With permission, I am sharing this from Mark File's RomanticAsheville website. Look at the difference in the fall leaf color between 2014 and this year for September 28th. Quite amazing. Thanks to Mark for finding these amazing photos.
I received a note from the NC Climate Office and they say that in Sep we have had minimum temperatures averaging about 7F above normal, and max temperatures above normal, although not by that much. Washington, DC has had it's rainiest and near warmest Sep in history. All of these factors combine to delay the onset of fall leaf color.
Departures from Normal This Fall for the Weather
I accessed the The Southeast Regional Climate Center’s “Climate Perspectives” tool (http://www.sercc.com/climper/Map.php?frame=true#) and analyzed all 10 weather stations in the mountains above 2,000’ elevation for the temperature departures from normal for this fall season (Sep 1 to Oct 2).
The MEAN MINIMUM TEMPERATURE (usually, but not always, at night) was 61.2F (16.2C). This is a whopping 9.2F (5.1C) above normal for this period. For every station accessed, with the exception of Highlands, NC, this departure was the warmest EVER ON RECORD. In Highlands, it was the 7th warmest, but that still put it in the 94th percentile of all years.
The MEAN MAXIMUM TEMPERATURE during this period was not less extreme at 3.5F (1.9C) above normal, with an average temperature of 77.9F (25.5C). This ranged from the 5th warmest ever in Asheville to the 38th warmest in Highlands.
This means that the current fall weather is unprecedented for as far back as weather measurements have been made. That being said, it also means that the responses of the trees to this unusual weather is also unprecedented, since they have never experienced this before.
My initial thoughts on this are that it sends a signal that the nighttime temperatures may play a much more important role in the development of fall leaf color than daytime temperatures, since the daytime temperature departures are not as extreme as those for the night.
So, time to do some temperature experiments on trees to see how it affects leaf color and retention!