Textbook storm across US – Tornado update 401 pm

Upper-level winds (500 mb, about 18,000 ft); surface pressure, and surface temperature at 7 am CDT
Visible satellite picture over US (courtesy WU)

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

Twitter @timbhm

Severe weather analysis – 430 pm

Current radar

…Tornado Watch in effect for much of north and west Alabama…includes Huntsville, Birmingham, and Tuscaloosa…

Right now, we have an area of showers with some claps of thunder moving through central Alabama. One of the storms in this band earlier prompted a tornado warning for counties in extreme north Alabama, but that warning is gone and I have not heard of any reports of damage.

We have an environment today that has limited instability for updraft growth (surface temperatures are only in the low 70s in most areas, with dewpoints in the low to mid 60s). An area of dry air is parked over central Alabama, and as it gets mixed down to the ground, it limits our instability. The largest instability (CAPE values) are currently in northeast MS and extreme NW AL, where the dry air has not made it (and may not).

CAPE right now

This storm system is mainly scary at all because it has very large wind shear for storm rotation (and the dynamics created by the wind shear can help updrafts grow also). 850 mb pressure level winds (about 5,000 feet) are about 60 mph over the Birmingham area, and increase to 80 mph over NE MS in the mid-level jet max. This is producing the large wind shear. 0-1 km storm-relative helicity values, a really good measure of wind shear for storm rotation, is extremely high, with areas in NW AL near 400 m2/s2. It is near 250 m2/s2 here in BHM.

850 mb (5,000 ft) winds
0-1 km storm-relative helicity

The storms that are currently moving through central Alabama should move on out in the next hour or so without incident. It is that next line of storms in northern MS that is somewhat concerning. However, the sun sets in one hour, so as it gets dark it will be difficult for the atmosphere to destabilize any further. However, the strongest wind shear will move into NW Alabama around 6:00 pm with those storms, and the most unstable air is also up there (areas like Florence, Moulton, Russellville, Hamilton, Double Springs, and Vernon). So, the largest risk for tornadoes in Alabama will be in those areas between 5 pm and 8 pm.

As the storms move farther southeast toward Birmingham and Tuscaloosa, the instability will be weaker. However, given the strong wind shear in the environment, we will likely see some damaging winds and we can’t rule out a tornado here in central Alabama this evening. The storms will move through the BHM metro between 10 am and 2 pm.

This does not look like it will be a tornado outbreak, especially not here in central Alabama. We will have to watch the storms coming out of north Mississippi into NW Alabama very carefully over the next 2 hours. We are under a tornado watch, so have a safety plan in mind for wherever you are. Remember, lowest floor, middle of the building, away from windows and doors, protect your head.

Dr. Tim Coleman

Twitter @timbhm

Weather update – 707 pm

Tornado on US 72 near Burnsville, MS (courtesy UAH SWIRLL)

…TORNADO WATCH UNTIL 10 PM…

Several tornadoes have occurred over Mississippi and Alabama today.  The one above in north MS, then we have always gotten reports of severe tornado damage in Columbus, MS.  There are also reports of wind damage in parts of Lamar, Fayette, and Marion Counties in Alabama.

The initial line of storms that brought funnel clouds, wall clouds, and torrential rainfall to parts of Tuscaloosa, Walker, Jefferson, and St. Clair counties this afternoon has moved out.  The main area of storms is now moving through western and central Alabama.  Currently, there are no tornado warnings in Alabama.

The strongest storms extend from Huntsville to Decatur, through Winston County near Smith Lake, and moving into Walker County.  That Walker County storm is rotating and is moving toward Cordova and Parrish.

The instability in the atmosphere has decreased due to rainfall and sunset.  CAPE has decreased to around 500 J/kg.  But, wind shear actually increased a little bit with sunset (something we are researching at UAH), and with the man storm system approaching.  So, a few storms could still rotate, and an isolated tornado can not be ruled out this evening. 

But, the threat for tornadoes has greatly decreased and will decrease through the evening.  However, so much rain has fallen the past few days, and now with dark, flash flooding will be the biggest threat tonight.  Do not cross any roads with water over them.  It is not worth it to drown.  The governor has issued a state of emergency for the northern 1/3 of Alabama due to long-term flooding.

The whole thing will be over in Birmingham by around midnight.  Sunny and cooler weather for early week!

Dr. Tim Coleman

http://www.colemanandknupp.com

Twitter @timbhm

Update – 345 pm

Reflectivity (left), velocity (right) from BMX radar

…TORNADO WATCH UNTIL 10 PM…

…TORNADO WARNING CULLMAN COUNTY NOW…

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.

Dr. Tim Coleman

http://www.colemanandknupp.com

Twitter @timbhm