Satellite imagery late Friday evening showed a swirling cloud mass moving southward out of Canada. This feature represents a disturbance associated with cold temperatures in the mid- and upper-levels of the atmosphere. By Sunday afternoon, the core of this cold pool is forecast to move across the upper Great Lakes.

Over Chicago, temperatures just 7,000 feet above the ground are expected to be at or below freezing. This will cause relatively warm, 70-degree air at ground level to ascend into towering cumulus clouds. Scattered afternoon and early evening showers and thunderstorms can be expected both Sunday and Monday. Brief, heavy downpours are possible in localized areas. Jet stream flow is forecast to become westerly by midweek, allowing the return of more summerlike weather.