Strong storms and potentially heavy rains are likely to occur across the Chicago area beginning this afternoon and running through tonight. Heat Indices should top out in the 100 – 110-degree range almost area-wide with temperatures expected to warm into the 90s and dew points in the very humid mid to upper 70s. This combined with a cold front sagging south out of Wisconsin makes for very unstable explosive atmospheric conditions.

There looks to be a very high probability (70% or greater) of thunderstorms within 12 miles of a given location over much of our area between 3PM and 7PM – see red-shaded area on the map below. The depiction between 7PM and 11PM indicates an additional 40 to 70% chance of t-storms. A third map shows our entire area under a Slight Risk of Severe storms today and tonight (yellow-shaded area).

The cold front may well move more quickly south over the cooler waters of Lake Michigan – this could bring early relief in the oppressive heat for Winnebago, Boone, Lake, McHenry, Cook, Kane, Du Page, Will Counties in Illinois and Lake and Porter Counties in Indiana counties, but set the stage for repeated storms along the frontal boundary and a great potential of Flash Flooding – thus that area is under a Flash Flood Watch from 2PM this afternoon to 3AM early Wednesday morning (see bottom depiction prepared by the Chicago National Weather Service).

Probability of t-storms 3PM – 7PM Tuesday
Probability of T-storms 7PM – 11PM Tuesday
Risk of Severe T-Storms 8AM Tuesday – 7AM Wednesday