"This is a particularly dangerous situation," says the Storm Prediction Center as a derecho is hauling eastward towards the Chicago metro area after producing 80 MPH winds across Iowa. The line should reach the Chicago metro area during rush hour, sometime between 530 and 630 Central Time this evening.
The "Particularly Dangerous Situation (PDS) Severe Thunderstorm Watch" is in effect for southeastern Wisconsin, northeastern Illinois, and northwestern Indiana through late this evening. In addition to the likelihood of "scattered gusts to 80 MPH," the storms could also produce ping pong ball size hail and a couple of tornadoes.
[Radar image from 4:28PM CDT]
As the structure of the storm is continuously changing, the worst of it could barrel right into Chicago proper or skirt to its north, hitting Madison and Milwaukee instead. The entire area is covered by the PDS watch, though, so it's prudent to stay alert even if you don't think you'll get the worst of the weather.
A derecho is an organized line of thunderstorms (called a "mesoscale convective system," or MCS) that produces widespread damaging winds over a path hundreds of miles long. This complex often produces significant wind gusts in excess of 70 MPH, and that is certainly the case this afternoon.
It is very rare for the Storm Prediction Center to call a system a derecho while it is still ongoing, which speaks to the power of the storm and the meteorologists' confidence that it will cause major problems for parts of Wisconsin and Illinois.
Here is the unusually strong language that the National Weather Service included in an earlier warning for eastern Iowa ahead of this storm:
This is an extremely dangerous situation with tornado-like wind speeds expected. Mobile homes and high profile vehicles are especially susceptible to winds of this magnitude and may be overturned. For your protection, move to an interior room on the lowest floor of a building. These storms have the potential to cause serious injury and significant property damage.
[Images via GOES and Gibson Ridge]