...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...now supported by warm air advection ahead of Isaac's remnant as mentioned in paragraph P2. This leaves sprawling upper vorticity in relatively lower pressures south of the upper ridge and located across the Caribbean. Hurricane Kirk and Tropical Storm Leslie t-storm latent heat release continues to locally inflate the upper ridge into an anticyclonic center in the central Atlantic...with relatively lower pressures west of this anticyclone still supporting adjacent upper trough SW of Kirk (and NW of Leslie)...merging with afoermentioned Caribbean upper vorticity. Relatively lower pressures east of this upper anticyclone continues supporting a large inverted upper trough moving west from Cape Verde Islands that is transforming into a large upper vortex as the upper vortex in paragraph P4 dives south into it.

P7...As mentioned in the previous two discussions...I had been using Meteosat-9 satellite animation to track what I believe was a tropical wave that had recently emerged Africa...and had estimated it to be SSW of the Cape Verde Islands yesterday. NHC TAFB this morning has located a tropical wave SSW of the Cape Verde Islands (new on their maps)...which either means the wave has been stationary for 24 hours...or that on my own I was positioning this wave too far west.


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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