Written by: William R. Deedler, Weather Historian SEMI - 10/12/2011
/*Latest Update - 10/19/2011/
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 or Indian Summer-like weather (depending on location) experienced recently.
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 18-26th). Unfortunately, it looks as though the beautiful gorgeous weather of the past ten days or so is on the wane, because of rain and waves of colder weather. For the next week or so, periods of wet weather along with progressively colder temperatures will spoil our peak autumn color season at least, somewhat. This next weekend will be nothing like last weekend with damper, breezy and colder weather to hold sway over Southeast Michigan. Best chance for rain will be late this week. Parts of the weekend may be salvageable for color viewing but the wind may screw that up too.
After, early indications are that the models intimating another more powerful cold air mass to blow into Southeast Lower Michigan by mid-late week /18-21st/ and this would follow the trends written in the Outlook. Preliminary indications a cold rain should move across the region sometime Tuesday into Wednesday with brisk wind conditions and unfortunately, this lousy weather will accelerate leaf drop. At their coldest, highs should be in the 40s to around 50 with lows in the 30s (some 20s are even possible if we clear out one night and winds decouple by the weekend). Normal highs at this time are in the upper 50s to lower 60s while lows fall into the lower to mid 40s.
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*Update 10/19 on predicted storm:
Deep storm with heavy rain and strong winds is on track to affect the entire region through Thursday (for current forecast, see: NWS Southeast Lower Michigan ).
She's a beaut' of a storm and note its placement late Wed evening on the12z 10/19/11GFS with a projected central pressure of 988MB /29.22"/ in or near SE Mich. The record low pressure for Detroit in October was way back on 10/14/1893 in an intense low pressure of 985MB /29.09"/. In more modern times, back in La Nina Fall (which ran from JJA 1970 – DJF 1971/72 & ONI of -1.3) on 10/16/1972, the pressure crated to 985MB /29.12"/ for the second lowest pressure at Detroit. Of course, remember these readings don't have to necessarily denote the central pressure of that particular storm since the center of the low pressure didn't have to pass right over Detroit. We will have to see how low the pressure actually gets at DTW. Mother nature does seem to have a preference toward spinning up the deepest storms for October in Southeast Lower Michigan during mid month.;-)
Latest GFS model guidance intimates we a not done with these cold October blasts nor storms. Another powerful cold jet core is being suggested on the GFS model diving out of northern Canada and into the Midwest/East next week (and this is not the first model run this has shown up for mid next week). If this happens, it would open the door for more storminess and subsequent cold air. HOWEVER, the European model is much weaker and further west with this cold air plunge and trough. If this is right, warmer weather will push out ahead of the system with a ridge developing over the East. So, the fun has just begun! Check back to see how this one plays out too, if this continues to be forecasted. --------------------------------------------------------------------------------------------------------------
After the gorgeous stretch of weather thru October (basically thru the 11th with temperatures averaging some 6-7 degrees above normal and virtually no rain) mid October offers quite a change and things will go downhill fast (at least for awhile).
From the Autumn Outlook issued early September:
The thing that is worth noting were the sharp contrasting cold snaps that periodically erased the longer warm spells. However, in spite of those sharp cold snaps throughout the fall, it also should be noted that October and in some years, early November had the best chance to contain some of the nicest weather. However, on the flipside, these sharp cold air masses tended to bring the risk of snow and the start of winter as a whole, earlier than on average.
So, the moral of this fall’s story remains the same:
Enjoy the periodic nice weather & Indian Summers while you get the chance!
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