Winds will shift around to the northwest Tuesday night following a cold front that brought scattered showers and thunderstorms during the late afternoon and evening. Within the northwest flow aloft, there will be weaker low pressure systems – upper level disturbances – that rotate around the main low near Hudson Bay. Tuesday morning will likely start off with cloud cover, but we should see some sunshine later in the morning and afternoon, warming temperatures into the upper 70s.
Temperatures aloft will be cooler as the northwest wind brings in a cooler air mass to the Midwest and Great Lakes. That change in temperature from the surface to the middle levels of the atmosphere will help increase instability across northern Illinois and southern Wisconsin. The greater the change in temperature, the steeper the instability. When that instability is combined with moisture and an incoming upper level disturbance, isolated showers and thunderstorms can sometimes form during the afternoon and early evening. This usually happens during the peak heating of the afternoon, mid to late afternoon.
Shower activity will decrease during the evening and overnight before picking back up again during the afternoon Wednesday. This means you’ll need to keep a watchful eye to the sky as showers will be possible both days. Overall coverage will be scattered and there may be some locations that don’t see much, or any, rainfall during that time. Drier air does move in Thursday with partly cloudy skies and temperatures expected to warm into the low 80s.