A significant tornado outbreak is expected to unfold through the day tomorrow across much of the Deep South. A strong low pressure system, the same one that will bring snow to a long stretch of the Midwest, is expected to form over Texas today and drag a cold front along to its south.
Due to a combination of very strong wind shear and enhanced lift along the front, severe thunderstorms will occur tomorrow in two rounds. The first round will arguably be the most dangerous -- supercell thunderstorms forming out ahead of the cold front, which will pose the greatest risk in terms of violent, long-track tornadoes. The second round will be a squall line along the leading edge of the cold front itself, which will bring a risk of damaging winds.
In anticipation of these significant tornadoes and damaging winds, the Storm Prediction Center has issued a moderate risk for severe weather in the areas shaded in red. A slight risk for severe weather exists for the areas shaded in yellow. A risk for general non-severe thunderstorms exists in the green shaded areas.
Jump the fold for a more detailed explanation of what's expected on Christmas...
Round 1: Tornadoes
A low pressure system will form over Texas today and tonight, moving east into Louisiana by tomorrow morning. The track of the low will bring warm, moist, unstable air inland off the Gulf of Mexico and park it in the southern parts of Louisiana, Mississippi, and Alabama. This unstable air will encounter a large amount of forcing from the cold front, resulting in very strong lift along the area at greatest risk for severe weather. This strong lift will help to fire off strong thunderstorms starting tomorrow afternoon.
These storms will rotate due to strong wind shear. A jet stream 30,000 feet up in the atmosphere will move in from the west, bringing 100-120+ MPH winds over the area. This jet stream coupled with gusty surface winds from the south will create very strong vertical wind shear at the same time the thunderstorms develop. The updrafts in these thunderstorms will tap into the wind shear and lead to the development of supercells.
NOAA diagram of how a supercell thunderstorm develops
The combination of strong lift and strong wind shear will lead to these supercells producing tornadoes. Given the predicted nature of the atmosphere, some of these tornadoes may be large, strong, and have tracks dozens of miles long.
In addition to tornadoes, the supercells will be capable of producing damaging winds and large hail.
Round 2: Squall Line
After the supercells roll through, the main squall line along the cold front will sweep through. This squall line should be capable of producing damaging winds in excess of 60 MPH. After this line of storms moves through, the front should pass and you should be in the clear.
Weather Channel's TOR:CON
Since many of you request Dr. Greg Forbes' TOR:CON index (a 0-10 scale ranking how likely tornadoes are in a given area), here it is for tomorrow. A 6 means that there's a 6 in 10 chance of seeing at least one tornado in that area. I've found that this index is surprisingly good considering it's from The Weather Channel.
AL central, south – 6Here's a graphical representation of the TOR:CON...
AL north - 2 or less
FL panhandle - 5
LA central – 5
LA south – 4 to 5
LA northeast – 4 to 5
MS central, south – 6
MS north – 2 or less
TX east-central – 5 morning
TX upper coastal – 4 morning
Other areas – 1 or less
Why is this happening?
Fall and winter make somewhat of a second tornado season in the southern United States. The drastic temperature differences between the south and the north allow semi-regular severe weather outbreaks when a potent storm system (such as the one forming now) moves through.
However, it isn't every winter day that we talk about spring-like supercell thunderstorms producing long-track, violent tornadoes. It's not excessively rare, but it's not common.
I'll try like hell to keep a liveblog going tomorrow for anyone not out getting sloshed on eggnog or trying not to burn their house down via turkey. Please try to spread the word to your southern friends and family about this dangerous outbreak. You (and they) NEED to keep an eye on this. Any tornado is dangerous, let alone large and violent ones.
National Weather Service Main Page
National Weather Service -- Houston TX
National Weather Service -- Little Rock AR
National Weather Service -- Shreveport LA
National Weather Service -- New Orleans LA
National Weather Service -- Lake Charles LA
National Weather Service -- Memphis TN
National Weather Service -- Jackson MS
National Weather Service -- Huntsville AL
National Weather Service -- Birmingham AL
National Weather Service -- Mobile AL
Storm Prediction Center Main Page
Storm Prediction Center -- Current Severe Weather Watches
Storm Prediction Center -- Convective (Severe Weather) Outlooks
Storm Prediction Center -- Mesoscale Discussions
Storm Prediction Center -- Storm Reports
Storm Prediction Center -- Mesoscale Analysis Pages
Wunderground's Detailed Radar (click the + nearest to you to see your local radar)
12:11 PM PT: As FishOutOfWater reminds us in the comments, this tornado outbreak will continue into the Carolinas on Wednesday, with the SPC noting a "significant tornado threat," including violent long-track tornadoes, in the black hatched area.
3:46 PM PT: Jim Cantore will be in Mobile, AL tomorrow. Run.