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.
Dr. Tim Coleman