Thursday, March 26, 2009

Severe Weather Threat in the South

After the rain ended late this morning, skies became partly cloudy and there was a lot of sun in North and Central Alabama. The atmosphere even destabilized a bit with CAPE values reaching 500+ j/k in Southwest Alabama. Temperatures soared into the 70's.

This is setting the stage for two more rounds of storms. Round two will come tomorrow morning. Round three will be the greatest threat and models have pushed the timing back a bit and prolonged the event. The low pressure that is expected to travel through the Memphis area will be be slow to move out to the northeast. Instability and shear values across Alabama are projected by the Short Range Ensemble Forecast models to remain in place for a long time. The air mass in Alabama might be potentially primed for severe weather to occur in the state anywhere between 6PM Friday and 3PM Saturday. A big question is how the atmosphere will be able to maintain that level of instability for an extended period of time.

The models show the greatest threat of tornadoes will be over East Louisiana and West Mississippi late tomorrow afternoon. That area is currently in a "Moderate Risk" according to the SPC and there is a large "Slight Risk" area covering much of the South. It is possible that the SPC could upgrade the risk to "High" and/or expand the Moderate Risk area in the morning. This is a complex weather system and forecast confidence may be higher in the morning.

All the parameters needed for severe weather seem to be in place: strong low pressure system to our northwest (we will be in the right rear quadrant), strong low level jet, high CAPE (instability, or convective available potential energy) values, and high shear values. It looks like there will be parts of the South where the ingredients may come together more perfectly but it is difficult to pinpoint which areas at this time. The only possible inhibiting factors for some areas may be long periods of heavy precipitation which may stabilize the air, the possibility of storms in the morning inhibiting the flow from the Gulf, and to a lesser extent, the nocturnal timing of the storms in some areas.

Flooding is also a concern. Many creeks are already rising high and some flood warnings have been issued. Flooding problems will only get worse in the next 48 hours. Please do not cross roads that are covered in water.

As we have already seen with multiple storm reports this morning and tornado confirmations, the atmosphere is very volatile in the Deep South, and it will be more so over the next 36 hours. Residents should take any storm that develops seriously and heed any warnings that are issued for your area. If you do not already have one, now is a very good time to invest in a NOAA Weather Radio.

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