2012 Atlantic Hurricane Season Birdseye Discussion #139

By: NCHurricane2009 , 3:22 AM GMT on October 22, 2012

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...SUNDAY OCTOBER 21 2012...11:22 PM EDT...
Tropical disturbance Invest 99-L in the central Caribbean Sea has not gotten any better organized in the last 24 hours. However...tropical cyclone formation from this system is still likely. See the Invest 99-L special feature section below for details.

Disturbance Invest 90-L in the open Atlantic...associated with an upper vortex interacting with a tropical wave...has potential for subtropical or tropical cyclone formation in the next 24 hours. See Invest 90-L special feature section for additional 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...CARIBBEAN DISTURBANCE INVEST 99-L...
The disturbance continues to have a broad surface low consisting of a tropical wave entering the central Caribbean and a western Caribbean surface trough. In the upper-levels...the disturbance continues to be ventilated by the outflow of an east-west upper ridge. The focal point of this system continues to be the tropical wave...but the system has become less organized in the last day while the western Caribbean surface trough had flared up. Perhaps the surface trough had taken away from the focal point of the disturbance...but with the surface trough removed from TAFB maps by 1200Z...this system may proceed to re-organize and develop into a tropical cyclone.

Their continues to be two possible solutions as follows...

The eastern solution is for gradual tropical cyclone formation while the east US surface ridge in paragraph P2 builds to the north...and a cut-off upper trough from the paragraph P2 upper trough amplifies to the NW (thanks to equally amplifying paragraph P1 upper ridge). In such a scenario...tropical cyclone intensity is highly uncertain...as the tropical cyclone either suffers from SW vertical shear delivered by the cut-off upper trough...or thrives under the upper ridge...depending on exactly where the surface center consolidates. In such a solution...it appears the tropical cyclone would be vertically coupled enough to be steered by the cut-off upper trough such that the tropical cyclone first drifts NNE toward Jamaica...eastern Cuba...the eastern Bahamas...and western Hispaniola while the tropical cyclone is trapped by conflicting steering between the surface ridge and cut-off upper trough. Afterwards the surface ridge gets knocked out in between the paragraph P2 cyclone and paragraph P1 surface frontal system...sending the tropical cyclone ENE into the open Atlantic.

The more western solution is a track toward Jamaica...Cuba...the Cayman Islands...the western Bahamas...and southeastern Florida. Computer models show a stronger tropical cyclone when depicting this western solution. This solution would verify if the cut-off upper trough is less influential. A less influential cut-off upper trough means less NE steering and more NW steering from the surface ridge...and also means a stronger tropical cyclone less affected by vertical shear from the upper trough.

The GFS model initially switched from the eastern to the western solution...but now shows a solution that is a compromise between these extremes. The CMC and Euro models have joined the current middle-ground solution of the GFS. The NOGAPS currently agrees with the western solution. Residents in Hispaniola (Haiti and the Dominican Republic)...Jamaica...Cuba...the Cayman Islands...the Bahamas...and southeastern Florida are urged to keep tabs on this developing situation until it is clearer which solution will play out.

...SPECIAL FEATURE...OPEN ATLANTIC UPPER VORTEX DISTURBANCE INVEST 90-L...
In the Atlantic tropics east of the Lesser Antilles...cut-off upper vortex persists. The tropical wave exiting the area and moving toward the Lesser Antilles has dissipated as of 0600Z TAFB analysis. The tropical wave/low near 45W-17N in the previous discussion is now the dominant surface feature of this system while centered near 50W-20N tonight. This surface low has become better organized with a small t-storm cluster enhanced by split flow upper divergence between the NW quad of the upper vortex and SW quad of the paragraph P2 central Atlantic upper ridge. Based on this organization...I forecast a fairly high chance of subtropical or tropical cyclone formation in the next 24 hours. Since the aforementioned split flow upper divergence supporting the surface low is created by both the cold core upper vortex and adjacent warm core upper ridge...subtropical or tropical status will be dependent upon whether more credit is given to the upper vortex or upper ridging. Such a tropical or subtropical system would first track west on the north side of the upper vortex and south of the paragraph P3 1016 mb ridge...then track northward by 48 hours while getting swept into the paragraph P2 cyclone's cold front coming in from the west. Such a forecast track keeps this system well away from land areas and over open waters.

...MID-LATITUDES DISCUSSION...
P1...Next surface frontal system and upper trough in the mid-latitudes continues entering from SW Canada and the western US in the upper-left corner of the above atmo chart. Low-level warm air advection ahead of the frontal system supports an upper anticyclone over Hudson Bay and upper ridge shifting into the central US.

P2...1001 mb deep-layered cyclone/upper trough centered over the Great Lakes in the previous discussion has shifted into SE Canada. The surface center has strengthened from 1001 to 994 mb while tapping into divergence on the east side of the upper trough. Upper convergence on the west side of the deep-layered cyclone supports Gulf of Mexico and W Atlantic dry air...as well as surface ridge (currently 1021 mb) over the eastern US. Warm air advection ahead of the deep-layered cyclone supports an upper ridge shifting from the western to the central Atlantic.

P3...Upper trough in the vicinity of southern Greenland has shifted into the NE Atlantic. Western convergence of the upper trough supports dry air and 1016 mb central Atlantic surface ridge. The remnant surface gale of Rafael continues on a southeast track in deep-layer NW flow on the back side of the upper trough and east of the aforementioned 1016 mb ridge. The gale has weakened from 979 to 988 mb in the last 24 hours while beneath the non-divergent upper trough axis.

P4...Upper trough and surface frontal system entering western Europe in the previous discussion has exited the picture...except for a 1011 mb surface low over NE Spain in the upper-right corner of the above atmo chart. Western convergence of this upper trough formerly supported what is now a weakening surface ridge in the central and eastern Atlantic. This surface ridge is currently weakening in upper divergence ahead of the paragraph P3 upper trough.

...TROPICAL BELT DISCUSSION...
P5...Large-sized upper ridge across the eastern Atlantic has eroded out of the subtropical latitudes due to the paragraph P3 upper trough. What is left of the upper ridge in the tropical latitudes is enhancing the upper outflow of a tropical wave W of the Cape Verde Islands that has been added to TAFB maps as of 1800Z.

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3. WunderAlertBot (Admin)
7:19 AM GMT on October 23, 2012
NCHurricane2009 has created a new entry.
2. KoritheMan
3:52 AM GMT on October 22, 2012
99L reminds me a bit of Michelle, although not quite of the same intensity.
Member Since: March 7, 2007 Posts: 602 Comments: 21302
1. wxchaser97
3:38 AM GMT on October 22, 2012
Thanks NC09, I agree 90L should become Sandy soon.
Member Since: March 16, 2012 Posts: 127 Comments: 7972

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