This summer was notable because of drought conditions in June and July, and searing heat in July, but that won't limit the beauty of Pennsylvania's fall foliage.
Instead, it's the next few weeks' weather that will be determinant, according to a forest expert in Penn State's College of Agricultural Sciences. And long-term weather forecasts point to mild weather—60- to 70-degree days and nights in the mid-40s—through the end of October, with no frosts.
For fall foliage lovers, that's not good, warned Marc Abrams, professor of forest ecology and physiology.
Although many wish for extended summer this time of year—with warm autumn days and mild nights—if you want to see really brilliant foliage in October, you should hope for cooler weather, Abrams explained.
"Fall weather is very important," he said. "We need to have cool temperatures falling into the 30s or low 40s at night, as well as bright sunny days with little rain."
If temperatures cool in coming weeks with little rain falling, fall foliage watchers in Pennsylvania should be treated to a superior display of color this year, Abrams predicted.
"If we cross our fingers and Mother Nature cooperates, we should have great color by the middle of October," he said. "Despite the weather extremes, most trees held up well and benefited from abundant rainfall later in the summer."
Persistent drought in other regions of the country will affect fall foliage, of course, but in Pennsylvania and the mid-Atlantic region, forests are in pretty good shape. Only the far western counties of the state remain dry, but probably not enough to affect the fall foliage display.
"It seems the cooler weather that came into Pennsylvania in the last week or so might be an aberration," Abrams said. "From past experience, most trees will keep their green leaf color as long as temperatures stay relatively high. We really need nighttime temps to start getting into the low 40s and 30s to bring out peak colors."
Still, whether it cools down sufficiently or not, Pennsylvania's forests are amazingly resilient and dependable, so there will be colorful foliage in mid-October, he pointed out. It just remains to be seen whether this will be one of our better foliage years.
For more than two decades, Abrams has studied how seasonal precipitation and temperature influence timing and intensity of fall colors in central Pennsylvania. "We believe that clear, bright days, low but not freezing temperatures, and dry but not drought conditions promote the best fall colors," he said.
Cooler temperatures signal deciduous trees to stop producing chlorophyll, the green pigment responsible for photosynthesis, he explained. The chlorophyll breaks down and disappears, unmasking other leaf pigments. It's these other pigments—called xanthophylls and carotenes—that create the yellows and oranges seen in the leaves of yellow poplar, hickory, sycamore, honey locust, birch, beech and certain maples.
After chlorophyll production stops, trees also produce another pigment in their leaves called anthocyanin, according to Abrams. The anthocyanins create the brilliant reds and purples seen in maple, sassafras, sumac, black gum and purple oak.
The amount of anthocyanin produced each year is related to starch levels in the tree. Trees often produce less starch during droughts.
"One thing that I have been impressed with in my 20 years of gauging foliage is the resiliency of the display," Abrams said. "Year after year, despite the conditions, there are places where the trees show good color, but perhaps not great color.
"People should go out and search for those pockets of bright color, because they will be there."
Explore further: Wild yaks: Shaggy barometers of climate change