2012 Atlantic Hurricane Season Birdseye Discussion #59
...JULY 21 2012...9:40 PM EDT...
Still monitoring the next tropical wave that has emerged from Africa (see paragraph P10). Disturbed weather in northern Gulf of Mexico...near Louisiana and Mississippi...still does not appear to have tropical development potential (paragraph P6).
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1922Z-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...Surface frontal system and its supporting upper trough over central Canada continues pushing east. Upper convergence on the west side of the upper trough supports a 1019 mb ridge near the North Dakota/Minnesota border. Warm air advection ahead of this frontal system has supports an upper ridge over eastern Canada.
P2...Strong frontal cyclone southeast of Greenland has rapidly intensified to 966 mb...thanks to strong upper divergence on the east side of its supporting upper trough moving offshore from E Canada. Cool air advection behind the surface cyclone has amplified the upper trough into an upper vortex directly aligned with the 966 mb center...and now we have a deep-layered vortex SE of Greenland. The 966 mb center should now begin weakening with a much less divergent environment beneath the upper vortex. Upper convergence behind the deep-layered vortex supports a 1026 to 1019 mb ridge over SE Canada. Upper anticyclone over the west US has grown into the central US. Cold front trailing from the 966 mb cyclone has a frontal depression and its supporting upper trough over the Ohio Valley. There are also several NW Atlantic frontal depressions along this trailing front...supported by divergence between northerlies flowing around the Bermuda-area upper ridge (paragraph P6) and westerlies flowing into the 966 mb deep-layered vortex.
P3...Cut-off upper trough south of Bermuda persists...and has an east-west extent while compressed between the Bermuda-area upper ridge in paragraph P6 and east Atlantic upper ridge in paragraph P7.
P4...Open Atlantic surface ridge is supported by a few upper convergent sources this evening. Convergence south of the central US upper anticyclone (paragraph P2) supports 1020 mb centers over the SW US and Gulf of Mexico. Convergence SE of Bermuda-area upper ridge (paragraph P6) supports a 1025 mb center east of Bermuda. Convergence east of the north cell of the upper ridge in paragraph P7 supports a 1026 mb center just south of the Azores. In conjunction with the south side of the upper ridge in paragraph P7...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.
...TROPICAL BELT DISCUSSION...
P5...Upper ridge over Central America has de-amplified further into a strong westerly jet as the westward-retrograding Caribbean upper vorticity mentioned in paragraph P7 pushes in.
P6...Upper vorticity in the southern Gulf of Mexico is weakening over the Yucatan and SE Mexico....and the SE US upper trough it left behind has been absorbed by the Ohio Valley upper trough mentioned in paragraph P2. Bermuda-area upper ridge persists east of all this upper vorticity...and westerly advection of warm air associated with the upper ridge...ahead of the NW Atlantic frontal depressions in paragraph P2...has spread this upper ridge eastward. T-storm activity continues persisting over the MS/LA area with associated surface trough. Continued latent heat release from the t-storm clouds continues inflating the south lobe of the central US upper anticyclone (paragraph P2) such that the disturbance has upper anticyclonic outflow that is allowing the disturbance to survive. However...land interaction and high surface pressures from the paragraph P4 ridge continue to limit tropical development potential. If at any time this disturbed weather loses aforementioned latent heat release driven upper outflow...it will then quickly succumb to upper convergence on the south side of central US upper anticyclone and dissipate. Just like last night...there is potential for an impulse of this disturbed weather to shift SW into the SE corner of Texas. The surface trough of this disturbance may soon get absorbed into adjacent 1013 mb low...which is a breakage of the trailing cold front discussed in paragraph P2.
P7...East Atlantic upper ridge persists. Upper vorticity is still west of the upper ridge...located across the Caribbean Sea. A NW Caribbean upper vortex from this vorticity is retrograding westward about the SW lobe of this east Atlantic upper ridge. Although the tropical wave in paragraph P8 has moved out of the area...t-storms persist over the Jamaica...east-Cuba...east-Bahamas area thanks to split flow divergence between the SW lobe of this east Atlantic upper ridge and Caribbean upper vorticity. This east Atlantic upper ridge is now split into three cells thanks to embedded upper trough originating from Canary Islands (which is now breaking into a few upper vortices). The northern cell is near west Europe while associated with warm air advection ahead of the 966 mb cyclone in paragraph P2...the SW cell has an anticylonic center just N of the Lesser Antilles...and the SE cell is located toward Africa.
P8...Tropical wave in the central Caribbean Sea in the previous discussion is now in the western Caribbean and pushing into Central America/SE Mexico. It is in a hositle westerly vertical shear environment thanks to jet mentioned in paragraph P5..although the jet looks directionally divergent based on the 200 mb wind barbs shown in the above atmo birdseye chart. This divergence supports t-storm activity over Panama/Costa Rica and 1008 mb surface low in the ITCZ. Normally I do not mark ITCZ features in the above charts...but this time I have in case it is associated with this tropical wave's south end.
P9...Tropical wave approaching the Lesser Antilles in the previous discussion is now crossing those islands into the eastern Caribbean Sea. It has produced some scattered t-storm clouds...but the expanse of its t-storm activity is limited by the fact it is below non-divergent uniform easterly flow on the south side of the upper ridge in paragraph P7.
P10...The tropical wave (with t-storms and low pressure spin) that rolled off of Africa in the previous discussion is now over the Cape Verde Islands. Low pressure spin is analyzed at 1011 mb per 1800Z NHC TAFB map. Satellite shows yet another spin with this tropical wave that just rolled off of Africa...so either this is the far east extent of this tropical wave or perhaps a highly-amplified mid-level AEJ (African Easterly Jet) trough whose western divergence spawned the tropical wave while it was over Africa. Satellite also shows a decrease in t-storm activity in the last 24 hours...so it may seem the dry air (mentioned in paragraph P4) is affecting this tropical wave. However...the above thermo charts shows moisture with this tropical wave is pushing out some of this dry air...so instead the decrease in t-storms may be from recently moving over the relatively cooler (more stable) low-level marine air as opposed to the hotter low-level (and more unstable) air over African land it was recently over. As the GFS model has been predicting...amplified Canary Islands upper trough in paragraph P7 is breaking into SW retrograding upper vortices. The tropical wave could soon move directly beneath the SE cell of the upper ridge in paragraph P7...the cell directly overhead and upper vortices to the northwest enhancing the upper outflow of this tropical wave. It is also still possible that dry air in paragraph P4 stops this tropical wave from developing altogether.