2012 Atlantic Hurricane Season Birdseye Discussion #32

By: NCHurricane2009 , 9:55 AM GMT on June 20, 2012

...JUNE 20 2012...5:55 AM EDT...
Subtropical low Invest 95-L becomes Tropical Storm Chris in the northwest Atlantic...south of Newfoundland. See 1st special feature section for current details on this system.

Future of tropical disturbance in the western Caribbean Sea becoming uncertain as it pushes into the Gulf of Mexico. See 2nd special feature section for further details.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1308Z-released HPC analysis.

In light blue is upper air anlaysis, 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 indicating surface lows, Hs indicating surface highs.


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).

When we last talked about subtropical surface low Invest 95-L in the NW Atlantic (discussion #31)...it was back to a favorable (and less shearing) position with respect to its parent upper vortex...and was taking advantage of split flow upper divergence on the east periphery of said upper vortex. I had said there was a small chance of 95-L becoming a subtropical or tropical cyclone over Gulf Stream waters (albeit in the low 20s of deg C). Such tropical development over waters less than 26 deg C is possible as the parent upper vortex's cold temps can de-stabilize the atmosphere despite the mild waters. Indeed this happened yesterday afternoon...and we now have Tropical Storm Chris. According to the 1st NHC (National Hurricane Center) public advisory on this system...Chris is the 3rd earliest 3rd tropical storm in the Atlantic on record.

Computer models had been predicting an eastward turn along 40N latitude while rounding the north side of the subtropical ridge in paragraph P4...and indeed Chris started to follow this forecast. Then Chris started tracking ESE such that it became south of the computer model forecasts and first NHC track forecasts. When tropical cyclones develop from parent upper vortices (like Chris did)...they seem to be steered by their upper-level parent early on as the parent feature is deep-layered enough to do so. Chris's parent upper vortex recently merged with the tail end of a NE Atlantic upper trough (paragraph P5 in mid-latitudes discussion)...and I think Chris's early ESE track was caused by northwesterly flow on the back side of this upper-level merger.

Chris's early ESE motion has ended...now back on an east heading like it should have been. Upper trough of third Canadian frontal cyclone (paragraph P1) is digging into the parent upper vorticity over Chris. This...coupled with the amplifying upper ridge to Chris's west (paragraph P2)...is causing the upper vorticity over Chris to amplify...which should cause Chris to eventually turn north on the upper vorticity's east side as shown in Figure 1's forecast. Looking at current upper-level wind streamlines of the steering upper vorticity...the NHC track forecast basically follows...so I see no reason to disagree with it...especially as Chris is also following the NHC forecast track on satellite animations. Another sign that Chris will turn north is the blocking surface ridge of 1024 mb to the northeast (paragraph P2).

My intensity forecast in Figure 1 is a copy of the NHC's...as I also agree with it. Chris should hold its strength...then get stronger as it becomes non-tropical and takes advantage of upper divergence ahead of the upper vorticity that is steering it.

Figure 1: Tropical Storm Chris Forecast

Figure 2 below is my current best guess on how this system evolves over the next days. In the previous discussion...it was good to forecast an initial NW track instead of WNW as I was expecting a northward reformation toward the best-curved cloud bands (located toward Cuba...W Bahamas...south Florida). In fact...my forecast NW track was not north enough...so the current position of the disturbance (marked by surface trough in latest TAFB analyses) is a bit more NE of where I expected it to be by now. I maintain the NW track...then bend it northward and slow it by 0000Z June 22 as the steering surface ridge in paragraph P2 gets knocked out by fourth frontal cyclone mentioned in paragraph P1.

This disturbance is struggling to push out the Gulf of Mexico upper trough (paragraph P3) with its warm core upper ridge..and as a result it no longer appears it will develop by June 22. I hinted at the possibility of this disturbance struggling with the Gulf of Mexico upper trough in earlier discussion #30 paragraph P11.

After June 22...models show the upper winds remaining unfavorable...with upper westerly winds from the fourth frontal cyclone's upper trough (paragraph P1) shearing this system. I bend the track back westward by 0000Z June 24 as a strong surface ridge builds behind the fourth frontal cyclone. With the expected westerly shear...I keep the system a mere surface trough through this time.

After June 24...computer models still dump in a west Gulf of Mexico upper level low (ULL)...a cut-off feature from the upper trough that supports the fourth frontal cyclone. Currently...I project that the disturbance will regenerate slightly northeastward by 0000Z June 25 while taking advantage of upper divergence at the NE periphery of this ULL. The ULL could help reduce the westerly shear by this time...so I think there is a remote chance the disturbance becomes a weak tropical cyclone while steered westward on the north side of this ULL. Such a track would take this system into the Texas coast by 0000Z June 27...but this scenario is superficial at this time as no models show this happening...and moreover forecast skill that far out is difficult.

Some computer model runs propose that the east side of this disturbance consolidates into another surface low by June 24 that tracks NE across Florida and then parallel to the east coast of the US (also illustrated in Figure 2). This appears to be driven by upper divergence ahead of the upper trough associated with the fourth frontal cyclone in paragraph P1. While this surface low may be tropical early on...I think it is likely to become non-tropical quickly as such a development method is a non-tropical method.

Finally...computer models suggest some upper ridging to the south of the forecasted June 25 to 27 ULL (upper level low)...located over the Bay of Campeche. They hint at a tropical cyclone in the Bay of Campeche by that time while taking advantage of this upper ridging.

This is a highly uncertain situation with multiple scenarios possible...

Figure 2: My current best guess forecast for west Caribbean disturbance (emphasis on guess) this morning.

P1...Third Canadian frontal cyclone in recent days (and its supporting upper trough) is now over the E coast of Hudson Bay with lowest surface pressure of 996 mb. As the associated upper trough continues east...it is expected to interact with Chris and transition him to non-tropical (extratropical) status (see 1st special feature section for more detials). Meanwhile...fourth frontal cyclone in recent days is entering the above birdseye charts from the western US (994 mb center in W Nebraska) with its supporting upper trough.

P2...Upper ridge over the eastern US has re-amplified as forecast...thanks to warm air advection ahead of fourth frontal cyclone mentioned in paragraph P1. The upper ridge's eastern convergence continues to support a surface ridge. This surface ridge currently has a 1022 mb center offshore of North Carolina and a 1023 mb center offshore of Massachusetts. A fragment of the upper ridge has split off into a shortwave upper ridge over E Canada associated with warm air advection ahead of the third cyclone mentioned in paragraph P1. Eastern convergence from this shortwave upper ridge supports 1024 mb surface ridge located well NE of Tropical Storm Chris.

P3...Upper trough over the W Atlantic and Gulf of Mexico persists. Peripherial upper diveregence on the east side of its upper vortex continues to support subtropical low Invest 95-L S of Newfoundland which has made the transition to Tropical Storm Chris (see 1st special feature section for further details). The Gulf portion of this upper trough remains split off...the split occuring as upper ridging from the latent heat release of the western Caribbean disturbance (2nd special feature above) expands.

P4...Surface 1027 mb ridge over the open Atlantic persists. From what I can gather from the latest 200 mb upper winds...it is supported as upper southerlies from E Atlantic upper ridge (paragraph P8) converges with upper westerlies from NE Atlantic upper trough (paragraph P5).

P5...Longwave upper trough persists in the NE Atlantic...with divergence ahead of this upper feature supporting a surface front stretching from the W Mediterranean all the way towards Tropical Storm Chris. 1018 mb frontal depression that was west of the Azores has moved across the Azores and is 1012 mb in the above birdseye charts. Divergence ahead of this NE Atlantic longwave upper trough also supports a relatively new extratropical (non-tropical) surface low seen on satellite imagery W of the British Isles...and it appears to have intensified quickly based on its satellite organization.

P6...Upper ridge over the Caribbean persists...enhanced by latent heat release of the t-storm clouds of the west Caribbean disturbance covered in the above 2nd special feature section.

P7...Inverted upper trough E of the Lesser Antilles persists.

P8...Expansive east Atlantic upper ridge continues. In conjunction with the 1027 mb surface ridge mentioned in paragraph P4...deep-layered easterly flow exists south of this upper ridge such that dry air from the African desert is being advected westward across the Atlantic tropics (brown shading in the above thermo birdseye chart). In the midst of this dryness...there are batches of moisture from surface convergence/lift associated with the tropical waves in paragraphs P10 and P11. This upper ridge seems to have concentrated into two centers...the western of which is associated with warm air advection ahead of Chris's deep-layered cyclonic circulation...the eastern of which is located toward Africa. A new inverted upper trough profile (midway between the Lesser Antilles and Cape Verde Islands) shows up in between these two upper ridge centers.

P9...Tropical wave approaching the Lesser Antilles in the previous discussion has entered the eastern Caribbean Sea. It had been suppressed by the inverted upper trough in paragraph P7 and dry air advected westward from the African desert. This is likely the last discussion of this tropical wave as it has been removed from TAFB analyses this early morning. This is probably because the tropical wave has dissipated.

P10...Vigorous tropical wave midway between the Antilles and Cape Verde Islands in the previous discussion is now nearing the Lesser Antilles. It has generated some increased cloudiness with the help of enhanced poleward upper outflow...genearted by upper southerly winds on the west side of the upper ridge in paragraph P8.

P11...New tropical wave has rolled off the west coast of Africa. It features a t-storm burst along the surface ITCZ. While it is in favorable low shear (thanks to deep-layered easterlies mentioned in paragraph P8)...if the tropical wave in paragraph P10 is a good analogue...than we can too expect this tropical wave to struggle with dry air.

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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