The fall foliage peak is weeks away around Philly, and the weather will be a factor

While the curtains of darkness are encroaching on the daylight, one of nature’s most anticipated annual dramas is getting ready to take over the stage.

And after those sizzling summer hot spells and a bone-dry August, September’s weather turnaround may be just what the burgeoning fall foliage season needs, the experts say.

“The recent spell of cooler days and chilly nights has already sparked some early changes,” said Ryan A. Reed of the Pennsylvania Department of Conservation and Natural Resources, author of the state’s weekly foliage updates. The first update was due to start Oct. 2, but he says he might move that up a week.

Brittle leaves have been littering the ground for weeks. Flecks of yellow have been appearing in the trees. This week, an air mass fresh from central Canada (this one smoke-free) is going to make it feel like October. And by the end of the month, woodlands of far northern New England will be transformed into bouquets of russet, scarlet, cinnamon, and gold.

The waves of the magic show will migrate southward, and we’ll get our turns at the end of October, perhaps lapping into early November if the weather allows.

No one is quite sure how this season will turn out. Veteran foliage observers say that some of the weather factors in recent weeks may have been unfavorable in key peeping zones in the Mid-Atlantic and Northeast.

But for the fate of the foliage show, in terms of the atmosphere’s effects the playoff season has begun.

“The most important month is September in most locations,” said Howard S. Neufeld, a biology professor at Appalachian State University in the spectacular Blue Ridge Mountains of North Carolina, where the colors can last until Thanksgiving in the lower elevations.

So far, so good in the key viewing corridors in the Philly region and the rest of the East, and the trends look to continue at least until the end of next week.

One concern is August dryness in the East that has reduced soil moisture levels from the Blue Ridges to northern New England, according to the National Oceanic and Atmospheric Administration’s Climate Prediction Center.

Locally, that has been evident in the premature leaf fall, said Jason K. Parker, district manager of the Davey Tree Expert Co. in Warminster. Some of the trees are in a stressed state, and for fall color, that’s “not a great place to be.”

He added that any rain — no downpours, please — would help with leaf retention.

On the plus side, he said the temperature trend that began in late August and is due to continue “absolutely is perfect.”

Reed said the dust of August wasn’t necessarily a deal-killer. Late-summer dry spells into September are not all that unusual, he said, adding: “This can intensify the color.”

In posting his annual foliage outlook in Yankee Magazine, meteorologist Jim Salge concurred, saying: “Some drought can make colors really bright.”

Dryness and sunshine can help promote the complex chemical reactions needed to color the leaves.

Some have tried, but forecasting color intensity appears to be even more elusive than forecasting the weather.

“Predicting the quality of the season using numerical models has been attempted, but the results are not compelling,” said Marc Abrams, a longtime foliage observer and Pennsylvania State University forestry professor emeritus.

The changes are driven more by light than by weather conditions, and it still is not clear precisely how the effects of the warming planet are affecting the season.

You may have noticed that the sun angles are getting lower, the days shorter. We are losing about 2½ minutes of daylight a day.

With the dimming of the light, green chlorophyll recedes from leaves, yielding to the more flamboyant pigments, such as the strawberry-red anthocyanins, the carrot-colored carotenoids, and the corn-like yellows.

Once the colors start to take over, sunlight makes the leaves more resistant to allowing the colors to escape, Neufeld said.

He added that the weather the next few weeks would bear watching.

“If it becomes warm and/or rainy,” he said, “the color display will be duller … and delayed.”

The progress of the foliage colors varies radically with elevation, but generally the wave of color proceeds from northeast to southwest. In the Philly region, the climax on average occurs around Halloween.

Hard data are lacking, and in predicting onset and peak times, one might have to resort to that dreaded adverb: anecdotally. For evidence of seasonal variability, check out the chart kept for the last 50 years by Polly’s Pancakes in upstate New Hampshire tracking the leaf color timing. The beginning of the peak periods has varied from Sept. 21 to Oct. 10.

While various websites offer timelines, be aware that, like snowflakes and storms, foliage seasons may look alike but no two are identical.

Various websites purport to show timelines, but the periods of onset and peak can vary tremendously from year to year and among elevations.

That said, several websites offer graphic images of the expected progress of the season.

The weekly Pennsylvania Department of Conservation and Natural Resources updates and forecasts, assembled from on-site reports from forestry officials, will begin Sept. 25 or Oct. 2.

Only a few regions on the planet — in the known universe, for that matter — can match the Eastern United States for autumnal brilliance, and Philadelphia is in the thick of it.

It is about equidistant from Front Royal, Va., the gateway to the Blue Ridges, and the Berkshires of southwestern New England. And Philly isn’t all that far from the Adirondacks and the Green and White Mountains.

If you don’t mind waiting and would prefer to save the gas, the Philadelphia region has several woodlands and walking and biking trails that become all the more picturesque as the leaves litter the pathways.

And while color intensity is always unpredictable — how does one even measure it? — you can never go wrong admiring how the setting sun animates the treetops.