Severe weather analysis – 215 pm

NOAA/SPC convective risk outlook

It has been an active year for severe weather in the Southeast, and we have another system to deal with later today and tonight.

Right now, a big area of rain and thunderstorms is occurring just east of the Mississippi River. A tornado watch is in effect for much of central and south Mississippi, and several storms have tornado warnings on them right now near and south of Jackson, MS.

This entire system is reflected by a slow-moving front and a surface low pressure area forming along it in south Louisiana. Note that pressures are falling rapidly across eastern MS into northern AL, so that is where the low pressure area in Louisiana will likely track toward over the next several hours.

How far north the low tracks is critical to how far inland the tornado threat can make it. With the low pressure area intensifying and creating imbalances in the atmosphere as it moves to our NW this evening, wind shear for storm rotation will be very high. Storm relative helicity values could be over 400 m2/s2 here in central Alabama between 7 pm and midnight.

The only limiting factor may be a lack of instability for storm updrafts. First of all, the air over central Alabama is warm right now, but somewhat dry, with dewpoints ranging from upper 50s in Anniston and Montgomery to lower 60s in Tuscaloosa and Demopolis. That makes the air more stable. Higher dewpoint air will likely move in this evening as south winds pick up, destabilizing the air somewhat, but it will be at night by then and cooling off. Also, temperatures aloft are not that cold and do not get cold rapidly with height. That will also limit instability somewhat, as shown in the expected atmospheric temperature profile for Tuscaloosa at 7 pm.

Computer model temperature profile vs. height at TCL at 00 GMT (7 pm CDT)

Given current information, it looks like the greatest threat for tornadoes will be south of I-20, especially in areas like Jackson, Meridian, Montgomery, and Troy, where it will be warmer and more humid. Storms will likely move into west Alabama by 5 pm, and the main line of storms will come through Birmingham metro between 7 pm and 11 pm. Despite the more stable air, with the wind shear in place, tornadoes will be possible even as far north as Birmingham, and I expect us to be under a tornado watch this evening. Even if we don’t get tornadoes, straight-line winds over 60 mph are likely.

Review your safety plan. Lowest floor, small interior room, no outside windows or doors, wear a helmet, etc. Make sure you have a NOAA weather radio with alert feature to get warnings from, and hopefully a phone app. I use the app called “tornado” from the American Red Cross.

Dr. Tim Coleman

Twitter @timbhm

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