After a tranquil Monday, ingredients are coming together that portend a stormy Tuesday. Late Monday, satellite imagery showed a large cloud mass moving southeast from the northern Rockies. Meanwhile, a low overcast was streaming northward across the Midwest, and thunderstorms had erupted across southern Illinois. These features are to whip into a storm system that is to bring heavy snow to our north, and damaging thunderstorms to the south. The catalyst for this activity will be a sharp temperature contrast along the storm’s path. Low pressure is forecast to pass just south of the metro area, with afternoon temperatures to varying by as much as 20 degrees from the Wisconsin state line to the far south suburbs. February-level cold and a chance of snow are to arrive as the system heads east Tuesday night.