2012 Atlantic Hurricane Season Birdseye Discussion #155

By: NCHurricane2009 , 5:56 AM GMT on November 09, 2012

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...FRIDAY NOVEMBER 9 2012..1:10 AM EDT...
Subtropical cyclone development in the open eastern Atlantic becoming more likely at a location west of the Cape Verde Islands. See the special feature section below for additional details.

Computer models also agree that a surface low will develop in the western Atlantic by 120 hours. However the dynamics are not as favorable for subtropical cyclone development as highlighted in paragraph P2.

Nor'easter continues impacting the coastal areas in the northeastern United States hit by last week's Hurricane Sandy as mentioned in paragraph P2.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1917Z-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).

...SPECIAL FEATURE...EASTERN ATLANTIC SUBTROPICAL SYSTEM...
Southern half of paragraph P4 upper trough has cut-off into an upper vortex in relatively lower pressures south of the paragraph P3 deep-layered ridge. This upper vortex is currently centered west of the Cape Verde Islands in the open eastern Atlantic. Upper divergence on the NE quad of the upper vortex supports a well-organized and large comma shaped t-storm mass and associated new surface trough. Because this surface trough and cloudiness is supported by a cold-core upper vortex...this system currently has the characteristics of a subtropical feature. Meanwhile...the surface trough midway between the Cape Verde Islands and Lesser Antilles...supported by divergent upper northerly flow between this cut-off upper vortex and east end of the paragraph P5 upper anticyclone...and mentioned in paragraph P4 of the previous discussion...has dissipated.

Computer model runs of the CMC...GFS...and NOGAPS continue to support potential subtropical cyclone development from this system in what is now the 56 to 72 hr timeframe. However given the well-organized comma shaped cloud mass and developing surface trough...I have already upgraded this to a special feature as I believe the potential for development could occur sooner than this and as I believe the potential for development is becoming likely. Expect for the next days this system to first track westward about the southern side of the paragraph P3 deep-layered ridge...then turn northward into the open Atlantic while influenced by the east side of forecasted W Atlantic surface low mentioned in paragraph P2.

...MID-LATITUDES DISCUSSION...
P1...The large surface gale that entered western Canada and the western US in the previous discussion has split into two gales. The northern split and associated upper trough has pushed into Hudson Bay...with western convergence of the upper trough supporting a strong 1040 mb w Canada surface ridge not yet in the scope of the above charts. The southern split and associated upper trough is moving into the western US...with the pressure gradient between the southern gale and aforementioned 1040 mb ridge set to create gusty winds across Montana. The gusty winds and snow showers from the southern gale will create blizzard conditions across parts of Montana where blizzard warnings are in effect...with winter storm warnings in states adjacent to Montana. See www.nws.noaa.gov for info on the hazardous winter weather expected in parts of the western US. Finally...low-level warm air advection ahead of the southern gale supports western US upper ridge that has shifted into the central US. Eastern convergence of the upper ridge supports a 1025 mb eastern US surface ridge.

P2...Southern Greenland upper trough is beginning to advance eastward into the NE Atlantic high seas. Eastern divergence of the southern Greenland upper trough has intensified the associated surface gale from 988 to 983 mb. However...the gale should begin weakening now that it has whirled beneath the less-divergent axis of the upper trough. Western convergence of this upper trough supports 1031 mb east Canada surface ridge beginning to move offshore. Elsewhere...the NE North Carolina upper vortex has moved NE to Massachusetts...while the upper trough extending from the upper vortex to the Yucatan is now shifting into the NW Caribbean. The 996 mb gale centered just offshore of the US east coast...a "nor'easter"...continues to impact coastal areas hit by Hurricane Sandy with gusty winds and rain/snow showers. In the last 24 hours...the nor'easter intensified further to 989 mb thanks to continued support from the eastern divergence of the upper vortex now over Massachusetts. With the nor'easter whirling into a position beneath the upper vortex...it should begin weakening with a lack of divergence directly below the vortex. Computer models agree that by 120 hours...a southern portion of the upper vortex/trough should become cut-off...with the eastern divergence of the cut-off supporting a W Atlantic surface low. However unlike the above special feature in the eastern Atlantic...this W Atlantic surface low will be exposed to high SW vertical shear as the cut-off never closes off into an upper vortex...and the surface low will track northward quickly into cooler waters below 26 deg C. Therefore...this surface low is not expected to develop into a subtropical cyclone.

P3...SW-NE tilted upper trough located SE of Newfoundland persists. North Atlantic upper ridge in the previous discussion now thrives in supportive low-level warm air advection ahead of the nor'easter mentioned in paragraph P2. The north Atlantic surface ridge (currently 1032 mb) has become vertically stacked with the upper ridge such that a deep-layered ridge center is developing just SW of the Azores.

P4...Large eastern Atlantic upper trough persists. Eastern divergence of the upper trough continues supporting surface troughing over NW Africa and near southern Portugal. The development of the deep-layered ridge described in paragraph P3 has fractured the south end of this upper trough at a location W of the Cape Verde Islands. This fracture is currently associated with subtropical cyclone development potential...see the above special feature section for additional details.

...TROPICAL BELT DISCUSSION...
P5...Sprawling tropical Atlantic upper anticyclone continues covering the waters just east of the Lesser Antilles and the Caribbean Sea. As seen in the above thermo chart...dry air continues to whirling within the upper anticyclone...perhaps supported by converging upper northeasterlies on the SE half of the upper anticyclone. South-central Caribbean t-storms persist with the support of the outflow of the upper anticyclone.

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