Parts of the Midwest from Des Moines to the Ohio River are under the gun for severe weather today. While a large chunk of real estate is at risk, it looks like the worst weather — including tornadoes and hail larger than golf balls — seem to favor Chicago this afternoon.
The highest risk for severe weather today lies in the areas shaded in yellow on the map at the top of this post, which are pegged for a "slight risk" for severe weather by the Storm Prediction Center. A slight risk is a 2/5 on a scale measuring the threat for severe weather. The risk is "enhanced" around northern Illinois, Chicago, and northwestern Indiana, making it a 3/5 on the scale.
What will happen?
The thunderstorms today are expected to fall under the usual pattern of supercells during the day, coalescing into a squall line as the night progresses. The SPC expects the strongest supercells to form in and around northern Illinois — including Chicago — which is where the threat for tornadoes and extremely large hail exists.
About the hype...
As the supercells turns into a squall line, the threat for severe weather will transition over to damaging winds. There is a lot of buzz that the squall line might turn into a derecho, but that's a bunch of bull.
The term "derecho" has a very specific meaning that can only be determined after the storm happens. Weather weenies prematurely scream about storms being a derecho just to get attention. A squall line doesn't have to be a derecho to be dangerous.
What is the risk?
Here's the tornado threat today. 5% warrants concern; it means that the risk for tornadoes today is 5x higher than normal.
Here's the hail threat, showing the bullseye over the Chicago metro. As with tornadoes, the 30% shaded area means that the risk for large hail today is 30x higher than normal. The black hatching indicates the risk for significant hail, which is the size of golf balls or larger.
Why is this happening?
As you can see by my awfully-edited map above, there is a weak low pressure system over the extreme southern border of Manitoba and Ontario in Canada, from which a weak cold front extends southward into the United States.
At the same time, an area of high pressure is sitting over the Florida Panhandle, pumping warm, moist air into the center of the United States straight from the Gulf of Mexico.
The combination of instability, strong low-level winds from the southwest, and forcing from the cold front will help to set off severe thunderstorms across the risk zone today.
- 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.