It’s a late spring afternoon along the southwestern coast of Lake Michigan, and people from Chicago to Milwaukee are out enjoying the sun after another long Upper Midwest winter. The temperature creeps up to 80 degrees Fahrenheit, and continues to climb, unseasonably. The promise of an early summer lures out gardeners, cyclists, and beachcombers. Then, it happens. A wall of cold lake air slams into the coast and moves rapidly inland. People scramble for shelter, or at least a sweatshirt. The temperature plummets 20, 30, even 40 degrees in minutes….
