On September 22 at 10:29 p.m. EDT, the autumn equinox occurred, which marked the beginning of the Fall season. The equinox marks the point when the sun is on the same plane as the Equator. This can be bitter-sweet as this is a sign that much cooler weather is on the way; however, with the onset of Fall we also begin to see many of our trees display absolutely amazing foliage. According to the Department of Natural Resources in Grand Rapids, there are many factors that can affect the timing and the brightness in the displays. Temperature, precipitation amounts as well as frost and freeze dates are the main meteorological factors that contribute to these amazing fall colors.
Generally the colors begin to peak over interior western portions of Upper Michigan as well as northern areas of Minnesota during the late September and early October timeframe, while areas farther south generally peak by mid October.
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The image below shows the average freeze dates across much of the Upper Midwest. This image was added as a freeze can potentially limit the duration of the brilliant color displays as mentioned under the What Causes Color Change (Minnesota DNR) link. Many of the inland west areas of the Upper Peninsula have already experienced frost this year, with a couple locations getting very close to a freeze over the past week.