A strong weather system, with an upper-level trough and associated low pressure area, is moving across the U.S. tonight. Warm, moist air has moved as far north as Michigan, and tornadoes have been reported in parts of Illinois and Missouri tonight, with at least two fatalities. In addition, a 95 mph wind gust was measured just west of Chicago. The radar picture below is from 835 pm. This weather system will move east tomorrow.
As indicated by SPC in the graphic at the top of the page, the greatest risk for severe weather, including tornadoes, will likely stay to the north of Birmingham with this system. The main upper-level energy will stay to our north, and the surface low pressure area will move through the Great Lakes area tomorrow. This will keep wind shear here in Alabama quite a bit lower than further to our north, closer to the low pressure area. This system looks more like mid-April than March 1.
Tonight, it will be tough for storms to form over Alabama because there is a cap of warm air in place up near 5,000 feet. But, there will be some wind shear for organized, rotating storms over Alabama out ahead of the cold front, and the cap will go away tomorrow morning as forcing ahead of the cold front moves in. The helicity, or wind shear for storm rotation, will not be super high tomorrow afternoon, and will decrease by late afternoon, but it will be sufficient for rotating storms and even a few tornadoes over parts of north Alabama, with values over 200 m2/s2 around noon.
With even moderate wind shear like this, we have to be concerned because the air will be very warm and humid. Temperatures tomorrow will rise into the 70s, and dewpoints out in the Gulf are currently in the 70s. Trajectories show that our low-level air tomorrow will have come from the near the Florida Keys. That means it will be unstable for updrafts tomorrow afternoon, with CAPE values rising to near 1000 J/kg.
The instability will not be extreme with upper-level temperatures not too cold, and the wind shear will be moderate due to the center of the main storm being 1,000 miles to our north. But, there will be enough shear and instability for a few supercell storms to form over north and central Alabama tomorrow, anytime after 10 am or so, then the main line of storms will move through during the late afternoon or early evening. The storms tomorrow will be capable of producing isolated areas of heavy rain, dangerous lightning, damaging straight line winds, and even a few tornadoes.
Remain aware of the weather all day tomorrow! Have a source of weather information available and review your tornado procedures for work, home, and even in the car, in case a tornado warning is issued. This should not be a major severe weather outbreak for Alabama, but it could be the most significant in several months, and the system has already caused injuries and fatalities to our northwest, so we have to watch it carefully. Colder air will follow for the rest of the week.
Dr. Tim Coleman