2012 Atlantic Hurricane Season Birdseye Discussion #5

By: NCHurricane2009 , 4:24 AM GMT on April 17, 2012

...APRIL 16 2012...
This discusion has been released much later than intended due to power outage at my residence. It has actually been released just after midnight into the first minutes of April 17 2012.

1013 mb frontal low east of Bermuda deepens to 998 mb, a rate of 15 mb over the previous 30 hours, while becoming well-organized. Subtropical cyclone possible in the next 36 hours.

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


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1928Z-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 vigorous central US 989 mb cyclone that produced a severe weather risk across parts of Texas, Arkansas, Missouri, eastern Nebraska, and Minnesota has lifted NE into the Great Lakes region as a 994 mb cyclone as marked presently in the above charts. A summary of yesterday's severe weather produced by this 994 mb cyclone is provided in the final section of today's discussion. The equally vigorous upper trough supporting this cyclone follows behind while positioned across the central US. Cold front extends south from the 994 mb cyclone into eastern Texas, where a 1018 frontal mb low is present and also supported by divergence east of the central US upper trough. An upper convergent environment behind the central US upper trough is supporting a disorganized 1024 mb western US surface ridge.

Mexican monsoonal surface trough/low that is normally outside of the scope of the above charts has moved back out of scope (unlike 24 hours ago). Its barely visible in the above charts as a surface trough in east-central Mexico as of 1800Z TAFB.

Upper ridge over eastern US continues to amplify due to continued immense warm air advection ahead of aforementioned 994 mb cyclone presently over the Great Lakes. The amplification of this upper ridge has resulted in a variety of features in the west Atlantic Ocean which are covered later in today's Open Atlantic Waters Discussion below.

As predicted yesterday, 999 mb cyclone over the S tip of Husdson bay has intensified while moving east as it successfully advected in enough cold air behind it to carve a supporting shortwave upper trough which is now present over eastern Labrador. The 999 mb cyclone itself has moved rapidly offshore from Labrador and deepened to 992 mb as labeled in the above charts. Convergence behind the shortwave upper trough supporting 1020 mb ridge center over north-central Labrador.

985 mb cyclone near S Greenland has ejected east-northeast toward the British Isles of Europe as it weakens to 997 mb as shown in the above charts. The northern fragment of supporting west Atlantic upper trough has fractured away while moving with the 997 mb cyclone. The remaining southern fragment of the west Atlantic upper trough has evolved into a cut-off upper low supporting subtropical cyclone formation as has been forecasted by computer models for the last several days.

Deep-layered blocking ridge persists over the Azores...featuring presently an impressive 1036 mb surface center. The S Greenland cyclone is now passing north of the deep-layered ridge...thus it is unable to provide additional warm air advection to support the ridge's upper layers. Warm air advection is now being supplied by southerly flow ahead of developing subtropical cyclone east of Bermuda. Leftover central Atlantic upper ridge axis to the SW of the deep-layered ridge also persists, also supported by warm air advection ahead of developing subtropical cyclone.

Cut-off upper low persists in the far east Atlantic midway between Azores and Cape Verdes now drifting east toward Morroco. Divergence east of the cut-off upper low has been battling dry air that was once produced by what is now the deep-layered ridge over the Azores. The lift created by the upper low's eastern divergence has been successfully "moistening away" the dry air in the region when animating the thermodynamics charts over the last 72 hours. Satellite imagery shows no weather associated with the divergent region of the cut-off upper low, but the observed moistening may allow this to begin at some point.

Upper ridge over the eastern US continues to greatly amplify due to strong warm air advection ahead of aformentioned 994 mb cyclone over Great Lakes. The west Atlantic 1029 mb surface ridge supported by the eastern covnergence of the upper ridge remains stationary and is now at 1028 mb. The amplification of the east US upper ridge has allowed the southern fragment of west Atlantic upper trough to amplify into an upper low supporting subtropical cyclone formation east of Bermuda. Although not mentioned in yesterday's discussion, yesterday's charts showed that the amplification of the east US upper ridge carved out a shortwave upper trough located NW of Bermuda whose divergence supported a light convective mass north of Bermuda. This short-lived shortwave upper trough has been absorbed by the new cut-off upper low vortex E of Bermuda associated with subtropical cyclone development...while the convective mass itself has spun up into a weak 1022 mb low pressure swirl well-defined in visible satellite photos earlier today. This 1022 mb low is not tropical in nature...it got its support from the shortwave upper trough that has been absorbed and therefore it will soon dissipate or get absorbed into the developing subtropical cyclone E of Bermuda.

Hot topic today is aformentioned cut-off upper low vortex that formed E of Bermuda today...and the subtropical cyclone formation its supporting at the surface. The 1013 mb frontal low that formed 1200Z yesterday has intensified to gale force with 998 mb thanks to plenty of divergence east of the cut-off upper low. The 998 mb low has developed an occluded front, which is the first step to developing a low-level warm core for subtropical cyclone formation. It has also whirled NNW into a position beneath the cut-off upper low. The cut-off upper low center itself is non-divergent, but the 998 mb surface low may be allowed to deepen a bit more thanks to divergence caused by split flow between the north side of the upper low and west side of the Azores ridge. Albeit, this split flow divergence is north of the 998 mb center rather than above it, so it may/may not be helpful in deepening the 998 mb low further.

The NNW track and intensification of the 998 mb low has placed it at a thermodynamic disadvantage for subtropical development. Using today's thermodynamic chart, the NNW track has placed it over 19 deg C water, located betwen the 20 deg C isotherm to its south and another 20 deg C isotherm to its north associated with the Gulf Stream. It takes a very cold temp upper low to de-stabilize the atmosphere for convection to begin with water temps that cold. However, even if convection began, it would be cloudless convection at the center of circulation. This is because the 998 mb cyclone wrapped in a significant amount of dry air to its west while intensifying over the last 30 hours. The source of ths dry air is mentioned in yesterday's Caribbean/Gulf discussion...which mentioned the east US upper ridge/west Atlantic surface ridge produced sinking dry air that had spread the northern Gulf dry air mass into the Atlantic S of Bermuda.

Yesterday's discussion mentioned subtropical cyclone formation was not expected before the 17th (tomorrow)...which indeed has become true. The above paragraph suggests it may never happen. However, the NNW track of the 998 mb low is forecast to bend W and even SW as it continues clockwise beneath the cut-off upper low, which will take it away from the 19 deg C isotherm and back to 20 to 21 deg C waters. Water vapor imagery also shows the 998 mb cyclone has warded off the earlier-ingested dry air by wrapping in the moist air induced by the eastern divergence of the cut-off upper low. These factors place the 998 mb low increasingly back toward a more favorable thermodynamic picture.

What happens in the next hours is crucial. If convective cloud tops (with the newly-moistened air) do not develop when the 998 mb low arrives toward 20 to 21 deg C waters, this means the cut-off upper low above it is not cold enough to de-stablize the atmosphere at those water temps, and subtropical cyclone formation will no longer be possible. On the other hand if convective cloud tops begin developing near the center, a special brief update will be issued on this blog declaring imminent risk of subtropical cyclone formation. Given the thermodynamics were unfavorable over the prior 30 hours as discussed above...subtropical cyclone formation potential is now delayed and now possible in the next 36 hours as of this writing. This means it could be as late as midday April 18 before a subtropical cyclone forms.

East-to-west oriented shortwave upper trough in the Gulf of Mexico has been lodged SE into the Bahamas and just N of all the Caribbean islands due to amplifying east US upper ridge. Upper convergence north of this upper trough axis, upper convergence southeast of the east US upper ridge, and surface divergence of the west Atlantic 1028 mb ridge are all teaming for dry sinking air in the N Gulf and. west Atlantic waters S of Bermuda.

Northern South Amercian monsoon upper ridge continues to be choked by west Atlantic upper trough which has now amplified into upper low vortex supporting possible future subtropical cyclone.

New upper ridge amplifying over the Yucatan peninsula of Mexico due to warm air advection ahead of 1018 mb frontal low over E Texas mentioned in today's North America discussion.

A new shortwave upper trough has been carved out in the central Caribbean...in between the new shortwave upper ridge over the Yucatan and upper ridge associated with aformentioned South American monsoon.

(Supercell #1) Weak tornado near Ericson in NE Nebraska. The tornado dissipated, and then the supercell re-produced another brief tornado near the NE Nebraska/SE South Dakota border. The cell continued NE and produced yet another brief tornado in SW Minnesota well to the south of Hutchinson before dissipating.

(Supercell #2) New tornado-producing supercell in SW Minnesota to the east of supercell #1. Signature was just north of Glencoe and not far west of Minneapolis/St. Paul, and then tornado signature quickly dissipated.

(Supercell #3) New tornado warning around 6:21 PM CDT on a disorganized supercell heading ENE in direction of Batesville, Arkansas. The cell quickly became disorganized and elongated north-south by 6:38 PM CDT at a location west of Batesville.

(Supercell #4) Supercell explosively develops just SW of Victoria, Texas tracking ENE by 6:35 PM CDT. Supercell was well-organized with hook echo but not yet tornado warned. At 6:36 PM CDT...it received severe thunderstorm warning. The cell continues east with its hail core passing over Victoria by 7:32 PM CDT. The south end of the hail showed another tornadic-like hook just SE of Victoria at this time. The hook echo dissipated, and the supercell became less organized.

(Supercell #5) Just SW of Supercell #4...a new supercell erupted and also had a severe T-storm warning by 6:53 PM CDT. The cell was located just south of Goliad, TX. It later merged with the remnants of supercell #4.

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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