Peak Fall Color Nears

Written by: William R. Deedler, Weather Historian
National Weather Service Detroit/Pontiac, MI
October 15, 2003
Originally written Fall, 1997

The Summer of 2003 in Southeast Lower Michigan will be remembered for its brevity and relatively cool and comfortable weather. It seems as if the summer took forever to get here and when it finally did arrive, it stayed only a couple of months.. The main problem was that we couldn't shake a fairly persistent northwesterly wind flow in the jet stream. This pattern was actually carried over from the winter and spring, where here too, it was the main culprit our winter was cold and spring was such a late visitor.

Besides being a bit cooler than normal, the summer also had a notable dry period in many areas late in the summer. The rains reappeared, somewhat, in September and October and  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 weather experienced recently during the second week of October).

These 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 xanthophyll 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 about the third week of the month (or approx the 20-26th).

For the remainder of October, the National Weather Service is looking for temperatures to average near normal to below and rainfall near normal. Generally across Southeast Lower Michigan during peak color season (or third week of October) 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.


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