Wednesday, 1 April 2009

More US severe weather

Well, Spring is well and truly here now, so it should come as no suprise to see a series of powerful troughs pushing through the USA, bringing rounds of severe weather. Some of these in recent days have not had huge amounts of Gulf moisture to feast on, meaning fairly low-end severe events. However, on Thursday, such a trough will be accompanied by plenty of moisture, as strong return flow from an open Gulf streams into portions of the south and south-east.

Basically anywhere from around the Mississippi Delta region eastwards, and northwards towards Tennessee is at risk from severe weather as the potent upper trough and associated surface low move through the area. 500-1500 J/Kg of CAPE are expected, along with strong 0-6 and 0-1km shear values - more than sufficient for supercell structures, although given the intense forcing, a fairly complex scenario is expected. Also, surface winds are expected to be a little veered - however, I recall seeing something similar on the Super Tuesday outbreak of Feb 2008, and there were numerous strong tornadoes, and very long-track tornadoes.

The warm front currently looks like lying through central Alabama (E-W, ish!) by around 21z/close to peak heating. Should this occur, low-level helicity would be maximised just ahead of the front. Storms crossing this front would have an enhance chance of producing tornadoes, although could become more elevated as they continue to move NE into the cooler air.

At this stage, although a large area is at risk, I would place the Birmingham - Montgomery - Columbus area of Alabama in the highest risk area for strong tornadoes Thursday afternoon/evening, although anywhere from Mississippi eastwards could be in for a rough ride.

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