The intense weather system is rapidly developing over the southeast US The thunderstorms we are getting in central Alabama right now are elevated because they are north of a warm front that is currently over south Alabama. These storms are producing heavy rain and lightning, but because they are not surface based, they present little or no tornado threat.
We at UAH are conducting limited operations (limited because of COVID-19 restrictions). The picture below shows the data from a recent weather balloon released from SWIRLL, our primary radar and lightning lab on campus. The profile of temperature (left) shows a stable atmosphere at low levels (no CAPE), but the hodograph, right, shows winds increasing and changing direction with height very quickly, and a storm-relative helicity of 510 m2/s2.
To look at a view of CAPE and helicity across the Southeast, we go to the SPC mesoanalysis. One can see the warm front clearly here also, as CAPE rapidly increases in the warm, humid air over south MS and south AL. CAPE values of 1500 J/kg are already as far north as Jackson, MS and Evergreen, AL. Note also the extreme wind shear in place over a large part of LA, MS, and AL, with SRH values generally 400-600 m2/s2. This means the atmosphere has a lot of the type of wind shear that will cause storms to rotate, and produce tornadoes once they become surface based.
Even though the storms over Alabama right now are noisy, they are not dangerous (except for the lightning). But, the surface low pressure area is currently centered in NE Louisiana, and it will move toward west TN over the next few hours. This will keep the wind shear high around here. The wind shear will be enhanced along the warm front itself, as these fronts and the rapid warm air advection along them cause this. As the upper-level system continues to intensify over the next 3 hours, a strong southerly wind surge will occur ahead of it, bringing the warm front into central Alabama. Warm fronts sometimes have a hard time moving north when storms are to its north, but with the dynamics of this system, I think it will make it to I-20 by 4 pm, and then to the Tennessee border by 6 pm.
Back in NE LA, south of the warm front, damaging tornadoes have already occurred this morning. There are reports of houses blown away in Monroe, LA. A tornado watch is in effect for north and central Mississippi now, and it is the rare “PDS” watch (particularly dangerous situation). Visible satellite shows a few breaks in the clouds south of the warm front over southern MS. If that happens here, warming up the air even more at low levels, it could cause additional instability.
As the warm front moves north, the atmosphere over central Alabama will become very volatile for storm development, starting around 3 or 4 pm, as the warm front gets close. Supercell storms may form over Mississippi in the next couple of hours and track into Alabama. Then, the main line of storms just now crossing the Mississippi River will move through here with the possibility of widespread, significant damaging winds (75 mph or higher) and additional tornadoes, between 8 pm and 11 pm.
Go ahead and prepare now. Please have at least two sources of weather information with you at all times. A good phone app that will alert you of tornado warnings where you are, and a properly programmed NOAA weather radio. Make sure your phone is charged and you have batteries for your radio. Also, flashlights, helmets, etc. Just stay calm, and will we get through this.
For additional safety information, see the end of my previous blog, at https://ckweather.com/2020/04/10/tornado-analysis-for-sunday/.
I will be monitoring the situation from my UAH research office here in Trussville this afternoon, helping to watch our crews’ backs and keep them safe.
Dr. Tim Coleman
Coleman and Knupp, LLC