The storm system that has developed over the continental United States over the past 24 hours is truly remarkable. It is ironic that it developed on March 13, 2019, because the most perfect atmospheric storm/heat engine that I have seen in my career occurred on March 13, 1993 (known to most southerners and the northeast megalopolis as “The Blizzard of 1993).
This one has some different characteristics, including the way it initially developed, as a “lee cyclone”, in the lee of the Rocky Mountains. However, the surface low interacted with a strong zone of temperature gradient over the Plains, and the upper-level cyclone deepened rapidly as cold air was pulled southward and warm air was pulled northward by the surface cyclone. The positive feedback loop began, and the perfect heat engine developed, transporting warm air away from the tropics toward the poles, and cold air from the poles into the midlatitudes. These big storms keep our atmosphere in balance. This storm has caused record low surface pressures in Oklahoma, blizzards in Colorado and Nebraska, flooding, and is now causing severe weather in the east.
A cold front is moving eastward, and the upper-level low continues to produce strong wind shear and forcing for ascent from Michigan to the Gulf Coast. Warm, moist air has moved northward and has produced an unstable air mass for thunderstorm updrafts through much of Indiana, Kentucky, Tennessee, Mississippi, and Alabama, and severe storms have developed. A tornado passed very near the Paducah National Weather Service office, and blew commercial air conditioning units off a mall. There have been numerous reports of straight-line wind damage and large hail, and the storms are still going.
Zooming into Alabama, NW Alabama is under a Tornado Watch, and I expect that to be extended eastward to include Birmingham soon.
A line of severe storms extends from near Huntsville to Cullman to Jasper to Reform, then on into Mississippi. There is strong wind shear, with 0-1 km helicity above 300 m2/s2 over much of western and central Alabama. The most unstable air is over south Alabama, but with the cold upper level temperatures and surface temperatures in the 70s with dewpoints in the 60s as far north as Huntsville, CAPE values are 500-1000 J/kg all the way into Tennessee. The temperature is 77 at BHM and 82 in TCL, but the NWS balloon release at 1 pm CDT showed a weak temperature inversion at mid-levels, that may be inhibiting storm development somewhat. Still, the storms near Cullman, Jasper, and just south of Fayette show signs of large hail and all 3 are rotating.
As the afternoon goes on, the air will destabilize further over central Alabama, but the wind shear will begin to pull away as the upper storm system races toward Canada. However, there will be a time overlap between now and about 7 pm when intense storms with large hail and damaging winds are likely in central Alabama, and a couple of tornadoes are likely also. The biggest threat here in the Birmingham metro area will be around 7 pm.
This is not a big outbreak of tornadoes, but be prepared in case a tornado warning is issued for your area. Have at least two ways to get tornado warnings in addition to outdoor tornado sirens. A cell phone app and NOAA weather radio that alert you are best. Remember, lowest floor, smallest room, center of the building, away from doors, windows, protect your head with a helmet or pillows.
Dr. Tim Coleman