A SEVERE THUNDERSTORM WATCH has been issued for Carroll, Jo Daviess and Whiteside counties in northwest Illinois until 7pm Tuesday. An extremely hot and humid air mass has developed across northern Illinois and southern Wisconsin as temperatures have soared into the low 90s and the heat index well into the low 100s.
Thunderstorms to the north in Wisconsin have been mostly moving to the southeast, towards southeast Wisconsin. Those storms, however, have produced an outflow boundary that is currently located north of Janesville and has prompted a few showers to begin forming along it within the last 30 mins. There is also another outflow boundary in southwest Wisconsin; both boundaries have had an increase in the cumulus field early this afternoon.
Another area of concern are severe warned storms near Waterloo, IA, moving further east into northeast Iowa. These storms are likely to interact with a surface boundary, as well as the outflow boundaries, to increase thunderstorm development over southwest Wisconsin and northwest Illinois within the next couple of hours. Biggest threats with any thunderstorms would be the risk for wind gusts to 70 mph, as well as for golf ball sized hail. The timeline for the storms out west to reaching northwest Illinois would roughly be from 4pm/5pm to 7pm. Make sure to keep an eye on the radar later today and have a place to seek shelter should severe weather occur.