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By: NCHurricane2009 , 6:52 PM GMT on May 12, 2012
...MAY 12 2012...
Surface low pressure center with characteristics of a subtropical storm is south of the Azores. It is now a subjective call on whether or not the National Hurricane Center will declare this system. See special features section below for further details.
This is the tenth 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 0600Z, and the 0727Z-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...SUBTROPICAL LOW SOUTH OF THE AZORES...INVEST 92-L...
For much of the first half of May...a semi-permanent upper trough spanned the Caribbean Sea northeastward into the Atlantic for several hundred miles. The above charts today show another upper trough/surface frontal system off of the eastern United States coast...with warm air advection ahead of this system promoting a deep-layered ridge presently northwest of the Azores. The development of this deep-layered ridge has cut-off the semi-permnanent upper trough into a few features:
The first feature is a remnant weak upper trough over the Lesser Antilles (in the above charts) whose eastern divergence is promiting weak cloudiness east of the Lesser Antilles.
The second feature is far more significant...a deep-layered low pressure south of the Azores that has managed to gain tropical characteristics over cooler waters around 20 to 21 deg C. The thermodynamics for such mild-water tropical development are no stranger to this part of the world (e.g. Hurricane Vince 2005 and Tropical Storm Grace 2009). In these cases...a cut-off upper low produces a surface frontal low with its boundary upper divergence...and the surface frontal boundary low eventually whirls into a position beneath its cut-off upper low (hence the formation of a deep-layered low with low wind shear). The cold temps of the upper low allow for instability despite the mild surface water temps...and sustained convective thunderstorms develop at the surface low. The surface low...which develops a low-level warm core due to its occluded front...sees the warm core grow vertically due to the latent heat release of the thunderstorms...and a subtropical depression/subtropical storm forms.
These processes have taken place with the 1009 mb surface low in the above charts south of the Azores. The Naval Reasearch Laboratory (NRL) took notice and upgraded the system to Invest 92-L this morning. As breaking news...the National Hurricane Center issued a Special Tropical Weather OUtlook while I was writing this discussion (at 2 PM EDT)...giving this a 40% chance of become a subtropical cyclone in the next 48 hours.
My personal opinion is more aggressive...as I believe this is already a subtropical storm. This is because the surface low's circular convective canopy (albeit a small diameter one) has been seperate from all (non-tropical) frontal boundary clouds...and moreover this ball of convective clouds has been sustaining itself for several hours. Features of this convective ball of clouds are shown in Figure 1.
Figure 1 shows a faint banding-type eye...but more impressively signs of anticyclonic cirrus outflow in the northern semicircle shown by the light blue arrows. This could be a sign that the surface low's vertical warm core has grown enough in height to promote high pressure upper-level anticyclonic outflow as a fully tropical system has.
This surface subtropical low is 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. The next feature that could affect its steering is the upper trough/surface frontal system off of the eastern US...but this next system is highly amplified and somewhat cut-off from the jet stream while forming a cut-off upper low just N of Maine. 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. If it ever does begin moving toward Europe...the water temps are much colder...so whatever tropical characteristics the subtropical low has now will be long gone by that time.
Figure 1: Features shown by the clouds of subtropical low Invest 92-L
The upper-levels in this part of the world today are in a west-to-east ridge-trough-ridge pattern...and so today's general discussion will sweep from west-to-east.
A large upper trough over the central US supports a very long surface cold front Hudson Bay in Canada (near a 1010 mb low)...through a 1017 mb frontal low in Wisconsin...to a 1005 mb low at the California/Arizona border. This upper trough has recently absorbed the upper-levels of a cut-off system that had been producing severe weather over Texas over the last days. What's left of this cut-off system at the surface is a 1013 mb weak frontal low in SE Texas. Convergence behind (west) of the upper trough axis supports a 1029 mb surface ridge over W Nebraska.
Southerly advection ahead of the large central US frontal system has pumped in warm air...resulting in a prominent and amplified upper ridge axis starting from Guatemala...across the Gulf of Mexico...across the eastern US...and into eastern Canada. Eastern convergence of the upper ridge supports a surface ridge of 1026 mb over the Virginias...and another 1025 mb center on the Carolina coast. Eastern convergence of this upper ridge and surface divergence from the 1025 and 1026 mb centers supports sinking, dry air in the western Atlantic...the driest in the basin.
Upper trough/surface frontal system is in the western Atlantic off of the eastern US. The amplification of the aforementioned upper ridge to its west has caused the upper trough itself to also get more amplified...resulting in an upper low along the upper trough axis just N of Maine. The remainder of the upper trough has fractured away to a position just SW of Greenland...with divergence east of this fracture supporting a 1011 mb surface frontal low just SE of Greenland. The upper low just N of Maine has caused a 1009 mb low to whirl cyclonically. A cold front extending from this 1009 mb low reaches the waters just N of the Bahamas. It is here where there are two features ahead of the cold front.
The first feature is a weak surface trough and band of T-storm clouds near and over the eastern Bahamas...both of which are supported by the frontal upper trough's divergence.
The second feature is more interesting...a mass of T-storm clouds east of the Bahamas and N of the Lesser Antilles. Very recently...1200Z TAFB added a surface trough below these clouds (not shown in the above charts). This more impressive T-storm mass was initialized by eastern divergence ahead of the frontal upper trough. Latent heat release with these T-storm clouds has caused upper-level warming such that an outflow enhancing upper ridge is forming. If this upper ridge continues to amplify...it could promote yet another tropical disturbance in this area in addition to today's special feature Invest 92-L.
The remaining features in the eastern half of the Atlantic are associated with what was once a semi-permanent upper trough and now is a weak upper trough over the Lesser Antilles and 92-L south of the Azores. These are covered in the special feature section above. However...what is not mentioned in the special feature section is that the upper low above 92-L is supporting a surface trough off on the NW African coast with its eastern boundary divergence.
Deep-layered ridge SE of Newfoundland and NW of the Azores has formed due to warm air advection ahead of aforementioned 1009 mb cyclone near N Maine. This deep-layered ridge is also noted in the special features section.
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