NCHurricane2009's Blog

2012 Atlantic Hurricane Season Birdseye Discussion #169A (Special Update)

By: NCHurricane2009, 2:08 AM GMT on December 06, 2012

...WEDNESDAY DECEMBER 5 2012...9:00 PM EDT...
Non-tropical remnant surface low of former subtropical disturbance Invest 91-L...and associated upper vortex...has entered the high-latitudes northwest of the Azores as remarked in paragraph P3 of full discussion #169.

Eastern divergence of cut-off upper troughing from from the eastern Caribbean...extending east-northeast to the central Atlantic...continues to support open Atlantic surface troughing and thunderstorms southwest of ex-91L as remarked in paragraph P5 in full discussion #169. The surface low along the surface troughing has intensified to 1006 mb while moving quickly northeast. Even though westerly shear ahead of the cut-off upper troughing has reduced due to its amplification...and even though the 1006 mb surface low is over sub-20 deg C water capable of supporting a subtropical disturbance...the surface low is moving quickly northeast into even cooler waters and is becoming non-tropical while merging with the front extending southwest from ex-91L. Therefore subtropical development from this system is not anticipated.

In the northern Gulf of Mexico...a well-organized comma shaped area of scattered thunderstorms has developed in assocation with eastern divergence of a highly-amplified cut-off upper trough and surface frontal depression. The surface frontal depression is along the cold front extending from the non-tropical cyclone mentioned in paragraph P1 of discussion #169...while the cut-off upper trough is a southern fracture of the central United States upper trough mentioned in that same paragraph. The south half of the disturbance is over waters in the 20s of deg C...the surface frontal depression is currently under low shear beneath the highly-amplified cut-off upper trough...and a comparison of surface air temps from Texas to Florida show little air mass contrasts across the surface fronal depression. While these are principles that could convert a surface frontal depression to a more subtropical feature...westerly vertical shear is expected to increase quickly as the cut-off upper trough de-amplifies thanks to warm air advection ahead of the next frontal system moving into the western United States. Therefore no subtropical development is expected here either.

With no indications of subtropical or tropical development in the near-term...I have stopped full birdseye chart discussions for the Atlantic hurricane season. Full birdseye chart discussions will resume on this blog when the Atlantic hurricane season re-starts in June 2013. Special updates and or full birdseye discussions could resume before that time if once again any out-of-season tropical or subtropical activity occurs in the Atlantic basin.

Sometime early in 2013...I will be releasing post-storm reports for the 2012 Atlantic Hurricane Season which will include an evaluation of how my storm forecasts (issued on this blog) compared with that of the official forecasts from the National Hurricane Center.

2012 Atlantic Hurricane Season Birdseye Discussion #169

By: NCHurricane2009, 1:56 AM GMT on December 05, 2012

...TUESDAY DECEMBER 4 2012...8:45 PM EDT...
Second central Atlantic subtropical disturbance within a week possible in the next 48 hours. Therefore I have resumed full birdseye discussions until this pattern of central Atlantic subtropical disturbances subsides. See paragraph P5 below for details on the next potential subtropical disturbance.

...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).

...HIGH-LATITUDES DISCUSSION...
P1...Upper trough has moved from the western to central US in the last 24 hours. Its eastern divergence supports a surface 992 mb frontal cyclone centered over southern Canada with its eastern upper divergence. Western convergence of the upper trough supports western US surface ridge. Warm air advection ahead of the frontal cyclone supports upper ridge along the east coast of North America.

P2...Shortwave upper trough entering the NW Atlantic in the previous discussion has moved into southern Greenland. Eastern divergence of the upper trough supports 999 mb surface frontal cyclone that has also moved across southern Greenland.

P3...Non-tropical remnant surface low of subtropical disturbance Invest 91-L is centered just NW of the Azores this evening while continuing NNE into the high latitudes. Associated cold core upper vortex has become vertically stacked over the surface center such that a lack of divergence directly below the upper vortex should limit additional strengthening of the surface low. The western convergence of the above-mentioned upper vortex supports greater-than-1024 mb surface ridge just west of the surface low.

...SUBTROPICAL DISCUSSION...
P4...Warm air advection ahead of the paragraph P3 surface low (ex-91L) has supported a subtropical-latitude upper ridge axis midway between the Azores and Cape Verde Islands...the eastern convergence of which supports a large area of dry sinking air and 1031 mb surface ridge centered just north of the Canary Islands. Relatively lower pressures east of the upper ridge axis has been supporting a cut-off eastern Atlantic upper trough near the Canary and Cape Verde Islands.

P5...Cut-off upper troughing persists from the Caribbean ENE to the open central Atlantic. A south fragment of the paragraph P2 upper trough has merged with this upper troughing. Western convergence of the upper troughing supports large area of Caribbean and W Atlantic dry air seen in above thermo chart...as well as a 1026 mb ridge centered offshore of the Carolinas formerly supported by the western convergence of the paragraph P2 upper trough. Meanwhile...eastern divergence of the cut-off upper troughing supports a wide area of open Atlantic surface troughing and thunderstorms south and SW of paragraph P3 surface low ex-91L. A surface 1009 mb low is embedded along the surface trouhing. Currently westerly vertical shear...driven by upper westerlies ahead of the SW-NE tilted upper troughing...is preventing the 1009 mb surface low from collocating with the thunderstorms such that subtropical or tropical development is prevented. However...watching to see if surface low becomes strong enough to advect in cool air to amplify the SW-NE upper troughing into an NW-SE upper trough...or even an upper vortex. Such an amplification would reduce the westerly vertical shear. Coupled with sea-surface temps in the sub-20 deg C range...perhaps another central Atlantic subtropical disturbance such as the recent 91-L is possible in the next 48 hours.

2012 Atlantic Hurricane Season Birdseye Discussion #168

By: NCHurricane2009, 6:12 AM GMT on December 04, 2012

...TUESDAY DECEMBER 4 2012...1:01 AM EDT...
Even though surface low Invest 91-L has converted to a non-tropical feature...another central Atlantic subtropical disturbance is possible on its heels in the next 72 hours. Therefore I have resumed full birdseye discussions until this pattern of central Atlantic subtropical disturbances subsides. See paragraph P6 below for details on the next potential subtropical disturbance.

...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).

...HIGH-LATITUDES DISCUSSION...
P1...Upper trough swinging eastward across the western US supports surface 990 mb frontal cyclone centered over southern Canada with its eastern upper divergence. Western convergence of the upper trough supports western US surface ridge. Warm air advection ahead of the frontal cyclone supports upper ridge across the Gulf of Mexico and eastern US.

P2...Shortwave upper trough is currently positioned over eastern Canada and is currently entering the NW Atlantic. Eastern divergence of the upper trough supports 994 mb surface frontal cyclone moving into southern Greenland...and supports NW Atlantic surface trough just north of Bermuda. Western convergence of the upper trough supports 1027 mb surface ridge centered just offshore of the Carolinas...and 1020 mb surface ridge over eastern Canada.

P3...Large upper trough and associated surface cyclone is beginning to exit the picture from the top-right of the above charts as this system moves into western Europe.

...SUBTROPICAL DISCUSSION...
P4...Impressive surface low...formerly designated as subtropical disturbance Invest 91-L during special discussions #167B and #167C...is intensifying quickly this early morning while located west of the Azores. Northerly flow on the back side of the surface low has drawn in enough high-latitude cold air such that the cold core upper trough supporting this system has amplified into an upper vortex. Westerlies streaming into the paragraph P3 upper trough heavily diverges with the flow around the upper vortex...and this highly-divergent upper atmosphere is why the surface low is intensifying quickly and why this surface low continues to have strong thunderstorms despite having moved NNE into waters in the teens of deg C. The surface low has wrapped in enough cold air such that the surface trough attached to it has been upgraded to a cold front...so despite the rather impressive comma-shaped t-storm activity persisting with this system...the cold front prevents classifying this system as a subtropical or tropical feature. Elsewhere...the western convergence of the above-mentioned upper vortex supports 1025 mb surface ridge/sinking...dry air band just west of the surface low.

P5...Warm air advection ahead of the paragraph P4 surface low (ex-91L) has supported a subtropical-latitude upper ridge axis midway between the Azores and Cape Verde Islands. Relatively lower pressures east of the upper ridge axis has been supporting a cut-off eastern Atlantic upper trough near the Canary and Cape Verde Islands whose western convergence supports 1036 mb surface ridge centered just north of the Canary Islands. This upper convergence also supports an area of eastern Atlantic dry air seen in the above thermo chart.

P6...Cut-off upper trough...originating from the paragraph P3 upper trough...is currently positioned from the Caribbean ENE to the open central Atlantic. Western convergence of the upper trough supports large area of Caribbean and W Atlantic dry air seen in above thermo chart. Eastern divergence of the cut-off upper trough supports an ever-widening area of open Atlantic surface troughing and thunderstorms south and SW of paragraph P4 surface low ex-91L. Currently westerly vertical shear...driven by upper westerlies ahead of the SW-NE tilted upper trough...is preventing the surface troughing from collocating with the thunderstorms such that subtropical or tropical development is prevented. However...computer models suggest this surface troughing will organize into a surface low. If the surface low becomes strong enough...it could advect in enough cool air to amplify the SW-NE upper trough into an NW-SE upper trough...or even an upper vortex...as we recently saw with paragraph P4 disturbance Invest 91-L. Such an amplification would reduce the westerly vertical shear. Coupled with sea-surface temps in the sub-20 deg C range...perhaps another central Atlantic subtropical disturbance such as the recent 91-L is possible in the next 72 hours.

2012 Atlantic Hurricane Season Birdseye Discussion #167C (Special Update)

By: NCHurricane2009, 3:58 PM GMT on December 03, 2012

...MONDAY DECEMBER 3 2012...10:50 AM EDT...
Surface low Invest 91-L...centered near 43.5W-20N on Sunday December 2 5 AM EDT...is now centered near 40W-38N. It continues moving north-northeast and will pass west of the Azores today. Although the surface low continues to have an impressive comma-shaped area of thunderstorms to the northeast of center...the National Hurricane Center has been continually downgrading its chances to become a subtropical or tropical cyclone since previous update #167B was released. Therefore I did not resume full birdseye chart discussions last night even though update #167B stated I would. The National Hurricane Center is now citing this as a surface low becoming non-tropical (or frontal). The surface low and strong thunderstorms continue to be supported by upper divergence east of the associated upper trough...which is also zooming north-northeast with the surface low.

Will assess the large-scale patterns in computer models to see if their are any chances for additional disturbances like 91-L in the next days. If their is potential for such additional disturbances...then I may decide to resume full birdseye chart discussions.

2012 Atlantic Hurricane Season Birdseye Discussion #167B (Special Update)

By: NCHurricane2009, 10:31 AM GMT on December 02, 2012

...SUNDAY DECEMBER 2 2012...5:30 AM EDT...
Formation of a post-season subtropical or tropical cyclone in the middle of the open Atlantic becoming very likely this morning...

Despite the official end of the 2012 Atlantic Hurricane Season on November 30...a surface low currently centered near 43.5W-20N is gaining tropical characteristics with an impressive comma-shaped area of thunderstorms in its northeast half. Hovmoller diagrams from the National Hurricane Center (http://www.nhc.noaa.gov/tafb_latest/gehov2latest. gif) indicate that the south end of an upper trough cut-off in the middle of the Atlantic around November 27...upon since when the cut-off upper trough's eastern divergence supported a distinct area of thunderstorms and gradual formation of this surface low.

According to the latest surface maps from the National Hurricane Center TAFB...the system has no surface fronts attached to the surface low. Coupled with the impressive comma-shaped thunderstorm mass this morning...the system at least appears to be a subtropical cyclone...which is a system that has a shallow vertical warm core from thunderstorm latent heat release but the thunderstorms are generated by upper divergence and-or instability from a cold core upper trough/vortex.

Despite the fact that the thunderstorm activity is currently supported by an upper trough...the comma shaped area of thunderstorms has generated anticyclonic warm core upper outflow across the east half of this system according to the latest satellite-derived 200 mb upper winds. Warm core upper anticyclones are more indicative of a fully-vertical tropical warm core...and therefore this system may become fully tropical despite the water temps currently below 26 deg C.

This feature is currently designated as Invest 91-L on the Navy NRL site. Latest info of this system can be found on www.nhc.noaa.gov. In lieu of this system...I will be resuming full birdseye discussions late tonight.


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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