Actually last year, my first seed sprouted about May 2nd. Mainly perennial rye. And the growing degree days that had accumulated were 81. This was in the spring of 2013.
So I guess I am being optimistic if I expect seed to germinate in the second week of April.
On average, using base 50 degrees, about 50 Growing degree Days is reached sometime between April 1 and 9.
Due to the influence of Lake Michigan we should be slightly warmer in the late fall and slightly cooler in the late spring...than nearby areas.