Heavy rainfall may produce some flash flooding this evening and overnight, particularly for those West of Rockford. Slow-moving thunderstorms with very heavy rainfall rates may produce some torrential rainfall across parts of the region.
So far this afternoon, coverage of storms has remained relatively limited, with only a few storms developing along an outflow boundary. These storms centered from around Walworth, WI down toward Sublette, IL will slide East and continue to weaken a bit, but additional coverage will be possible later this evening.
Additional development will begin after sunset from the West. Exact location of where these storms will initiate is still unknown, but slow-moving thunderstorms one right after another will be responsible for producing heavy rainfall across a narrow corridor.
These thunderstorms will likely persist for several hours into the night with the highest coverage developing across parts of Iowa where conditions will be more favorable to sustain storms longer. As such, West of Rockford will be most favored to see the extremely heavy rainfall, with some spots picking up over 2" of rain.
A crude computer model of rainfall accumulation shows this off well, with likely a sharp cutoff of heaviest rainfall the further South and East you get from Rockford. Highest confidence on 2" or more of rain is centered mainly near and West of Freeport.
The highest potential for flash flooding will be centered across Iowa, Southern Wisconsin, and Northwestern Illinois, where a Moderate Risk for excessive rainfall has been issued from the Weather Prediction Center. This equates to a 40% chance to reach flash flooding guidance, where instantaneous rainfall rates could exceed 1.5"/hour.
Those same locations are under a flood watch through Monday. Green and Rock Counties in Southern Wisconsin along with Jo Daviess, Stephenson, Carroll, and Whiteside Counties in Northwestern Illinois are included in this flood watch. Be sure to keep watch for flooded roadways and heed any flash flood warnings that are issued!
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