WEATHERAmerica Special Statement, Wednesday Evening, April 11, 2007, 7:20 PM ET

[Larry_Cosgrove]

Multifaceted Storm Threatens Much Of Southern And Eastern U.S. With Wild Variety Of Weather, Including Severe Thunderstorms, Tornadoes, Strong Winds, Heavy Rain, And Coastal Flooding This Weekend

(Disclaimer And Copyright Statements At End Of The Discussion)


The disturbance that is now arriving on the West Coast is not just your "garden variety" storm.

Rather, before it exits the scene into the Atlantic Ocean sometime Tuesday night, it will leave a wake of destruction, broken weather observation records, and economic loss. And this chaos will happen in a variety of ways: from severe weather to conditions associated with late season Nor'easters: torrential stratiform rain and coastal flooding associated with winds generated by a tight pressure gradient.

As viewed on satellite, the core of the system is now over WA and OR. But the low is represented aloft by a full-latitude trough. Also connected to the trough complex is the subtropical jet stream, which acts in two ways upon approach to the lower Great Plains: driving a hot and dry cT regime from northern Mexico into the NM and W TX while at the same time supplying moisture from the equatorial Pacific Ocean and eventually the Gulf of Mexico. The latter process is helped by a strong, flat heat ridge lying across the Caribbean Sea.

The actual surface reflection of the cyclone will probably emerge near Tucumcari NM within the next 24 hours. During the following two days, we can expect to see the low move eastward, very close to the KS/OK border and then into S KY. A very intense center of vorticity at 500MB (an astounding 66 units according to the NAM will accompany the disturbance, along with a textbook mPk/cT/mT convergence point, and a 130 knot jetlet at 250MB. North of the low, heavy stratiform rain will likely affect KS and MO. Where capping is not an issue (above a line from Stephenville TX to Mansfield LA), supercell thunderstorms will organize. The intense convection should continue eastward through Friday and Saturday, involving most of Dixie (with a particularly dangerous situation looming for E GA....N FL....GA....SC Saturday afternoon and evening). On the night of April 14, the clusters of thunderstorms will give rise to secondary cyclogenesis near Columbia SC. And thus will begin the final and possibly most intense phase of this feature.

Tapping both deep tropical moisture and grabbing an easterly fetch off of the Atlantic Ocean (set up by the pressure gradient against the lingering anticyclone in QC), heavy rainfall will spread from NC into VA....DC....MD....DE....S NJ on Sunday morning. Some of the rain may be accompanied by strong thunderstorms, as the low center tracks just west of the Atlantic shoreline. Since the low will be vertical with its very cold 500MB counterpart (530dcm), the thermal conflict and unstable setting will make for strong UVVs. The rain should reach Boston MA late Sunday night, and most of the Interstate 95 corridor from Richmond VA to Portsmouth NH will be hit with anywhere from 3 to 6 inches of rain. The super-tight pressure gradient will mean strong winds and coastal flooding into Monday, at which time the low will stop its northward march and begin to wobble eastward. Still, impressive northerly winds and moderate to heavy rain bands will likely not cease in the coastal areas of the Mid-Atlantic and New England states until the morning of April 17, and cooler than normal temperatures drawn down from QC and ON may stick around the eastern half of the nation through the remainder of the week.


Prepared by Meteorologist LARRY COSGROVE on
Wednesday Evening, April 11, 2007, 7:20 PM ET

Disclaimer:

The previous statements are my opinions only, and should not be construed as definitive fact. Links provided on this newsletter are not affiliated with WEATHERAmerica and the publisher is not responsible for content posted or associated with those sites.

Copyright 2007 by Larry Cosgrove

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