2012 Atlantic Hurricane Season Birdseye Discussion #9

By: NCHurricane2009 , 5:56 AM GMT on April 29, 2012

...APRIL 29 2012...
Caribbean Sea tropical disturbance still not expected to develop into a tropical cyclone. This is the last discussion until Atlantic Hurricane Season starts June 1 2012.

This is the ninth 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 1929Z-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 disturbance over the last 24 hours has transitioned to a more subtropical state. Instead of being supported by outflow beneath the warm core upper ridge displaced to the east (as in a tropical system)...the thunderstorms have gained greater concentration toward western Cuba and the Cayman Islands while now supported by divergence east of the cut-off upper trough axis located over the E Gulf of Mexico/Yucatan.

CIMSS products from University of Wisconsin show a decrease in the southerly vertical shear levels in a narrow swath along the E Yucatan shore associated with calm upper winds near the upper trough axis. But also... the upper trough axis has brought in dry air from the west which had been generated by convergence west of the upper trough axis. This dry air is why there is a distinct absence in thunderstorms in the NW Caribbean Sea.

With this disturbance now more supported by divergence E of the upper trough axis...the fate of it will be associated with how this upper trough evolves. First off...the TAFB analyses have dropped all surface troughs supported by the upper trough...suggesting insufficient divergence to support surface pressure drops. The GFS model however redevelops a surface trough in the Florida Straits. But also...the models de-amplify the upper trough and move it east which will increase westerly vertical shear and reduce the supportive divergence east of the upper trough axis...and no subtropical/tropical development will occur as a result. The GFS model eventually dissipates its Florida Straits surface trough while it moves west into the Gulf of Mexico while steered by the surface ridge due north.

As for the rainfall...the past 24 hours have seen the thunderstorms concentrate more westward over the Cayman Islands...west Cuba..the western Bahamas...and SE Florida while becoming more associated with divergence E of the upper trough axis. Based on the models drifting the upper trough eastward and de-amplifying it...I expect this rain mass to gradually shift east and weaken.

The 997 mb extratropical cyclone over E Maine has intensified to 982 mb and moved northward into the east coast of Ontario. Strong southward cold air advection behind the intensifying cyclone has amplified its supporting upper trough into an upper low vortex now just SW of the 982 mb center. The 982 mb cyclone has absorbed what was a 1003 mb cyclone in north-central Ontario 24 hours ago.

The western US upper trough mentioned in the previous discussion has become cut-off into an upper low vortex over the W Dakotas...to the south of a Manitoba upper ridge in the westerlies. Computer models re-link the cut-off upper low vortex with the westerlies while merging it with the next upper trough in the westerlies. Meanwhile the surface cyclone supported by this upper disturbance has weakened and moved NW while steered around the cut-off upper low vortex. The main center of this cyclone is 1006 mb over SW North Dakota. The cold front once attached to this cyclone has a 1005 mb center over S Oklahoma...1011 mb center over Missouri/Illinois border...and 1015 mb center over E Kentucky.

Convergence E of the aforementioned Manitoba upper ridge supports a strong surface ridge of 1030 mb just SW of Hudson Bay and another surface ridge center of 1028 mb near E Michigan.

The cold front mentioned in the previous discussion...running from Greenland to the waters E of Bermuda and N of Puerto Rico...has mostly dissipated with the exception of a segment marked toward the NE corner of the above charts. The position of this remnant segment is based on an eastward moving cloud band in satellite imagery. The upper trough associated with the diminished cold front has fractured into one upper trough axis NW of the Azores and another upper trough axis to the SW of the Azores. Divergence from the upper trough SW of the Azores is supporting cloudiness well S of the Azores and SW of the Canary Islands.

Surface trough located midway between the Cape Verde Islands and Lesser Antilles continues westward while steered by the greater-than-1028 mb surface ridge NW of the Azores. It continues to be inactive while beneath convergent upper-level NW flow.

Amplified upper trough in the E Atlantic continues. The NW Africa/W Europe cold front supported by its eastern divergence is now barely within the scope of the above charts as of 1800Z TAFB. Its divergence has also has generated a surface trough also marked in the above charts running from near the Azores to Portugal. Convergent upper-level NW flow west of this upper trough axis continues to support the greater-than-1028 mb surface ridge NW of the Azores.

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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