2012 Atlantic Hurricane Season Birdseye Discussion #157
...SUNDAY NOVEMBER 11 2012..10:50 AM EDT...
Computer models continue to agree that a surface low will develop in the western Atlantic in the timeframe that is now 60 hours. However the dynamics are not favorable for subtropical cyclone development as highlighted in paragraph P2.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0725Z-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...Gale pushing across Canada's Hudson Bay in the previous discussion has become absorbed by the 958 mb gale in paragraph P2. The gale centered over southern Wyoming in the previous discussion is maintaining strength at 997 mb while moving into NW Minnesota. This gale is maintaining strength from eastern divergence of an upper trough entering the top-left of the above charts from the western US. Low-level warm air advection ahead of the 997 mb gale supports a central US upper ridge. Eastern convergence of the upper ridge supports a 1030 mb eastern US surface ridge and 1030 mb eastern Canada surface ridge. Low-level southerlies between the 998 mb gale and 1030 mb ridge centers are in directional shear with respect to upper westerlies across the central US upper ridge. The shear coupled with any instability that develops from daytime heating of land could cause severe t-storms across the central US through the remainder of this weekend.
P2...Large upper trough spanning east Canada...the western Atlantic...and Caribbean persists. Eastern divergence of the upper trough has intensified a severe east Canada coast gale from 974 to 958 mb in the last 24 hrs while the gale has moved offshore toward southern Greenland. Expect rough seas in the Atlantic high seas...as well as high winds on the east coast of Canada...southern Greenland...and over open waters in the vicinity of this gale. Computer models agree that by 60 hours...a southern portion of the large upper trough should become cut-off...with the eastern divergence of the cut-off supporting a W Atlantic surface low. 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 forecast surface low is not expected to develop into a subtropical cyclone.
P3...Deep-layered north Atlantic ridge...currently with a 1033 mb surface center...persists near the Azores.
P4...Eastern Atlantic upper trough is moving into western Europe. Eastern divergence of the upper trough supports surface cold front over western Europe mentioned in paragraph P2 of the previous discussion.
...TROPICAL BELT DISCUSSION...
P5...Upper ridge axis over the central tropical Atlantic persists. Upper divergence west of this upper ridge and east of the paragraph P2 upper trough continues to support a surface trough over the Lesser Antilles.
P6...Cut-off upper vortex NW of the Cape Verde Islands persists. Expansive upper divergence east of the upper vortex continues to support a large comma shaped t-storm area that continues overspreading the Cape Verde Islands and eastern tropical Atlantic. The surface low formerly supported by this same upper divergence in the previous discussion has continued westward while steered by 1033 mb ridge in paragraph P3. On this track...the surface low has continued westward across the south side of the upper vortex where westerly vertical shear is inhibiting it. It has also moved into dry sinking air supported by the western convergence of the upper vortex. As a result...the surface low has weakened into an inactive surface trough currently located midway between the Cape Verde Islands and Lesser Antilles.