2012 Atlantic Hurricane Season Birdseye Discussion #162

By: NCHurricane2009 , 1:13 AM GMT on November 17, 2012

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...FRIDAY NOVEMBER 16 2012...8:14 PM EDT...
Weather in the western Atlantic Ocean to deteriorate in the next days due to two strong surface lows forecast to develop.

Computer models agree that by 80 hrs...a surface low will form east of Bermuda and along the western Atlantic front mentioned in paragraph P2. Tonight's 18Z GFS model shows the surface low forming in eastern upper divergence of a southern fragment of the paragraph P2 upper trough. At the same time...the northern fragment of the upper trough is expected 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 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.

While the first surface low then moves out to sea...computer models develop a more impressive second surface low just offshore of the southeastern United States beginning in 111 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 upper trough is expected to amplify into an upper vortex over the surface low...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 1929Z-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 is entering the top-left corner of the above charts from western Canada...but is not quiet yet in the scope of the charts. Eastern divergence of the shortwave upper trough supports a 1015 mb depression over SE Montana and 1019 mb depression over Manitoba. Low-level warm air advection ahead of the depressions supports an upper ridge axis entering the central US.

P2...Large surface low east of Greenland...currently just outside of the top-center of the above charts...persists thanks to divergence east of the major North America upper trough. This large surface low is pushing a surface front into the NE Atlantic and western Europe seen in the top-right of the above charts. Lengthy W Atlantic surface cold front formerly extending from the large surface low persists. The Gulf of Mexico fragment of the front has decayed into surface troughs. Surface frontal cyclone over Hudson Bay in the previous discussion....also supported by the eastern divergence of the major upper trough...is moving into eastern Canada. Western convergence of the major upper trough continues to support strong surface ridging across the US. Eastern divergence of the major upper trough continues to support a W Atlantic surface low that has intensified to 1001 mb currently centered just SE of Newfoundland. This surface low continues to be located along the aforementioned W Atlantic surface front. The 1018 mb low just offshore of the Carolinas in the previous discussion...yet another feature along the surface front and supported by the eastern divergence of the major upper trough...has intensified to 1016 mb at a location just NW of Bermuda.

P3...NE Atlantic cut-off upper vortex midway between the Azores and Canary Islands has weakened into an upper trough heading toward western Europe. Surface frontal depression associated with the upper trough has intensified to 999 mb while tapping into the eastern divergence of the upper trough. This 999 mb depression will soon make 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 has supported an increase in t-storm activity across the central Caribbean and eastern Bahamas in the last 24 hrs. Upper convergence east of this upper ridge axis supports the north Atlantic surface ridge (currently 1027 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 t-storm clouds that continues overspreading the Cape Verde Islands and eastern tropical Atlantic. This upper divergence has also triggered new surface troughing midway between the Cape Verde Islands and the Lesser Antilles...while the previous surface troughing in this area has dissipated in the western convergence on the back side of the cut-off upper trough.

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2. WunderAlertBot (Admin)
4:19 AM GMT on November 18, 2012
NCHurricane2009 has created a new entry.
1. wxchaser97
1:58 AM GMT on November 18, 2012
Thanks NC09, I think the season is pretty much over but there could be one more surprise.
Member Since: March 16, 2012 Posts: 127 Comments: 7958

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