The threat for severe thunderstorms Friday afternoon and evening continues to grow as a large storm system moves across the Midwest. Damaging winds, hail, and tornadoes are all possible for northern Illinois and southern Wisconsin. However, there is a slightly higher risk for tornadoes west of I-39.
Skies remain cloudy Thursday evening as a warm front moves in from the south. Showers and mixed precipitation have developed along the leading edge of warm air aloft, moving through Minnesota and central Wisconsin. We may see a sprinkle or two during the evening, but isolated thunderstorms are likely after Midnight. Severe weather is not expected with those storms, but a few of the stronger ones may produce some small hail. Those storms will be weakening/fading by Friday morning, with a chance for a few scattered showers going into Friday morning.
Two rounds of thunderstorms are possible Friday, one for the early afternoon and then another late afternoon and evening. The first round is forecast to form along a weakly organized upper-level low. Our atmosphere is capped, meaning it has a lid on it, during that time. This would cause those storms to become elevated giving us a hail and gusty wind threat. If, however, that cap weakens or breaks then the risk for those storms to become severe would increase, producing a slightly bigger hail and wind threat. The window for those storms to move in is from 1-3pm.
The second round of thunderstorms pose more of a severe threat, assuming the first round clears out fast enough. These storms will initially develop in Iowa ahead of low pressure and a cold front, quickly turning severe early afternoon. As the storms move east, they’ll develop into a line crossing into northwest Illinois. There is a risk that a few isolated storm cells remain detached from this main line. If that occurs damaging winds and isolated tornadoes would increase. The greatest threat for that remains west of I-39.
The damaging wind threat will increase as the line of storms move east across northern Illinois and southern Wisconsin. The timeline for this second round is roughly from 4pm to 10pm. Once the storms move out winds will increase quickly behind the departing low pressure system. Cold air will rush in behind bringing temperatures down into the 30s through Saturday morning. Scattered snow showers are possible early Saturday with drier skies for the afternoon.
Please make sure to have multiple ways to receive watches and warnings Friday and know where to seek shelter should severe weather occur. Storms will be quickly moving from the southwest to northeast between 50-60 mph. This will not give you a lot of time to react when a warning is issued. Plan today, be ready to act tomorrow. Updates will continue regarding the severe potential, so be sure to keep checking back.