2012 Atlantic Hurricane Season Birdseye Discussion #23
...JUNE 7 2012...11:59 PM EDT...
After the unusual pre-season tropical activity in the Atlantic...no signs of new tropical activity as the Atlantic Hurricane Season officially starts.
The unusual pre-season activity appears associated with the equally unusual "loopy" jet stream (with high amplitude upper troughs and ridges) seen this year. Every instance of pre-season activity was associated with cut-off upper vortices left behind by the loopy jet stream. Divergence on the periphery of the cut-off upper vortex triggers a surface cyclone...the shear reducing when the surface cyclone aligns with the upper vortex. Cold temps of the upper vortex de-stabilizes the atmosphere for t-storms and tropical development of the surface cyclone...in some cases above waters less than 26 deg C. For example...see the following statements made earlier on the blog this year concerning how cut-off upper vortices triggered subtropical/tropical development:
Discussion #1...4th and 5th paragraphs of west-to-east discussion...for prelude to Invest 91-L
Discussion #2...4th 5th and 6th paragraphs of west-to-east discussion...for prelude to Invest 91-L
Discussion #4...5th and 6th paragraphs of Open Atlantic Waters Discussion...for prelude to Invest 91-L
Discussion #10...special feature section...for Invest 92-L
Discussion #12...1st special feature section...for prelude to Tropical Storm Alberto
Discussion #17...Special feature section....for Tropical Storm Beryl
As the jet stream lifts northward during the peak summer months...we are seeing fewer cut-off upper vortices capable of tropical development...and so the source of development is now transitioning mainly to tropical waves of African origin. As tonight's birdseye charts show...conditions are unfavorable for a tropical wave to develop. The eastern tropical Atlantic is covered by dry air (paragraph P6). The Gulf of Mexico and eastern Caribbean are seeing unfavorable vertical shear. The western Caribbean is seeing upper ridging (paragraph P5)...but the upper ridge is oscillating in and out of the western Caribbean. When it oscillates away into the eastern Pacific (like it is tonight)...it provides unfavorable westerly and northerly vertical shear. When it oscillates back into the western Caribbean...it reduces the shear and enhances outflow favorable for tropical development.
Concenring the tropical belt...the GFS model thru June 23 shows the aforementioned unfavorable patterns continuing...so the only possibility of tropical development during this window is if a tropical wave can sneak under the west Caribbean upper ridging on an oscillation back into the area. In fact...the GFS model today flares up the west Caribbean upper ridge and develops a tropical cyclone beneath it towards June 23.
Concerning the mid-latitudes...the GFS model has backed off developing a cut-off upper low in the open central Atlantic (June 13 to 15). Yesterday...the GFS model had developed this cut-off upper low from the weather system currently 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 1950Z-released HPC analysis.
In light blue is upper air anlaysis, 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 indicating surface lows, Hs indicating 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...West US upper trough supports a surface frontal cyclone in SW Canada. This cyclone in the last 24 hrs has actually tracked NW toward another moving into the west coast of North America. Warm air advection ahead of this cyclone still supports an upper ridge from Texas to Manitoba...and another upper ridge axis over New Mexico. An upper trough over S Texas formed between these two upper ridge axes yesterday...perhaps where warm air advection has not been as strong. This S Texas upper trough has amplified into an upper low.
P2...Longwave upper trough over eastern US is moving into the western Atlantic. It still consists of a SW-NE-tilted axis and upper vortex over Atlantic Canada (cut-off to the SW of paragraph P4 upper anticyclone). Divergence ahead of the SW-NE upper trough supports a stalled front over the SE US that extends to a 996 mb cyclone that has made landfall over SE Newfoundland. This surface cyclone has moved underneath the upper vortex...resulting in a deep-layered low altogether where the surface cyclone will now begin weakening benath the less divergent upper vortex. Convergence behind the SW-NE upper trough (with respect to paragraph P4 upper anticyclone) supports a 1021 mb central US ridge...1013 mb ridge over NE Canada...and 1020 mb ridge over S Greenland.
P3...Longwave upper trough regime across the entire high seas of the Atlantic continues breaking up...becoming replaced by a new upper anticyclone in the above birdseye charts...located S of the Azores...and caused by warm air advection ahead of the 996 mb cyclone in paragraph P2. Remnant upper trough and associated surface cyclone approaching western Europe have made landfall. Remnant cut-off upper trough in eastern Caribbean is moving W into the central Caribbean while steered by Caribbean upper ridging in paragraph P5. Remnant cut-off upper low (midway between Lesser Antilles and Azores) is being pushed south by new upper anticyclone mentioned earlier in this paragraph. Remnant upper trough over the Canary Islands is now cut-off over the Cape Verde Islands...to the SE of new upper anticyclone mentioned earlier in this paragraph. Surface 1027 mb open Atlantic ridge in this area has weakened to 1025 mb in the last 24 hrs.
P4...Full-fledged and anomalous upper anticyclone over the east coast of Canada persists.
...TROPICAL BELT DISCUSSION...
P5...Long-lived Caribbean upper ridging persists. Accelerationally-divergent upper westerly jet across the Gulf of Mexico persists due to the pressure gradient between this upper ridge...upper trough in paragraph P2...and S Texas cut-off upper low in paragraph P1. Disturbed weather and surface troughs produced by the upper jet's divergence is becoming assimilated into SE US stalled front mentioned in paragraph P2.
P6...Upper ridge in E tropical Atlantic is losing definition against new upper anticyclone mentioned in paragraph P3. Thermodynamics birdseye charts (such as the above) continue suggesting a westward expansion of dry air from Africa's desert...supported by deep-layered easterly flow south of the 1025 mb ridge (paragraph P3)...south of the E Atlantic upper ridge (mentioned in this paragraph)...and south of new upper anticyclone (paragraph P3).
P7...Tropical wave in the eastern Caribbean has been removed from TAFB analyses today as of 1200Z...and likewise has been removed from the above birdseye charts.
P8...Tropical wave that was S of the Cape Verde Islands 24 hrs ago has moved westward...and is now WSW of those islands. Its still suppressed by dry air mentioned in paragraph P6.
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