2012 Atlantic Hurricane Season Birdseye Discussion #168
...TUESDAY DECEMBER 4 2012...1:01 AM EDT...
Even though surface low Invest 91-L has converted to a non-tropical feature...another central Atlantic subtropical disturbance is possible on its heels in the next 72 hours. Therefore I have resumed full birdseye discussions until this pattern of central Atlantic subtropical disturbances subsides. See paragraph P6 below for details on the next potential subtropical disturbance.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0127Z-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).
P1...Upper trough swinging eastward across the western US supports surface 990 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 across the Gulf of Mexico and eastern US.
P2...Shortwave upper trough is currently positioned over eastern Canada and is currently entering the NW Atlantic. Eastern divergence of the upper trough supports 994 mb surface frontal cyclone moving into southern Greenland...and supports NW Atlantic surface trough just north of Bermuda. Western convergence of the upper trough supports 1027 mb surface ridge centered just offshore of the Carolinas...and 1020 mb surface ridge over eastern Canada.
P3...Large upper trough and associated surface cyclone is beginning to exit the picture from the top-right of the above charts as this system moves into western Europe.
P4...Impressive surface low...formerly designated as subtropical disturbance Invest 91-L during special discussions #167B and #167C...is intensifying quickly this early morning while located west of the Azores. Northerly flow on the back side of the surface low has drawn in enough high-latitude cold air such that the cold core upper trough supporting this system has amplified into an upper vortex. Westerlies streaming into the paragraph P3 upper trough heavily diverges with the flow around the upper vortex...and this highly-divergent upper atmosphere is why the surface low is intensifying quickly and why this surface low continues to have strong thunderstorms despite having moved NNE into waters in the teens of deg C. The surface low has wrapped in enough cold air such that the surface trough attached to it has been upgraded to a cold front...so despite the rather impressive comma-shaped t-storm activity persisting with this system...the cold front prevents classifying this system as a subtropical or tropical feature. Elsewhere...the western convergence of the above-mentioned upper vortex supports 1025 mb surface ridge/sinking...dry air band just west of the surface low.
P5...Warm air advection ahead of the paragraph P4 surface low (ex-91L) has supported a subtropical-latitude upper ridge axis midway between the Azores and Cape Verde 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 whose western convergence supports 1036 mb surface ridge centered just north of the Canary Islands. This upper convergence also supports an area of eastern Atlantic dry air seen in the above thermo chart.
P6...Cut-off upper trough...originating from the paragraph P3 upper trough...is currently positioned from the Caribbean ENE to the open central Atlantic. Western convergence of the upper trough supports large area of Caribbean and W Atlantic dry air seen in above thermo chart. Eastern divergence of the cut-off upper trough supports an ever-widening area of open Atlantic surface troughing and thunderstorms south and SW of paragraph P4 surface low ex-91L. Currently westerly vertical shear...driven by upper westerlies ahead of the SW-NE tilted upper trough...is preventing the surface troughing from collocating with the thunderstorms such that subtropical or tropical development is prevented. However...computer models suggest this surface troughing will organize into a surface low. If the surface low becomes strong enough...it could advect in enough cool air to amplify the SW-NE upper trough into an NW-SE upper trough...or even an upper vortex...as we recently saw with paragraph P4 disturbance Invest 91-L. 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 72 hours.