Tuesday, March 18, 2008

Severe threat today and tomorrow in the South

There is a threat of severe weather across the South today and tomorrow. At lunchtime today, the SPC has two PDS (particularly dangerous situation) tornado watches in place to the west of Alabama. Part of the area covered by these watches is under a moderate risk for severe weather.

Tomorrow the threat moves east. I expect storms to approach Alabama in the early morning hours and exit the state around midday. At the moment the SPC has placed Alabama and points to the east under a slight risk for severe weather. I would not be shocked if a part of this area was upgraded to a moderate risk.

The big question with this storm system seems to be instability. Models have been consistently predicting a tremendous amount of sheer but they show very little CAPE (convective available potential energy).

It seems, though the models have been underestimating the fact that with sufficient wind shear, tornadoes can occur with limited instability. I understand that dewpoints were in the 50's when an EF2 tornado struck downtown Atlanta on Friday night, March 14. Of course instability depends not only on temperatures and moisture levels at the surface, but how that contrasts with the air aloft. There needs to be a strong contrast to help provide the upward lift for thunderstorms.

Even with minimal instability, many storms this season have been "low-topped" but produced tornadoes regardless, due to the tremendous amount of shear in the atmosphere.

My "gut" tells me that this will be yet another significant tornado outbreak in parts of the South. Temperatures have been fairly warm over the past few days and with the strong amount of shear, there will be some tornadoes. Of course the $64,ooo question is "where?" No one knows.
One limiting factor for Alabama will be that the storms will be arriving in the morning hours. Of course, storms do not care about the clock, and if conditions are "right" they can occur anywhere at any time.

This is a situation that should be monitored very closely across the South during the next 36 hours!

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