Update 8:15PM CDT…
The Severe Thunderstorm Watch for western Illinois until 1AM CDT ( pink-shaded counties on the headlined map) has been extended east to include Grundy, Will, Kankakee, Kendall and Livingston Counties south of Chicago. Strong thunderstorms will develop in the newly -added counties this evening with damaging winds and large hail possible under some of the strongest storms. The Flash Flood Watch continues in the added counties, as well as those counties noted earlier (dark green-shaded in headlined map).
Severe thunderstorms will move out of Iowa and northern Missouri into western Illinois early this evening – Severe Thunderstorm Watch until 1AM CDT is in effect for that area (pink-shaded counties on the headlined map) – reaching as far east as LaSalle and Lee Counties.
While the Chicago area may experience some strong storms with damaging winds and large hail in spots later this evening, the primary threat appears to be heavy downpours with most of the Chicago area in a Moderate Risk for Excessive Flood-Producing Rainfall (see map issued by the National Weather Prediction Center below) – as much as 2 to 4-inches additional rains could accumulate in some areas where “training” storms repeatedly track over the same locations.
The Flash Flood Watch (green-shaded area on the headlined map) remains in effect into Friday morning. Heavy rains could cause many river segments to rise to bankfull or minor flood levels (see hydrologic river map below).