Monday, February 28, 2011

Winter Severe Thunderstorms in Middle Mississippi Valley

There were many severe thunderstorm reports yesterday from northern Oklahoma east-northeastward to Ohio. The SPC outlooks were similar during the day - the 20Z product is shown at the top. The current plot of yesterday's reported severe thunderstorm events is shown at bottom. The north half of the slight risk area caught most of the severe reports, but the south half of this area was mostly devoid of storm reports. The moderate area outlooked at 20 UTC also remained mostly devoid of severe storm reports.
A quick look indicates that most of the severe storms occurred along and north of the strong west-east frontal zone, with a large number of the storms being elevated and not rooted in the surface BL. The extreme eastward push of cT air - reported by Bosart on the MAPS chatboard - apparently lead to a rapid intensification of a capping layer above the BL across the south half of the outlooked severe area (the soundings taken at Little Rock, Arkansas, at 00, 03, and 06 UTC captured this capping process). The 500 mb S/W also took on a positive tilt during the day and slowed considerably, weakening forcing for vertical motion in the warm sector - thus, a missed short-term outlook worthy of more detailed examination.

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