A band of supercell storms has formed in western Alabama and is now moving into central Alabama. These storms are rotating.
The one with the most vigorous rotation, that has caused the NWS to issue Tornado Warnings for northern Walker, southern Winston, and now Cullman counties, is currently west of Cullman, approaching Trimble and possibly Exit 310 on I-65.
There is another one in southwest Jefferson county with rotation and a significant “hook echo” on radar, but so far the NWS has not issued any warnings on it. It went right over Tuscaloosa with no damage reports I am aware of.
It will be interesting to see what happens to these storms in Cullman and Jefferson Counties as they cross a thermal boundary between the cooler air trying to come in from Georgia and the warm humid air over west Alabama.
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.
This has been one of the rainiest weeks in north and central Alabama in a very long time. Some rainfall totals for the week include 3.63″ in Birmingham, 3.72″ in Tuscaloosa, and 6.89″ in Huntsville. It looks like the rainy pattern will end after a cold front moves through this weekend. But, as it does, it will bring our first real chance for severe weather in north Alabama in 2019.
A front has been stalled over Alabama and Tennessee for the past several days, with a strong ridge of high pressure at upper levels to its SE keeping it in place. This front, and upper-level disturbances moving along, it have given us all the rain and thunderstorms this past week. You can see the big upper ridge in satellite water vapor imagery.
But, a large upper-level trough is moving through the southwest US now, and will move out into the Plains states tomorrow. The upper divergence ahead of this trough will cause a surface low pressure area to form and move to near Kansas City by late tomorrow afternoon. It looks like this upper-level system will finally be strong enough to push the cold front through Alabama early Sunday morning.
With the deepening surface low to our NW, winds will become gusty out of the south tomorrow, near 30 mph at times. If the wind is strong enough to break the low clouds, temperatures could warm into the mid 70s in the Birmingham area, but even without sunshine it will be near 70 due to warm air coming from the south.
Those winds will also bring in some of the very humid air currently in place over Louisiana and the Gulf of Mexico, and with the ground as wet as it is, the moisture will not modify that much. Winds at 850 mb (about 5,000 ft) will reach 55 kt (65 mph) and help transport in the warm, humid air at low-levels, especially over Mississippi and extreme west Alabama. With the air fairly cold aloft, this warm, humid air near the ground will create some instability for thunderstorm updrafts. CAPE values over MS and western AL tomorrow afternoon will likely reach 1,000 J/kg, pretty high for February.
The areas in blue on the last panel will have CAPE above 1,000 J/kg. With the strong upper-level system there will be plenty of wind shear to support severe storm organization and potentially rotation, with helicity values over 300 m2/s2 over most of the northern half of Alabama tomorrow.
This combination of shear and instability means thunderstorms are likely. They will develop over Mississippi during the day, and a few may move into Alabama, especially west of I-65, before sunset. These storms may contain heavy rain, damaging winds over 60 mph, hail, and dangerous lightning. The significant tornado parameter is a combination of CAPE, helicity, and cloud base, and gives us an idea of tornado potential.
The map above is for 6 pm CST tomorrow. Note the best combo of shear and instability for tornadoes will be over northern MS, but anywhere shaded in blue has a value of 1, enough for the atmosphere to produce a tornado. We have some very high-resolution computer models now that actually simulate expected storm formation (this is fairly new). One of those shows some individual storms, or supercells, over west Alabama tomorrow at 6 pm. This is concerning because these lone storms often are the ones that produce tornadoes.
Taking a look at the expected atmospheric profile of temperature, humidity, and wind at that time, one can see temperatures getting cooler rapidly with height, and the wind direction veering sharply from SE near the ground to SW aloft.
As the cold front moves in and the strongest dynamics move away tomorrow evening, the storms should grow into a squall line that can still produce damaging wind but are less likely to produce significant tornadoes.
It looks like the highest risk for tornadoes will be over northeast MS and northwest AL, basically north and west of a line from Decatur to Jasper to Aliceville. But, we can’t rule out a tornado further east here in Birmingham or Tuscaloosa. The main threat time for tornadoes in west Alabama will be 4 pm to 9 pm. The squall line will move through BHM around 1 am, then it will finally get sunny and cooler on Sunday. The good news is that these storms will move through fairly quickly and we will not get a whole lot more rain.
This should not be a widespread outbreak of big tornadoes, especially in Alabama. So, don’t panic. However, there is a good chance that one or two tornadoes will hit in Alabama, and people west of I-65 and north of I-20 need to have the tornado plan in place. Lowest floor of your home, away from windows and outside doors, near the center of the building, in a small room like a closet if possible. Bicycle, football, or baseball helmets will protect your head from flying or falling debris. You can also get under a heavy piece of furniture or workbench to protect yourself.
I will update this blog again not later than noon tomorrow.