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2012 Atlantic Hurricane Season Birdseye Discussion #164

By: NCHurricane2009 , 5:13 AM GMT on November 19, 2012

...MONDAY NOVEMBER 19 2012...12:30 AM EDT...
Weather in the western Atlantic Ocean to deteriorate in the next days due to two strong surface lows forecast to develop. However...the models remain west with the formation of the first of the two lows such that solutions increasingly consider both features as one.

Computer models agree that by 30 hrs...a surface low will form west of Bermuda and along the western Atlantic front mentioned in paragraph P2. This surface low is expected to form with the support of eastern divergence of the southern upper trough in paragraph P2. In fact...the 1013 mb surface low offshore of the Carolinas...mentioned towards the end of paragraph P2...may be the beginnings of this surface low. In addition...the northern upper trough in paragraph P2 is beginning to create a tight pressure gradient with respect to the paragraph P4 upper ridge...resulting in a strong upper westerly jet just east of the developing surface low. Upper winds accelerating into the jet are expected to enhance the upper divergence over the surface low such that it develops quickly. Despite a forecast formation just north of the 26 deg C sea-surface temperature isotherm...this first surface low should not develop into a subtropical cyclone due to hostile shear from the upper jet.

Computer models develop a more impressive second surface low just offshore of the southeastern United States beginning in 60 hours. This second surface low is expected to develop in upper divergence east the upper trough currently associated with the western US frontal system mentioned in paragraph P1. This second surface low will either absorb the above-mentioned first surface low...the first one will jump left while evolving into this second surface low...or the first one will simply exit stage right. Because the upper trough is expected to amplify into an upper vortex during this phase...this could reduce the shear. Coupled with formation just north of the 26 deg C waters and potentially de-stabilizing cold upper air with the upper vortex...their is some potential for subtropical cyclone development with this second surface low.


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.


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).

P1...Shortwave upper trough previously over the north-central US is now over eastern Canada and will soon merge with the northern upper trough mentioned in paragraph P2. The surface depressions associated with the shortwave upper trough have been absorbed by a surface frontal system entering the top-left of the above charts from the western US. Low-level warm air advection ahead of the western US frontal system supports the central US upper ridge.

P2...Northern upper trough continues crossing the Atlantic high seas. Southern upper trough persists over the SE US...E Gulf of Mexico...and W Caribbean...with an upper vortex at the north end of this upper trough. Lengthy W Atlantic surface cold front persists while supported by eastern divergence of both upper troughs. Surface frontal cyclone moving across the southern tip of Greenland remains associated with the northern upper trough. Western convergence of the northern upper trough supports strong 1040 mb ridge over the NE US. Western convergence of the southern upper trough supports surface ridging over the western Gulf of Mexico and Central America. Surface low moving ENE across the Atlantic high seas...located along the aforementioned W Atlantic front...has become absorbed by aforementioned surface cyclone near the south tip of Greenland. The 1019 mb low ENE of Bermuda in the previous discussion...yet another feature along the surface front...has dissipated. The SW end of the front just offshore of the Carolinas has developed a surface low (currently 1013 mb) as anticipated in the previous discussion. This surface low is supported by eastern divergence of the southern upper trough.

P3...NE Atlantic upper trough and associated surface cold front is exiting the top-right of the above charts from western Europe.

P4...Upper ridge axis over the central tropical Atlantic persists. Upper divergence west of this upper ridge continues to support t-storm activity across the central Caribbean Sea. A 1010 mb low located along the surface ITCZ and near eastern Panama continues to be marked in consecutive 6-hourly TAFB surface maps...a feature perhaps also supported by this upper divergence. Meanwhile...upper convergence east of this upper ridge axis formerly supported the north Atlantic surface ridge (currently 1022 mb). This surface ridge is now supported by upper convergence on the back sides of the northern upper trough in paragraph P2 and upper trough in paragraph P3.

P5...Cut-off upper trough NW of the Cape Verde Islands has amplified into an upper vortex midway between the Cape Verde Islands and Lesser Antilles. Upper divergence east of this upper vortex supports a comma-shaped area of t-storm clouds. Surface troughing formerly supported by this upper divergence has shifted west beneath the upper vortex while steered by 1022 mb north Atlantic surface ridge mentioned in paragraph P4.

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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