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By: NCHurricane2009 , 8:14 PM GMT on September 16, 2012
...SEPTEMBER 16 2012...4:15 PM EDT...
A surface trough in the western Gulf of Mexico has been introduced into the National Hurricane Center Tropical Weather Outlook and is upgraded to tropical disturbance Invest 93-L. The surface trough appears to have originated from the decaying upper trough and surface frontal system driven by the remnant of Leslie days ago:
Paragraph P2 in mid-latitudes discussion...Discussion #104
Paragraph P2 in mid-latitudes discussion...Discussion #105
Paragraph P2 in mid-latitudes discussion...Discussion #106
Paragraph P2 in mid-latitudes discussion...Discussion #107
Paragraph P3 in mid-latitudes discussion...Discussion #108
It should be noted that by discussion #106...the decaying front had dissappeared from the Gulf of Mexico in surface anaylyses. However...analyses had added a surface trough in the western Gulf of Mexico by discussion #108 as seen in the atmospheric birdseye chart of that discussion. It now appears in the last 36 hours that this surface trough broke off from the main portion of the decaying frontal boundary while steered westward by the eastern US low-level ridge mentioned in paragraph P2 of discussion #108.
Currently this surface trough is under hostile westerly vertical shear and has unimpressive t-storm activity...but is becoming increasingly supported by the SW US upper trough mentioned in paragraph P2 of discussion #108 as the upper westerly flow is becoming increasingly divergent in advance of this upper trough. A check with the 12Z GFS model provides a 72-hour outlook. The SW US upper trough during this timeframe will be merging with the paragraph P1 upper trough...with the system moving northeastward into the northern US Gulf coast in deep-layer southwesterly flow developing ahead of the paragraph P1 frontal system. While eastern divergence ahead of the merged upper trough will provide a mechanism for increased t-storm activity...the merged upper trough is not forecast to develop a cut-off upper vortex that moves over the disturbance to reduce westerly shear for subtropical development. Instead of a fully-tropical solution...I prefer a more subtropical solution due to the interaction with the upper trough...but again without a shear-reducing cut-off upper vortex...I expect no development in this repsect either. Therefore...I am forecasting this to simply be a rainfall-enhancer for the northern US Gulf coast and much of the SE US...and will not be introducing this as a special feature in my next full discussion.
In the meantime...return to full discussion #108 for an assessment on the rest of the Atlantic tropics.
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