UNIVERSITY PARK, Pa. — Warm, wet weather is predicted to continue through the rest of September and most of October in the mid-Atlantic, Northeast and Great Lakes regions, and those soggy conditions likely will result in a subdued foliage display, according to a Penn State forest expert.

"This is the opposite of what is needed to bring out the best and timely colors, which require cool and dry conditions with the onset of fall," said Marc Abrams, professor of forest ecology and physiology in the College of Agricultural Sciences. "I predict that there will be a late — and muted — leaf coloration this October."

The foliage outlook is a bit better for the Great Lakes and Adirondack regions, Abrams noted, because although they received above-average amounts of precipitation, they were spared the exceptionally high amount of rainfall other regions received.

"I'm not expecting a total washout because even during the worst of times trees produce good to fair color," he said. "But it may take a bit more hunting to find the best color this year. What we need now — and what we are not likely to get this fall — is for cool to cold temperatures to arrive by early to mid-October to bring out the best colors."

For three 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. These other pigments — called xanthophylls and carotenes — are what 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, blackgum and scarlet oak.

"One thing that I have been impressed with in my 30-plus years of gauging foliage is the resiliency of the display," he said. "Year after year, despite the conditions, there are places where the trees show good color, but perhaps not great color every year."