Big pattern change means a big cool down later in the week

Temperatures will turn quite a bit cooler by the end of the week with highs struggling to even make it out of the 30s! Some of the coolest air we've felt all season is forecast to move behind another powerful Fall storm system later this week. Before the cool down temperatures are actually going to rise back close to 70 degrees by the middle of the week.

A fast-moving jet stream to the northwest of the Stateline has kept skies mostly cloudy for some, but has allowed some sunshine to appear across northern Illinois and southern Wisconsin. Temperatures Sunday warmed into the upper 50s, reaching 59 degrees in Rockford. Under a mostly clear sky Sunday night temperatures will fall to the low 30s. There will likely be just enough of a breeze to keep widespread frost from forming, but patchy frost in the typical cooler locations is possible. Sunshine returns Monday with highs right around average, warming into the low 50s.

High pressure will once again reside over the Southeast allowing a southwest wind to develop across the Midwest and Great Lakes. This will help bring temperatures back up above average, warming into the middle 60s on Wednesday and then possibly close to 70 degrees (if not low 70s) by Thursday!

A strong Fall storm system, however, will develop out west and quickly move through the High Plains and Upper Midwest. As it does a strong cold front will sweep through Illinois and Wisconsin either late Thursday night or Friday morning. The air mass ahead of the front will bring well above average temperatures, but the air mass behind the front will bring well below temperatures. 

Like this past Saturday, highs on Friday are likely to be achieved early in the day with temperatures quickly falling once the cold front comes through. This means afternoon temperatures will remain in the 40s and 30s. Skies will be quick to clear, but it'll remain cold and windy. The chill will last through the weekend - Saturday and Sunday - when high temperatures struggle to make it out of the 30s. Saturday's high is forecast to rise only to 36 degrees with Sunday's high only at 35 degrees. Overnight lows both nights will fall into the 20s with chills most likely in the teens, if not single digits.