Very isolated showers and thunderstorms have been popping up across far northern Illinois and southern Wisconsin Friday evening producing brief downpours. These showers will continue to bubble up across the area through much of the night.
We then turn our attention back out west in Iowa as thunderstorms have started to develop on the heals of an increasing low level jet (stronger winds in the lower levels of the atmosphere), as well as an increase in both warm air and moisture. These storms should begin to develop between 10pm and 11pm, moving east into northern Illinois after Midnight.
Even though the sun will have set and we will have lost our daytime heating, instability building in the mid levels of the atmosphere will continue to grow through the night. This combined with the increasing low level jet and upper level disturbance moving through Iowa will help ignite a few thunderstorms over north-central Iowa. While widespread thunderstorm activity isn’t expected, a few of the stronger thunderstorm cores could produce some hail, as well as a localized wind threat. Heavy rainfall will also be possible through the night which could pose a localized flash flooding risk. Any storms that do develop should be pushing more to the south by Saturday morning.
A few storms may be ongoing across the region Saturday morning before drying out mid to late morning. A warm front southwest of Rockford will begin to lift north/northeast during the morning and afternoon, slowing slightly if clouds and rain linger a little longer during the morning. By early afternoon the warm front should be positioned across south-central Wisconsin, down through north-central Illinois. Behind the front, heat and humidity will quickly build as temperatures rise into the low 90s and heat index readings reach the triple digits. A HEAT ADVISORY will go into effect for Stephenson, Jo Daviess, Carroll and Whiteside counties at Noon as the heat index could rise as high as 105 degrees.
An isolated thunderstorm or two remains possible during the afternoon Saturday as a rather strong upper level disturbance moves across the Plains and into the Midwest. Individual thunderstorms, known as supercells, will form from Minnesota to Wisconsin during the afternoon, merging into a cluster of storms by early evening. These storms will most likely move from west to east, entering into central and southern Wisconsin Saturday evening. The majority of the thunderstorms may remain north of the state line, but a southeast shift in the storm cluster is possible, putting far northern Illinois in line for a wind threat with any storm that moves through. The window for those thunderstorms would be roughly from 8pm – Midnight. As a cold front moves through Saturday night the focus will turn to heavy rainfall as moisture remains high in the atmosphere.
An enhanced risk for severe thunderstorms is in place from Minnesota, northern Iowa and Wisconsin with a slight risk extending south to near Highway 72 in northern Illinois. Locally, the biggest threat from any Saturday evening storms would be damaging wind gusts, with an isolated tornado risk in Wisconsin. We’ll continue to monitor the radar Friday night as thunderstorms begin to develop in Iowa, as well as storm development during the afternoon and evening Saturday. If you’re planning on being out Saturday please make sure you have a way to monitor the weather and receive any weather updates and/or alerts.