...FASTER FALL FOLIAGE?...
By: William R. Deedler, Weather Historian, WFO Detroit/Pontiac Mi
The Summer of 1997 in Southeast Lower Michigan will be remembered for its brevity
and unseasonably cool August. It seems as if the summer took forever to get here and
when it finally did arrive, it stayed only a month or so. 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 spring, when here too, it was the main
culprit our spring was such a late visitor. Simply put, when the dominant wind flow
is northwest across the region, one can expect cooler than normal weather for that
time period. Besides being cooler than normal, the summer also had a notable dry
period in July whereas in August, the rains reappeared but it was significantly
cooler than normal. Now, 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 in the late 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. 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.
Latest indications are this years color festival is slightly ahead of schedule. Looking
ahead through October, the National Weather Service outlook for calls for temperatures
to be near normal to below and rainfall near normal to above. Generally across Southeast
Lower Michigan during peak color season or mid-October, high temperatures rise into
the lower 60s and lows fall into the lower 40s. October is usually one of the area=s drier
months with an average rainfall of just over two inches.