2012 Atlantic Hurricane Season Birdseye Discussion #7

By: NCHurricane2009 , 2:52 AM GMT on April 19, 2012

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...APRIL 18 2012...
Invest 91-L running out of time to become a subtropical cyclone. This could be the final full discussion until hurricane season starts June 1 2012 (see today's special feature section below for details).

Invest 91-L has been moved from the Open Atlantic Waters discussion (where it has been mentioned in the last days) to its own special features section. This has been at the request of fellow blogger "KoritheMan." From now on each area of disturbed weather that is deemed a threat for tropical development on this blog will get its own special features section. All active tropical/subtropical cyclones will each get their own special features section as well. Of course this will add a challenge as what I deem a threat for tropical development may/may not align with the opinion from the National Hurricane Center and other fellow bloggers.

This is the seventh birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1916Z-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).

...SPECIAL FEATURE INVEST 91-L...
Hot topic continues to be surface low Invest 91-L east of Bermuda, which has weakened from 1002 mb to 1006 mb over 24 hours as it struggles in what is a non-divergent environment beneath its parent upper low. The long-awaited SW track into 20 to 21 deg C waters has finally occured, which was anticipated in the last two discussions of this feature. Yesterday's discussion of this feature was at a climax...waiting to see if persistent convective cloud tops were to develop when this low arrived to those waters. It was concluded that if convective cloud tops did not develop, the parent upper low above was not cold enough to de-stablize the atmosphere at those water temps and subtropical cyclone formation discussions would be cancelled.

Well...the surface low has actually developed bouts of convective cloud tops today in intermittent small circular shapes just E of center. Meanwhile clouds west of center are suppressed by neighboring dry air mass S of Bermuda (see final paragraph of today's Open Atlantic waters disucssion for source of this dry air).

Despite looking more tropical with these intermittent convective bursts, the National Hurricane Center TAFB analyses keep handling it as a non-tropical entity. Yes they have downgraded the original cold front...which is now a long surface trough (red dashed line) E of 91-L in the above charts. But the surface trough attached to 91-L's center in yesterday's charts has been upgraded to a cold/occluded front in today's analyses from TAFB. 91-L has reached the warmest waters it will ever see as its track is now bending E and eventually N as it rounds the SE side of its parent upper low. Therefore if today's developments are not sufficient for the National Hurricane Center to declare subtropical cyclone formation, 91-L then is running out of time to become a subtropical cyclone.

What is causing 91-L to run out of time is approaching central US upper trough and surface front from Florida panhandle-North Carolina-Newfoundland seen in the above charts and mentioned in today's North America discussion below. This approaching frontal system is expected to absorb all vertical layers of 91-L beginning April 19 (tomorrow) into early on the 20th. I plan to issue a special update around lunchtime tomorrow cancelling these Atlantic birdseye discussions altogether until June 1 2012 (when Atlantic hurricane season officially starts). I am not yet cancelling these birdseye discussions tonight as I am on standby...waiting to see if the National Hurricane Center unexpectedly updates this system into a subtropical cyclone between now and lunchtime tomorrow.

...NORTH AMERICA DISCUSSION...
A frontal system was newly-introduced to this discussion yeserday as it charged across the NW US with a 1012 mb low in NW North Dakota. With the support of its upper trough, this low has since deepened to 1003 mb over the W Great Lakes as shown in the above charts. Its supporting upper trough has merged with the central US upper trough also mentioned here yesterday. Cold front extends from the 1003 mb low to a new 1011 mb low over SE Colorado. The upper divergence supporting this 1011 mb low is a bit complicated. Based on the current state of 200 mb wind barbs, this low appears to have developed in split flow west of the two upper troughs just before they merged. The split flow was characterized by NW flow into the central US upper trough and W flow into the northern upper trough that has merged with the central US upper trough. Upper convergence west of the central US upper trough supports surface ridge centers of 1022 mb over east-central Mexico, 1023 mb at the Arkansas/Missouri border, and 1019 mb at the Dakotas.

Yesterday's 1007 mb cyclone near Maine is now over Newfoundland at 1015 mb. Yesterday it obtained its own supportive upper trough fragment that had split from the central US upper trough. This small upper trough is now over Ontario and Nova Scotia in the above charts. Upper convergence behind this upper trough supports surface ridge centers of 1027 mb over Ontario, 1026 mb near New York, and 1025 mb near N Virginia.

Lengthy cold front continues to extend from what is now the 1015 mb Newfoundland low to a 1016 mb low over North Carolina (was 1019 mb in yesterday's discussion) and 1013 mb low over the Florida panhandle (was 1018 mb over E Louisiana in yesterday's discussion). All of these surface features continue to receive supportive divergence from the central US upper trough.

...DISCUSSION OF OPEN ATLANTIC WATERS...
Yesterday's discussion mentioned a 1001 mb cyclone over SE Greenland that today is still in the above charts at that location and now at 1002 mb. Yesterday's discussion also mentioned it was sharing supportive upper trough divergence with the 1007 mb cyclone near Maine (that is now 1015 mb over Newfoundland as remarked in today's North America discussion).

Extratropical (non-tropical) cyclone that made landfall in the British Isles mentioned yesterday is still clinging on to the scope of above charts as 1800Z TAFB still shows its tailing cold front over the N Canary Islands and Morocco. The upper trough associated with it has absorbed the E Atlantc cut-off upper low that was on its way to Morroco mentioned in yesterday's open Atlantic waters discussion. This absorption has resulted in a long upper trough axis in the far east in the above charts.

As predicted in detail during yesterday's open Atlantic waters discussion, the deep-layered ridge over the Azores has re-developed. It has a 1026 mb surface center in the above charts. Leftover E Atlantic upper ridge axis to the SW of the deep-layered ridge also persists with the continual support of warm air advection ahead of Invest 91-L.

Upper ridge that entered the W Atlantic yesterday is de-amplifying today. The west Atlantic 1024 mb surface ridge once supported by the eastern covnergence of the upper ridge remains stationary and has weakened further to 1022 mb. The de-amplification of the upper ridge and weakening of surface ridge is occuring as it becomes squeezed between low pressure features....including Invest 91-L to its east...the surface front extending from Florida Panhandle-North Carolina-Newfoundland in today's North America discussion...and the central US upper trough also in today's North America discussion. Yesterday's Caribbean/Gulf discussion mentioned sinking motion from these ridges creating a lot of dry air in the waters S of Bermuda. This dry air continues today as seen in the above thermodynamics chart.

...CARIBBEAN SEA DISCUSSION...
Central Caribbean shortwave upper trough mentioned in yesterday's Caribbean/Gulf discussion has been absorbed by upper low vortex over Invest 91-L.

Upper ridge over the Yucatan Peninsula has accelerated east and is now over Cuba and near Jamaica in the above charts. Divergence west of this upper ridge and east of the central US upper trough is increasing the moisture content and convective cloudiness in the western Caribbean region.

South American monsoon upper ridge shifted westward toward Central America yesterday, but has re-shifted back east over South America today.

A new shortwave upper trough is marked in the above charts just north of Panama, carved out in between the South America monsoon upper ridge and aformentioned upper ridge near Jamaica and Cuba.

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