2012 Atlantic Hurricane Season Birdseye Discussion #22

By: NCHurricane2009 , 4:03 AM GMT on June 07, 2012

...JUNE 6 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 tropical belt discussion shows...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 (see paragraphs P5 and P7). 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 22 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.

Concerning the mid-latitudes...the GFS model shows a decent cut-off upper low in the open central Atlantic (June 13 to 15) originating from the weather system currently mentioned in paragraph P2. Could this cut-off upper low trigger tropical development like we have been seeing in the pre-season? So far...GFS does not show anything very impressive at the surface in association with the forecasted cut-off upper low.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1324Z-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.


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...Divergence east of a west US upper trough supports a surface frontal cyclone that has moved from Montana into SW Canada. Warm air advection ahead of this cyclone supports an upper ridge from Texas to Manitoba...and another upper ridge axis over New Mexico. An upper trough over S Texas has formed between these two upper ridge axes...perhaps where warm air advection has not been as strong.

P2...Longwave upper trough regime over eastern US continues. It still consists of a SW-NE-tilted upper trough and upper vortex near 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 997 mb cyclone offshore of Atlantic Canada. Convergence behind the SW-NE upper trough (with respect to paragraph P4 upper anticyclone) supports a 1022 mb Michigan ridge and 1026 mb ridge over the east coast of Canada.

P3...Longwave upper trough regime across the entire high seas of the Atlantic is breaking up today. Its upper vortex is no longer cut-off...and is now accelerating as an upper trough toward western Europe. Decaying surface cyclone and its cold front is also heading toward western Europe. Other features of this regime include an upper trough that is becoming cut-off into the eastern Caribbean...another upper trough midway between the Antilles and Azores that has become a cut-off upper low...and a third upper trough over the Canary Islands. Surface 1027 mb open Atlantic ridge was supported by convergence W of the Canary Islands upper trough 24 hrs ago...but is now being supported by convergence on the NW side of the cut-off upper low midway between the Antilles and Azores.

P4...Full-fledged and anomalous upper anticyclone over S Greenland 24 hrs ago is now over the east coast of Canada tonight.

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 and upper trough regime in paragraph P2. The divergent aspect of the upper jet supports continued t-storms across the Gulf of Mexico...and a new surface trough in the W Gulf of Mexico. Westerly aspect of the upper jet continues to shear these t-storms and prevent tropical development.

P6...Upper ridge in E tropical Atlantic has expanded westward toward the Lesser Antilles...the expansion caused by the weakening of adjacent upper trough regime in paragraph P3. Animation of thermodynamics birdseye charts (such as the above) since June 1 suggests a westward expansion of dry air from Africa's desert...supported by deep-layered easterly flow south of the 1027 mb ridge (paragraph P3)...and south of the E Atlantic upper ridge (mentioned in this paragraph).

P7...A tropical wave has crossed the Lesser Antilles (into the eastern Caribbean) and remains at the west end of paragraph P6 upper ridge...where it has found enhanced poleward outflow and hence why it has persistent t-storms. However...this tropical wave is battling southerly vertical shear from the east Caribbean cut-off upper trough (mentioned in paragraph P3).

P8...A tropical wave is S of the Cape Verde Islands and towards the east end of paragraph P6 upper ridge. This tropical wave is suppressed by dry air mentioned in paragraph P6.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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