Dangerous Tornado Outbreak Likely Across Southern US Today

by: weatherdude

Tue Jan 29, 2013 at 13:12:54 PM EST

(Also posted to the orange fainting couch.)

A strong cold front that is currently moving through the Central Plains states will act as the proverbial spark in the powder keg this afternoon, firing off severe thunderstorms across parts of the southern United States. Given the upper-level dynamics associated with this system, there is a moderate (several magnitudes higher than normal) risk of significant, long-track, violent tornadoes over a large area of real estate this afternoon and evening.

The Threat

The main threats today will be significant, long-track, violent tornadoes, as well as violent straight-line winds.

The SPC has issued a moderate risk for severe weather (red) across a large chunk of the south. The yellow area is a slight risk, and the green area indicates a risk for general, non-severe thunderstorms.

Here's the tornado probability chart. An area with a 15% chance of tornadoes means that there is a 15% chance that at least one tornado will touch down within 15 miles of any point in the shaded area. The black hatched area indicates the risk for significant (EF-2 or higher), long-track tornadoes.

Here's the severe wind (greater than 60 MPH) map. Percentages work the same as they do with the tornadoes. The black hatched area indicates a risk for severe thunderstorm winds greater than 75 MPH.

The Setup

A strong cold front is moving across the Central Plains states into a very warm, very soupy airmass being pumped in straight off the Gulf of Mexico:

Dew point map as of around 1130AM Central. The greener, the moister.

A cold front ramming into this warm unstable air will wreak havoc for the areas outlined by the Storm Prediction Center above. Winds at 850 millibars (about 5500 feet off the ground) are screaming along in what's called a low-level jet (LLJ), reaching speeds of almost 80 MPH. Winds at the jet stream level (30-35,000 feet off the ground) are moving along at almost 175 MPH.

This change in speed and direction with height is called wind shear, and along with the massive amounts of instability from the warm air, and the forced lift from the cold front, extremely strong storms are expected to develop this afternoon.

The storms are initially forming as individual supercells, which carry the greatest risk for tornadoes. Over time (probably later tonight) these supercells will start to merge with each other and create an intense squall line along the cold front as it heads into the southeastern and eastern parts of the United States.

CORRECTION: The supercells are forming out ahead of the main squall line along the cold front. The individual, more dangerous storms will come through first with the tornado threat, and be almost immediately followed by the damaging winds of the main squall line.

What we're most concerned about are the supercells that form before the squall line. The shear I mentioned before creates a horizontal rolling motion in the atmosphere -- think of a cardboard tube tilted on its side. This rolling tube of air can be tilted vertically by the strong updrafts in thunderstorms, making the entire storm begin to rotate.

This rotation is what facilitates the development of tornadoes. We measure this with something called "storm relative helicity." Essentially, the higher the number, the more likely we are to see strong tornadoes. The threshold changes based on a number of factors, but in this type of setup we typically see tornadoes begin to form with SRH values of around 250.

SRH values are expected to climb up beyond 500, especially over Arkansas, this afternoon.

The supercells, by nature, will eventually form into a strong squall line and move east. When this occurs, the tornado threat will die down and the damaging wind threat will ramp up.


National Weather Service Main Page
National Weather Service -- Central Oklahoma
National Weather Service -- Dallas/Ft Worth TX
National Weather Service -- Tulsa OK
National Weather Service -- Springfield MO
National Weather Service -- Arkansas (whole state)
National Weather Service -- Paducah KY
National Weather Service -- Shreveport LA
National Weather Service -- Memphis TN
National Weather Service -- Jackson MS
National Weather Service -- Huntsville AL
National Weather Service -- Birmingham 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)

NOAA Weather Models

TwisterData.com's excellent GFS/NAM/RAP model website.

I'll continuously post updates to my Facebook page on this and most other major severe weather outbreaks.
weatherdude :: Dangerous Tornado Outbreak Likely Across Southern US Today
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Mixed emotions: glad to see you here, weatherdude, but (2.00 / 21)
here we go again with severe weather. Right now it's 77 here in the Houston area, cloudy, just light winds, but there's an ominous feel in the air.

Stay safe out there, meeses!!

I never travel without my diary. One should always have something sensational to read in the train.

-- Oscar Wilde

Thanks, weatherdude. (2.00 / 18)
Arkansas looks really bad.  

Thank you weatherdudde (2.00 / 15)
It looks like spring storms here in North Texas.

They are training in the west and headed our direction but the rough stuff looks to be to our north at the moment.

Everybody stay safe and scrap the snow off of the cover of the storm cellar.

We had thunderstorms this morning in Madison. (2.00 / 15)
Flood watch from 3pm until 3am, winter storm watch for tomorrow morning 6am to 6pm.

Now I have to worry about tornadoes in January?

Words have meaning. Our words will reflect what is in our souls.

thanks weatherdude n/t (2.00 / 15)

Thanks (2.00 / 15)
According to our weather forecast here in Winston Salem, NC we are okay today but possibility of severe thunderstorms and high winds tomorrow. Sigh. Gee didn't I leave Indiana to get away from ice, snow, and tornadoes?

"A dreamer is one who can only find his way by moonlight, and his punishment is that he sees the dawn before the rest of the world." Oscar Wilde

Now we are under the gun until 8 pm cst. The watch stretches (2.00 / 14)
east thru St. Louis and into Illinois.  The highest tornado threat is still looking like southeast Missouri counties and stretching south into Arkansas.  We are having sporadic thunderstorms but there's a fairly tidy line of new cells on the march.

"I base most of my fashion sense on whether or not it itches"  -- Gilda Radner

I'm on the edge of the slight risk area in central KY. It won't get here until (2.00 / 13)
after midnight--I hate these kinds of storms after dark, at night when people are asleep. But the storms should weaken before they get here (hopefully). I think i'm going to sleep with my weather radio next to my head tonight.

I heard NWS in Louisville has called for a phone conference at around 2:30. When I find out what was said, I'll post it here if it looks important.

Glad to see you keeping us informed, weatherdude.  

From what I read on KY Mesotrack, NWS in Louisville seems to (2.00 / 6)
be downplaying the severity of this system somewhat. They are saying the main threat in the Louisville area (which includes some of the moderate risk areas) will mostly be damaging winds with a few weak, isolated tornadoes. We'll see--I don't know if it is wise of them to downplay this, considering the set-up with this system.

(A lot of the weather geeks here in KY seem to think Louisville NWS sucks half the time, while Jackson sucks all the time.)

[ Parent ]
Also posted to the orange fainting couch (2.00 / 13)
Made me LOL

Great Comment (2.00 / 10)
Thanks for pointing that out. I missed it the first time.

"A dreamer is one who can only find his way by moonlight, and his punishment is that he sees the dawn before the rest of the world." Oscar Wilde

[ Parent ]
Me too (2.00 / 10)

Or this?

Words have meaning. Our words will reflect what is in our souls.

[ Parent ]
Well played - with hovers as well!! (2.00 / 7)
I surrender to your brilliance, uh, oh, I'm feeling a little light headed.  I think I'm going to faint...  

[ Parent ]
Actually ... (2.00 / 6)
I am responsible for HoverMania™. I feel compelled to continue the tradition wherever possible.

Words have meaning. Our words will reflect what is in our souls.

[ Parent ]
With all due appreciation (2.00 / 12)
For the marvelous work, is it absolutely necessary to put the geographic center of all the red splotches on your maps directly over my sister's house in Memphis?

Seriously, thanks. I've shared this with her.  

Enduring security and lasting peace do not require perpetual war.

Barack Obama 1/21/2013

Just talked to a customer in Little Rock (2.00 / 11)
and he said that the whole city is freaking out expecting the worst and that they're all ready.

I told them all to act right because we are all watching them.

I'm in Alabama, (2.00 / 4)
so this is soooooooooo very useful. Thank you. Hoping the worst of it kinda misses our area. If it does, it won't be by much.

Come to me in my dreams, and then
By day I shall be well again!
For so the night will more than pay
The hopeless longing of the day.

Extreme winds here in Dandridge, TN at the moment. (0.00 / 0)
My partially-built office is rocking like a ship at sea. Hope the walls stay on.

John Askren - "Never get into a pissing match with a skunk."

More winds, 12:45am. (0.00 / 0)
Shaky shaky!

John Askren - "Never get into a pissing match with a skunk."

[ Parent ]

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