Sunday, October 24, 2010

10/24 - 11:40am - Severe Weather Risk Today Across the South

Severe weather could be on the way this afternoon and evening for a large chunk of the country. The Storm Prediction Center has issued a Slight Risk from just south of Chicago to Dallas/Fort Worth, to East Alabama, and southward to the Gulf Coast. Here in Starkville, MS, we're right in the middle of all this. This Slight Risk includes a 5% tornado risk, 15% hail risk, and 15% wind risk for us here in North Mississippi. We're already seeing a few small storms breaking out in the Delta region of Mississippi, but I don't expect anything substantial to form until this afternoon.

This is a fairly classic severe weather setup for the South today. From the west we're seeing the entrance of a shortwave trough into the region, which is causing divergence, or wind spreading, in the upper levels of the atmosphere. This spreading of the winds up there causes air at the surface to rise because something has to fill that low pressure void aloft created by the divergence. That rising motion, along with cooler temperatures in the upper atmosphere and increased heating and moisture at the surface act to create atmospheric instability, or the tendency for air to rise. Today we're already seeing instability or CAPE values above 1000J/Kg, and it's expected that we'll see values into the 2000J/Kg range. Anything above 1500J/Kg is considered to be adequate for a severe weather setup, so we won't be lacking in instability later on today and tonight. Wind shear, or the differences in wind speed and direction with height, is important because it helps sustain any storm that forms by blowing the storm's downdraft away from its updraft. This keeps the downdraft from contaminating the warm, moist air needed for the updraft. We'll have bulk shear values around 40-50kt this afternoon and evening, which is adequate for severe weather.

These storms will be in clusters when they form and come through the area tonight, but there's a chance that we could see some independent cells as well. This really isn't a big event for tornadoes, but there's a chance that a few could spin up later this afternoon and into the evening. I'm more concerned about the hail and high winds that these storms could bring, so be on the lookout for that. This risk extends well up into West Tennessee as well, but the severity of the storms will most likely be limited by lower dewpoints and the presence of the rain moving into that region this morning. The satellite image this morning shows that quite a bit of sunlight will be able to reach the ground today here in North Mississippi, which should crank up instability. This severe weather event won't be historic or record-breaking in any respect, but it does pose a general threat to parts of the South this afternoon and evening.

I'll be sending out updates on this event for the Starkville, MS / MSU area on Twitter and Facebook today. I'll also post if there are any major severe weather developments in other areas of the South as well.

No comments: