2012 Atlantic Hurricane Season Birdseye Discussion #163

By: NCHurricane2009 , 4:19 AM GMT on November 18, 2012

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...SATURDAY NOVEMBER 17 2012...11:30 PM 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 have shifted 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 54 hrs...a surface low will form west of Bermuda and along the western Atlantic front mentioned in paragraph P2. 24 hrs ago...solutions showed the formation of the low east of Bermuda...so this is a westward shift from the previous. This surface low is expected to form with the support of eastern divergence of the southern upper trough in paragraph P2...and with this upper trough currently more amplified than shown in the 18Z GFS model...the formation of this surface low could occur even further west toward the SE US coast. In fact...this surface low may already be in the formative stages as evidenced by the statements currently towards the end of paragraph P2. 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 84 hours. This second surface low is expected to develop in upper divergence east of an impressive upper trough that has not yet entered the scope of this discussion. Because the above-mentioned first surface low is now expected to form further west than thought before...this second surface low will either absorb the first one...or the first one will jump west while evolving into this second surface low. 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.

...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).

...MID-LATITUDES DISCUSSION...
P1...Shortwave upper trough has entered the top-left corner of the above charts and is currently located over the north-central US. Eastern divergence of the shortwave upper trough supports a 1020 mb depression over SE Manitoba and 1014 mb depression over the Dakotas. Western convergence of the upper trough supports 1026 mb ridge over the SW US. Central US upper ridge axis ahead of this system has split into one upper ridge axis over Mexico and the south-central US...and a second upper ridge axis over the Great Lakes.

P2...Large North America upper trough has split into two. The northern upper trough is entering the Atlantic high seas between Canada and Greenland. The southern upper trough is located over the SE US...E Gulf of Mexico...and W Caribbean. Lengthy W Atlantic surface cold front persists while supported by eastern divergence of both upper troughs. Surface frontal cyclone over eastern Canada in the previous discussion....now associated with the northern upper trough...is moving across the south tip of Greenland. Western convergence of the northern upper trough supports strong 1040 mb ridge over the NE US. Western convergence of the southern upper trough supports 1028 mb ridge over eastern Mexico. 1001 mb low centered just SE of Newfoundland in the previous discussion has raced ENE into the north Atlantic while becoming associated with the eastern divergence of the northern upper trough. This surface low continues to be located along the aforementioned W Atlantic surface front. The 1016 mb low just NW of Bermuda in the previous discussion...yet another feature along the surface front...has weakened to 1019 mb at a location ENE of Bermuda. The SW end of the front just offshore of the Carolinas has become kinked with increased cloudiness spreading onshore...a sign that yet another surface low maybe forming along the front. Such a surface low would be supported by eastern divergence of the southern upper trough.

P3...NE Atlantic upper trough is moving into western Europe from the Iberian peninsula. Surface frontal depression associated with the upper trough is currently 1002 mb while supported by the eastern divergence of the upper trough. This depression has made landfall across Portugal and Spain.

...TROPICAL BELT DISCUSSION...
P4...Upper ridge axis over the central tropical Atlantic continues to be expanded northward into the Atlantic high seas via warm air advection ahead of the W Atlantic surface front mentioned in paragraph P2. 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 has been marked in the last three 6-hourly TAFB surface maps...a feature perhaps also supported by this upper divergence. Meanwhile...upper convergence east of this upper ridge axis supports the north Atlantic surface ridge (currently 1028 mb).

P5...Cut-off upper trough NW of the Cape Verde Islands persists. Expansive upper divergence east of this cut-off upper trough supports a wide area of clouds that continues overspreading the Cape Verde Islands and eastern tropical Atlantic. This upper divergence continues to support relatively fresh surface troughing midway between the Cape Verde Islands and the Lesser Antilles.

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