The Storm Prediction Center has upgraded north MS to a moderate risk of severe storms, and extended the enhanced risk into NW AL. A tornado watch has just been issued for most of Mississippi. For a discussion of the larger scale features driving this event, see yesterday’s blog at https://ckweather.com/2019/02/23/severe-weather-analysis-for-tomorrow/
There are breaks in the clouds over west AL and east MS early this afternoon, allowing temperatures to really warm up. In east AL, a wedge cold front is keeping it much cooler. Currently, it is 79 degrees in TCL, 71 in BHM, but only 63 in Anniston (and 50 in Atlanta). Perhaps even more striking is that the dewpoint in TCL is already up to 67 degrees, almost like summertime, and south winds will continue to bring moist air northward through the afternoon. With all this warm, moist air coming north and the breaks in the clouds, the air is already unstable from just west of I-65 back into Mississippi. SPC mesoanalysis indicates CAPE >1500 J/kg as far east as Tuscaloosa and Hamilton.
These numbers are pretty close. A VORTEX SE balloon sounding from Starkville, MS at 11 am CST showed a CAPE of 1,370 J/kg and it has warmed up some since then. The observed 0-1 km helicity was 182 m2/s2, enough for rotating storms, but not quite as high as the computer models are putting out there, partially because the winds are so high at the surface.
As warm, moist air continues to pour into MS this afternoon and the forcing for ascent gets closer as the upper system and front move east, storms will likely form over MS in the next 2-3 hours out ahead of the main line of storms that is currently just west of the Mississippi River. Some of these storms will likely be supercells, capable of producing large hail and tornadoes.
As far Alabama, with the warm, humid air in place in Tuscaloosa, Demopolis, and Greenville, and that air moving north toward Jasper and Hamilton, storms could begin to fire as early as 4 pm, and they will increase as the forcing gets closer around 6-7 pm. The highest risk for tornadoes in Alabama is north and west of a line from Huntsville to Double Springs to Jasper to Aliceville, but we can’t rule out a tornado in areas farther east like Tuscaloosa, Jefferson, and Shelby counties.
The main line of storms will move through around midnight, with damaging straight line winds as the main threat. As my colleague Nathan Burks points out, the ground is so saturated from all the rain, trees will go down easier than they normally would.
This still does not look like it will be a major outbreak of tornadoes here in Alabama, as the system will weaken slowly as it moves in here this evening and grow into a squall line, and those are less likely to produce tornadoes than the isolated supercell storms we expect to the west. But, 2 or 3 tornadoes are likely in Alabama, mainly in the areas I outlined.
Just be ready, keep an eye on the weather this evening, and have a source of weather information including tornado watches and warnings. Know your tornado safety rules.
Dr. Tim Coleman
Coleman Knupp and Dice Weather Blog 