NEW: Severe Thunderstorm Warning issued for Jo Daviess and Stephenson counties until 6:45pm. Storm is moving to the east at 60mph. Quarter sized hail and wind gusts to 60mph possible with the this storm.
UPDATE: SEVERE THUNDERSTORM WATCH now issued to cover all of northern Illinois and southern Wisconsin through 11pm Tuesday evening. Isolated severe thunderstorms have developed over Wisconsin, producing primarily large hail Tuesday evening. As a cold front moves further southeast, large hail and damaging winds will be our primary threats with any storms that do develop. The cold front is expected to pass between 9pm and 10pm, ending our severe threat.
A Severe Thunderstorm Watch has been issued for Rock and Walworth counties in southern Wisconsin as well as for McHenry County in northern Illinois until 9pm. A cold front stretching through Wisconsin will continue to move southeast through Tuesday evening. It will move into a fairly unstable atmosphere, however, warmer temperatures aloft (known as a cap) are preventing rapid storm development as the front moves southeast.
However, a few isolated severe thunderstorms have developed near Milwaukee and will continue to push east through the evening. It’s possible that the cap further south may weaken a little over the next couple of hours, allowing storm development a little further to the south. Right now, it appears the best area for thunderstorms will reside over southeast Wisconsin and northeast Illinois.
If thunderstorms do develop over the Stateline, the greatest impacts would be hail and damaging winds. The cold front will move through much of the area between 9pm and 10pm, ending the storm threat overnight.