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4/19/09 MS, AL, TN, GA

Dave Marshall

I'm not terribly impressed with the models overall for Sunday, but it is an unusual setup for this part of the country, and a situation I have little experience forecasting/chasing. I'm very eager to hear how others who chase the plains and see this setup more often interpret it. I hope this stays within the scope of the Target Area rules.

Looking at the 12z/17Apr GFS, there appears to be a rather pronounced dryline setting up in central MS around 18z, with a nice surge of low level moisture ahead of it in western and central AL.

The mid levels east of the dryline are still pretty dry, as indicated by forecast soundings and the relative humidity above 700mb. Thats concern #1 for me. There's also a dramatic thermal gradient, with upper 70s to low 80s behind the dryline, and very cool, moist air left over at the surface from an MCS across northern AL ahead of the dryline.

There's a nice little corridor along the AL/MS border that hints at CAPE approaching 1000-1250j/kg in pockets, which is encouraging.

Shear throughout the air column looks reasonable ahead of the dryline, and my interpretation is there is a reasonable risk for severe weather along a narrow corridor from the MS/AL line east to Montgomery from about 1400 local until sunset. By 00z, the shear seems to run away from the other dynamics, and I suspect convection will die off pretty quickly as nocturnal cooling takes hold.

Model agreement isn't very good right now, with the GFS being much more pronounced than the NAM, and the NAM lagging the instability and lift by several hours.

If there's floaties in my tea leaves, someone set me straight here.
 
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The possibility of a prefrontal dryline is the most encouraging factor for me at the moment. I assume it will depend on what is left over after early morning convection departs the area. Photogenic supercells might be possible, though, (and we seem to be harvesting more of those than usual in Dixie Alley this year).
 
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