(Originally posted to The Motley Moose.)
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 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.
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.
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