The second episode in a days-long tornado outbreak will unfold today across parts of the Deep South known as "Dixie Alley," an area of the U.S. highly susceptible to intense tornadoes. The Storm Prediction Center is calling this a "particularly dangerous situation" due to the high likelihood of seeing intense, long-lived tornadoes like we saw in Arkansas yesterday.
For reference, "severe weather" is defined as thunderstorms that produce hail the size of quarters or larger, damaging winds in excess of 60 MPH, or a tornado.
Where is the outbreak expected?
The above map shows the Storm Prediction Center's current thinking on the tornado outbreak. The atmosphere is primed for a major severe weather outbreak in the area shaded in pink, which denotes a "high risk" for severe weather — a 5/5 on a scale measuring the threat for severe weather.
The worst storms will form across an area from northeastern Louisiana through south-central Tennessee, with the bulk of the outbreak affecting Mississippi and Alabama.
Tornadoes are definitely possible outside of the Deep South anywhere in the yellow-shaded area on the map at the top of this post, especially along the Mississippi River from St. Louis northwestward through south-central Iowa.
The Storm Prediction Center issued a statement at 12:05PM CDT saying that they're going to issue a tornado watch across the areas they've outlined in the image to the left of this paragraph. As the afternoon wears on, expected to see these storms potentially produce violent, long-lived tornadoes.
When will it start?
It's a few minutes after noon Central Time as I write this sentence, so the answer is really "any time now." The atmosphere over the risk area is no longer capped, meaning that the environment isn't resisting rising air anymore. As clouds over the region start to break up and the sun heats the air near the surface, air should begin to rise in earnest and severe thunderstorms will grow in coverage.
What are the chances?
The Storm Prediction Center issues maps that show the "probability" that an area will see different types of severe weather, and people misuse and misread these maps all the time. They're really useful if you know how to read them.
Here's today's tornado probability map:
A 30% risk for tornadoes means that there is a 30% higher-than-normal chance of seeing a tornado within 25 miles of any point within the shaded area. Since the percentage refers to the chance of seeing a tornado compared to normal, that's extremely high. A 5% risk for tornadoes usually warrants concern, so anything above that is just dangerous.
The black hatching indicates the risk for intense, long-track tornadoes like the one we saw in central Arkansas yesterday. This is an atmospheric setup conducive to those half-mile to one-mile wide "wedge" tornadoes that tear up entire towns. It is a dangerous setup and one that residents need to watch closely
What can I do to protect myself?
When you're at home, work, or school, take a look around and scout out the safest place to take shelter in case a tornado warning is issued and you need to get to safety. The best place to be in a tornado is underground either in a storm shelter or basement, but many buildings don't have them. If you can't get underground, you'' want to find a small, interior room that has as many walls between you and the outside as possible. Closets and bathrooms are usually the best, especially in townhouses, offices, and schools.
If you live in an apartment building and you're not on the ground floor, you need to make some friends downstairs and go to one of their apartments if a tornado threatens. Tornadoes can shear the top one or two floors off an apartment building with relative ease, so it is not a place you want to take shelter.
Mobile homes are the worst place to be in any type of severe weather, let alone during a tornado. Mobile homes are not built to withstand winds much higher than 60 MPH, and oftentimes they start seeing damage well before that. If you live in a mobile home and tornadoes threaten your area, leave and find a sturdy building. Go to a bank or local school — they will let you in to take shelter.
- The Storm Prediction Center is the official agency in charge of predicting severe thunderstorms in the United States.
- The National Weather Service issues local forecasts, as well as severe thunderstorm/tornado warnings across the U.S.
- Wunderground provides excellent weather radar imagery — click on the + closest to your location.
- NASA's website provides excellent satellite imagery from the GOES weather satellites.
- Dr. Greg Forbes of The Weather Channel is a world-class meteorologist who developed the accurate TOR:CON index that evaluates the threat for tornadoes on a 0-10 scale. Dr. Forbes' TOR:CON forecasts are found on weather.com.
[Images via SPC and College of DuPage]
UPDATE 12:43PM CDT: A Particularly Dangerous Situation ("PDS") Tornado Watch is now in effect for parts of Louisiana, Mississippi, and Alabama. The enhanced "PDS" language is only added to the most dangerous tornado outbreaks, and as such it's very rare to see.
UPDATE 3:11PM CDT: This post has been updated to reflect the upgrade from a "moderate" risk to a "high" risk for a dangerous tornado outbreak.