The fall foliage show’s intensity may be all about the next few weeks of weather

The leaves have been well-nourished with water during the last three months, and the woodlands in the Northeast and Mid-Atlantic have been spared prolonged excessive heat.

Those factors would bode well for the prospects of a vibrant foliage season around the Philly region and throughout Pennsylvania, say experts who have long tracked leaf-peeping seasons. The outlook for New England, inundated by excessive summer rains, is a bit more complicated.

Ultimately, however, the quality of the season is very much in the chaotic hands of the atmosphere, and what happens between now and the onset of color — a process that has started in northern New England — will determine just how good it gets. Typically, it begins around here in mid-October.

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“The foliage every year is a little bit of a mystery,” said William Keeton, professor of forest ecology and forestry at the University of Vermont, “sort of like this great reveal that happens. It’s this very delicate dance between a whole bunch of different factors.”

The foliage show is driven by the predictable changes in daylight and the not-so-predictable changes in weather.

As the days shrink, the chlorophyll that makes the leaves green in deciduous trees withdraws. It is replaced by those more-animated pigments — anthocyanins, which give apples their blush and the woods their fire, and carotenoids, think carrots.

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But the onset of color is never linear, nor is the arrival of the peak.

Forecasting the course of a given foliage season might be a simple matter if it were only about what happened during the summer, but the weather in the previous summer can also be a factor, said Keeton, something to watch around here.

Rainfall in July and August last year was dramatically below normal. That may mean “that the trees had less energy, less carbohydrate reserves in their root systems this spring,” he said. That could mean they produced less of the compounds that produce the colors.

That may explain some of the early leaf drop that has occurred in the region this year.

Marc Abrams, a forestry professor at Penn State, says that all things considered, the region and the state should be in for a “fine” foliage season.

Ryan Reed, with the Pennsylvania Bureau of Forestry, says that he is a bit concerned about the recent dry spell in the Philadelphia area, but that he believes “a good fall foliage showing is still probable.”

As for New England, the outlook is more problematic, said Keeton and Jim Salge, a meteorologist who is the foliage forecaster for Yankee Magazine and newengland.com.

“There’s a couple of serious concerns,” said Salge. Parts of New England experienced record rains during the summer, and excess precipitation can dilute leaf color, he said. All in all, Salge says he is expecting a longer, but more-muted season up that way.

The wild card, the experts agree, is what happens between now and when the season begins in earnest.

The optimal conditions would be sequences of cool, crisp nights and sunny days. The government’s outlook through the third week in September does favor below-normal temperatures in most of the Philly region, but above-normal for Maine and most of Vermont and New Hampshire.

And when have those longer-term outlooks been wrong?

All foliage seasons unfold differently, and localized factors, such as elevations, tend to imperil generalizations about timing. But typically, Keeton’s Vermont country can expect the peak during the first week to 10 days of October. The newengland.com site posts foliage updates through the season.

» READ MORE: Tom Gralish's photographic look at the changing seasons

For upstate New York, peak colors are more likely mid to late October. The state’s tourism site has last year’s timetable, which is a reasonable indicator of what to expect this year.

Similarly, for guidance you’ll find last year’s excellent week-by-week reports, compiled by Reed, on Pennsylvania’s Department of Conservation and Natural Resources site. The 2023 reports will begin appearing Sept. 28, Reed said Monday.

In Philly, it’s likely that the peak won’t arrive until the end of October.

Regardless of the intensity of the leaves, autumn colors are always spectacular at sunset, and on the mountains when the sunlight is playing peekaboo with the clouds.

Said Keeton, “It never ceases to please.”