2012 Atlantic Hurricane Season Birdseye Discussion #165

By: NCHurricane2009 , 9:40 AM GMT on November 20, 2012

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...TUESDAY NOVEMBER 20 2012...4:40 AM EDT...
Weather in the western Atlantic Ocean deteriorating due to anticipated formation of surface low as mentioned towards the end of paragraph P2. In the previous intro sections of these discussions....this event has been presented as one that will have two surface lows. However with model solutions moreso showing this as a single broad surface low event...will go ahead and now present this as a single surface low event.

This surface low has formed with the support of eastern divergence of the southern upper trough in paragraph P2. In addition...the northern upper trough in paragraph P2 has created a tight pressure gradient with respect to the paragraph P3 upper ridge...resulting in a strong upper westerly jet just east of the surface low. Upper winds accelerating into the jet are enhancing the upper divergence over the surface low such that it remains vigorous. Despite being positioned just north of the 26 deg C sea-surface temperature isotherm and being below a cold core upper vortex at the north end of the paragraph P2 southern upper trough...the current upper vortex does not appear cold (de-stabilizing) enough for subtropical cyclone formation as their is a lack of thunderstorms at the center of the surface low. In addition...the paragraph P2 southern upper trough will soon de-amplify overhead of the surface low...which will increase the shear across this system in the short-term.

Beginning in 24 to 36 hours...this surface low will receive boost from eastern divergence of the next upper trough. This next upper trough is currently associated with the western US frontal system mentioned in paragraph P1. This next upper trough is expected to amplify into an upper vortex over the surface low...which will once again reduce the shear over the system. If cold enough for instability...this upper vortex will be the final shot this system has of becoming a subtropical cyclone.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1927Z-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...Western US surface frontal system and attendant upper trough have entered the top-left of the above charts. These features will be crossing the central US later today. Western convergence of the attendant upper trough supports the 1028 and 1022 mb western US surface ridge centers shown in the top-left.

P2...Northern upper trough continues crossing the Atlantic high seas. Southern upper trough persists offshore of the SE US 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. This cold front continues to curl into surface frontal cyclone near the south tip of Greenland...and this cyclone remains associated with the northern upper trough. Western convergence of the northern upper trough supports strong 1032 mb ridge moving offshore into the NW Atlantic from the NE US. Western convergence of the southern upper trough supports surface ridging over the western Gulf of Mexico and Central America. The 1013 mb low offshore of the Carolinas in the previous discussion...located along the SW end of the aforementioned W Atlantic front...has been absorbed by larger/broader 1012 mb surface low that has formed more offshore. This larger/broader surface low is supported by eastern divergence of the southern upper trough.

...TROPICAL BELT DISCUSSION...
P3...Upper ridge axis over the central tropical Atlantic persists. Upper divergence west of this upper ridge supports scattered t-storm activity across the central Caribbean Sea. A 1008 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 1020 mb). This surface ridge is now eroding in advance of the large surface cyclone near the south tip of Greenland (paragraph P2)...and this surface ridge will soon be replaced by 1032 mb ridge mentioned in paragraph P2.

P4...Cut-off upper vortex midway between the Cape Verde Islands and Lesser Antilles persists. Upper divergence east of this upper vortex supports some scattered t-storm activity. Surface troughing formerly supported by this upper divergence currently remains beneath the upper vortex.

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