Wednesday, December 23, 2009

Christmas Eve Severe Potential Discussion

Severe weather has broken out in the ArkLaTex region near Shreveport and Texarkana. This is right along the main convergence from the warm air mass overtaking the cooler air mass. But I don't expect our severe weather until tomorrow as the cold front pushes through.

My timing tomorrow looks like 11am to 5pm. I know this window might seem too big for some, but some analysis shows me the potential for the front to move slower than expected. So whats the big deal?

Minimal, but sufficient, instability will be present in South Mississippi from Natchez all the way east towards Greene County and the Coast. The whole state will have plenty of storm relative helicity (rotation in the atmosphere) to go along with the instability. Also, plenty of moisture will be present for heavy rain. So everything seems to mark South Mississippi as the best potential for damaging wind and tornadoes. I'm looking at the area South of Hwy 84 as the geographical line for the highest potential.

A squall line will push across the state parallel to the cold front, and this is where major wind gust can be found. In front of the cold front, discrete supercells can produce isolated tornadoes within the unstable air mass.

My Concerns:
1) The instability has been my concern for the last several days, but I continue to lean on climatology and note the winter season doesn't need much instability to trigger severe events.

2)The timing has been difficult to forecast as well this week. Models are beginning to agree on an early afternoon frontal passage, but some current analysis of the action to our west looks a little slower than anticipated. Some notes have been made that the front could accelerate with the increased drying behind the front as it passes through the state.

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