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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.


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.


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...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.

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.

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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