With the notable dry period over most areas late in the summer and the rains reappearing somewhat in September, what does all this mean for the annual fall foliage display? Actually not as much as one might think. Fall leaf color is basically caused by lack of sunlight and to a lesser extent is influenced by the September and October weather.
Drier than normal weather late in the summer into early fall will tend to accelerate the leaf changing process, causing the leaves to fall prematurely. Likewise, a wet September and October will tend to produce fewer vivid colors and the leaves may also fall earlier due to the rain, wind and storms. The prime weather conditions which are conducive to brilliant fall colors are warm, sun dominant days and cool, crisp nights but without frosts or freezes; such as high temperatures in the 60s and 70s with lows in the mid 30s to around 50 (or similar to the our Indian Summer-like weather experienced recently). A true Indian Summer is briefly defined as an extended period of warm, mainly dry weather after a killing frost and or freeze. While some areas have had a few frosts and even freezes, others have not. This is typical in the autumn across Southeast Lower Michigan as frosts and freezes generally progress southeast across the region with time.
Sharp, daily temperature swings and more importantly, the decrease in sunlight, play vital roles in the development of the leaf color. This combination of weather and lack of sunlight, creates a blocking effect on the sugars which are manufactured in the leaves and keeps them from reaching the root system. Eventually, these sugars convert to pigments that produce the vivid and brilliant colors seen on many trees in the fall. Evidently, the green chlorophyll in the leaves begins to fade during the shorter fall days with subsequently, less sunlight. Thus, the other color pigments already in the tree leaves are exposed, come out and produce the fall color splendor. The yellow color seen in some leaves is created by the xanthophylls pigment, while the orange-red color is caused by the carotene pigment and the red-purple color can be attributed to the anthocyanin pigment.
While color peak may vary season to season across Southeast Lower Michigan, generally the maximum leaf color occurs during the second and third week in October. This appears to be close to schedule with the trees changing quickly now and most likely will peak around the third week of the month (or approx the 16th - 25th).
Generally Southeast Michigan's color season peaks the second to third week of October as high temperatures rise into the mid 50s to around 60, while lows dive into the mid 30s to around 40. October is usually one of the area's drier months with an average rainfall of 2 - 2 1/2 inches.