2012 Atlantic Hurricane Season Birdseye Discussion #11

By: NCHurricane2009 , 6:35 PM GMT on May 13, 2012

...MAY 13 2012...
Subtropical low south of the Azores not expected to be upgraded to a named storm or numbered depression. This is the last birdseye discussion until June 1 2012 unless yet another early tropical disturbance develops before then.

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

Any feedback on how to improve these discussions can be left in the comments section below...and some of these ideas may be implemented in time before hurricane season officially starts June 1. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0729Z-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).

The formation of this subtropical surface low is best explained in the special feature section of yesterday's discussion #10. This surface low has since moved north and then west around the northeastern semicircle of its parent upper low...which took it from waters 20 to 21 deg C twenty-four hours ago...to now waters of 19 to 20 deg C. Its central pressure has risen from 1009 to 1010 mb in the last day. This is because it has no upper divergence to work with as in a non-tropical system while beneath the non-divergent upper low center. Its convective clouds have also weakened such that no warm core upper-level outflow is present to strengthen it as in a tropical cyclone.

The loss in convective clouds may be associated with its travel over slightly cooler waters...and also the fact that the cut-off upper low (like all cut-off upper features) is gradually weakening. With the cold core upper low above it weakening (warming) and with cooler surface waters...the surface to upper air temp contrasts necessary for instability and convective clouds is diminishing...and it is very unlikely the National Hurricane Center will be upgrading this to a named storm or numbered depression. As noted in yesterday's discussion #10...the system had the characteristics of a subtropical storm (or even a tropical storm with cirrus outflow clouds)...so it is possible that the National Hurricane Center considers this an unnamed storm in the post-season analysis.

This surface subtropical low remains trapped in the southeastern "armpit" of a deep-layered ridge (with surface center of greater-than-1032 mb in the above charts). Moreover...the subtropical surface low is also trapped directly beneath its parent upper low. Both of these facts mean the subtropical low should meander for the next several hours while making cyclonic loops beneath its upper low. Although these cyclonic loops may take it intermittenly southward over warmer waters...honestly we are only talking about water temp fluctuations of +/- 1 deg C within the cyclonic loops...not enough to compensate for the fact that the upper low above continues to weaken (warm). Therefore...I am not expecting much re-development of convective clouds.

The next feature that could affect its steering is the upper trough/surface frontal system off of the eastern US (in the western Atlantic)...but this next system is highly amplified and somewhat cut-off from the jet stream as noted in today's west-to-east discussion below. Therefore it may take a while for this next upper trough/surface front to work its way eastward. If/when this next frontal system erodes the deep-layered ridge to the NW depends on if/when the subtropical low begins acceleration to the east-northeast towards Europe. It may very well dissipate before this next frontal system gets a chance to steer it toward Europe.

Large upper trough over the central US has fractured into two upper trough due to the strength of the adjacent upper ridge to its east. The northern fracture is over Canada's Hudson Bay whose eastern divergence supports a cold front (with 1016 mb low) and whose western convergence supports a 1026 mb ridge over Wisconsin. The southern fracture remains over the central US...and has picked up yesterday's weak 1013 mb frontal low over SE Texas and sent it northeastward into the SE US...and this frontal low is presently 1016 mb over Mississippi in the above charts. Divergence on the central US upper trough's SE end also supports a weak surface trough in the Bay of Campeche (in the SW Gulf of Mexico). Convergence on its west side of the central US upper trough has strengthened the 1028 mb W Nesbraska ridge to 1029 mb.

Southerly warm air advection ahead of the aforementioned 1016 mb frontal low continues to support an amplified upper ridge starting from Honduras/Guatemala...across the E Gulf of Mexico...and into the eastern US. Eastern convergence of the upper ridge supports a surface ridge of 1027 mb near Cape Hatteras, NC. Eastern convergence of this upper ridge and surface divergence from the 1027 mb center supports sinking, dry air in the western Atlantic. However...this dry air wedge has shrunk in size as upper westerlies from the upper ridge axis have wafted moist air eastward from the 1016 mb frontal low and into the wedge of dry air.

Upper trough/surface frontal system contines in the western Atlantic off of the eastern US. Its northern fracture SW of Greenland 24 hours ago has raced off to the SE of Greenland...and is now supporting a strengthening surface cyclone (1011 mb in yesterday's charts) all headed toward northern Europe. Behind this northern fracture...a shortwave upper ridge has built in SW of Greenland and cut-off the remainder of this upper trough in the western Atlantic. The 1009 mb cyclone associated with this cut-off west Atlantic upper trough (near N Maine 24 hrs ago) has weakened to a 1019 mb triplet of centers as shown in the above charts. Cold front continues to extend south of this now-disorganized cyclone. Two distinct features mentioned yesterday remain ahead of this cold front's south end:

The first feature was a weak surface trough near the east Bahamas created T-storm clouds over the east Bahamas and Hispaniola (Haiti and the D.R.) yesterday. This surface trough has dissipated and is no longer in the above charts.

The second feature was a mass of T-storm clouds N of the Lesser Antilles which was upgraded to a surface trough at 1200Z TAFB yesterday (and this surface trough is still in the above charts). Latent heat release with these T-storm clouds had caused upper-level warming such that an outflow enhancing upper ridge formed overhead...and I was watching this system for possible development into a tropical disturbance. This is no longer the case as the T-storm clouds have basically fizzled. It appears this system coughed on the dry air wedge to its west...this dry air wedge mentioned three paragraphs ago. The dissipation of T-storm activity has caused the overhead upper ridge to diminish...and upper winds are no longer favorable for any sort of tropical development.

The weak upper trough over the Lesser Antilles (mentioned in yesterday's special feature section and yesterday's west-to-east discussion) has actually amplified due to the formation of the upper ridge associated with the surface trough/T-storm mass that was located N of the Lesser Antilles. It has amplified to the degree there is a weak upper low at its north end presently marked in the above charts. Far to the west of the Lesser Antilles upper trough is a convective T-storm mass that has been over Colombia and Panama in the far south Caribbean...which is indirectly related to the upper trough itself. Northerly upper-level flow over Colombia and Panama heavily split...some splitting off eastward into the Lesser Antilles upper trough...some splitting off westward around the flow of the upper ridge mentioned four paragraphs ago. The highly divergent split upper flow coupled with surface ITCZ convergence is causing these T-storms over the Panama/Colombia area.

Deep-layered ridge SE of Newfoundland and NW of the Azores continues due to warm air advection ahead of the disorganized cyclone (with triple 1019 mb centers) mentioned four paragraphs ago. This deep-layered ridge is also mentioned in today's special feature section.

Finally...surface trough along NW Africa continues...and reaches into southern Spain in today's TAFB analyses. This surface trough continues to be supported by divergence along the eastern boundary of the parent upper low above Invest 92-L.

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