Philly’s fall foliage season is looking special, September weather permitting

The sequences of steamy days and downpours in the Philly region and elsewhere won’t rank among the cherished memories of summer, but they may well end up having been a boon to the imminent fall foliage season.

The consensus among the veteran experts who pay close attention to the behavior of the trees is that this is shaping up to be an excellent season for spectacular waves of fall colors that ripple from northern New England to North Carolina, cresting from early October into early November, with Pennsylvania and New Jersey in the middle of it all.

As much as some folks may have been fed up with the humidity banquet, the trees — at least those that survived the storms — have been partying. Unlike last year, characterized by an early leaf fall, the players are remaining on the stage.

“Despite a droughty start to the growing season and some frost damage, our trees are looking green and healthy now, overall,” said Ryan Reed, a forestry specialist with the Pennsylvania Department of Conservation and Natural Resources, who is “looking forward to a beautiful season.”

What’s more, the snows and Arctic freezes of winter also may have made a significant contribution: The long-lasting snowpack patiently watered the foliage as it melted, neutralizing drought conditions. That was especially true in New England, meteorologist Jim Salge said in his Yankee Magazine foliage forecast.

But the key to the foliage season would be what happens this month, said Howard Neufeld, a biologist with Appalachian State University, who analyzed weather-foliage patterns for the last several years around the campus area in the heart of the Blue Ridge Mountains and all but interrogated the trees to get at their myriad secrets, most of which they kept to themselves.

He holds that the main driver in the drama of the season is the production of anthocyanins, the reddish tints that are the show-stealer, and provide the tasteful contrasts to the yellows and russets.

Sunny days and cool nights are best, say the experts, but like everything else involving the atmosphere, this all gets very complicated.

As for whether the weather will cooperate, the burgeoning El Niño aside, the scientists who assembled the federal Climate Prediction Center’s fall outlook saw odds slightly tilted toward above-normal temperatures in the fall.

The day-to-day weather is going to influence when the leaves begin to turn and when they will peak. Just what effect long-term warming is having on the seasons, which are primarily light-driven, is unclear, and “each year brings subtle differences,” says William Keeton, forestry professor at the University of Vermont. Typically, the leaves up his way take on serious color by the end of September and peak the first or second week in October. Where Neufeld, an emeritus professor of ecophysiology, works in the Blue Ridges, whose ultra-popular foliage season was wiped out two years ago by Hurricane Helene, the peak can extend into early November.

On average, in the immediate Philly region, expect onset in mid-October, with peak around Halloween.

The higher the September temperature, averaged for the month, the later the onset of color, Neufeld said. A single degree could mean a difference of two days, he added.

Conversely, he found that higher daily maximum temperatures in September actually sped up the onset. (He didn’t know why, either.)

The key period for the weather conditions begins right about now, he said. A run of sunny days into October would be ideal for color, but a warm and rainy period would take the blush out of the reds and make for a duller show.

But that’s very much subject to variability: The trees have complex inner lives.

Without getting too deeply into the weeds (or the leaf piles), here are the basics of the process.

Along with the shorter days — Philly loses an hour and 15 minutes of sunrise-to-sunset time this month — the sun is losing some mojo. With the dimming of the light, green chlorophyll recedes from leaves, yielding to those carrot-colored carotenoids and flamboyantly reddish anthocyanins. And Neufeld holds the reds are the stars.

“The oranges and yellows tend to be the same from year to year,” he said, “but the reds vary.” When it’s sunny and cool, sugars accumulate on the leaves and the reds can go to town. But if it’s cloudy and warm, the sugar factories shut down.

Just why some leaves turn red, such as the maples, “is not completely understood,” Keeton said.

Neufeld, a biologist, noted that it doesn’t happen in Europe, and holds the redness is related to a form of malnourishment — a function of a lack of nutrients in Eastern North American soils.

Whatever the cause, the process has contributed to the health and well-being of spectacular falls in one of the world’s prime foliage viewing corridors.

This year is shaping up to be another one, said Marc Abrams, a Penn State forestry professor emeritus, who has been tracking the woods for over 30 years. “At this point,” he said, “all systems go.”

For an overview of the likely onset and peak times across the country check out explorefall.com/fall-foliage-map. Its custodians say it’s the product of climatology and computer modeling.

The show opens in northern New England, and the NewEngland.com site posts graphics that show average conditions for each day.

Starting Oct. 1, the weekly Pennsylvania Department of Conservation and Natural Resources updates and forecasts, assembled by Reed from on-site reports from forestry officials, will be up and running.

And check out The Inquirer’s guide on where to peep in the Philly region.