Cooler but still well above normal this week

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Fall's dazzling colors will soon transform woods and forests around the U.S., delighting spectators — if they can figure out when to see the fall foliage at the peak of their transformation. 

 is here to help, offering a best-guess guide to when aspen, birch, larch and maple trees will turn vivid shades of gold, orange and crimson.

The forecast produced by the Smoky Mountains website predicts much of the U.S. will start seeing the most dramatic colors by late September to mid-October. But fall foliage can be very tricky to predict, especially as climate change brings warmer temperatures and disrupts historic rainfall patterns.

The timing comes down to several weather factors

Areas where warm temperatures persist can see fall colors arrive later than normal, especially if they've had near-average amounts of rainfall. And right now, the U.S. is coming off its third-hottest summer on record, according to the 

National Oceanic and Atmospheric Administration

Dry conditions are another factor. In areas where rainfall has been scarce, leaves could start changing color early this year, and their effect might be less vibrant. 

 currently shows large sections of central and western states are experiencing moderate to exceptional drought. Much of the eastern U.S. is in better shape, but portions of the mid-Atlantic and Northeast, from Maryland up to Maine, are experiencing abnormally dry or drought conditions.

"We see a forest that is healthy in the far north but increasingly stressed by heat and drought in the south and near the coast," 

 magazine says in its report on its home region.

The folks behind the Smoky Mountains' map acknowledge that like any forecast, theirs might not prove to be 100 percent accurate for every area. But they say their map is based on a complex algorithm that generates a strong prediction.

"Our model ingests multiple data points including NOAA precipitation forecasts, historical precipitation, average daylight exposure and temperature forecasts," Smoky Mountains founder and map creator David Angotti told NPR. "We process hundreds of thousands of data points from a variety of private and government sources to accurately predict the precise moment fall will occur for the entire United States."

The best conditions: Cool nights and sunny days

"Sunny, but not hot, days and cooler nights provide the optimal conditions for fall foliage, with the lower temperatures slowing the production of chlorophyll — the pigment that gives plants their green color — and sunlight increasing sugar production," according to North Carolina State University's 

Dry conditions in parts of the Northeast have experts warning that people may need to temper their expectations for the fall show. 

"The timing might not be the same, the colors might be a little bit more muted and not as beautiful and robust as we usually get to experience here in New England," as Nicole Keleher, the Massachusetts Department of Conservation and Recreation's forest health program director, told NPR member station 

Some areas could be helped by more rainfall — as long as it doesn't come with strong storm winds that knock leaves down. 

"When there is more rain, the colors pop even more," the 

 says, adding, "When there is a cool fall, the red maple leaves appear brighter as well." 

When chlorophyll steps aside, other pigments shine

"Carotenoids are where we're going to get that yellow and orange color," as botanist and plant ecologist Tanisha Williams of Bucknell University 

. "So as the green pigment starts to break down, we see this yellow and orange color."

As trees prepare to conserve their energy for the following spring, some of them also produce another pigment, anthocyanin, that brings striking red and purple colors.

"The falling of the leaves and the changing of the colors are all in preparation for the tree to hunker down and basically hibernate in the wintertime," Williams said. 

Environmental News

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Bill Chappell

Author

Aspen, maple and other trees are about to turn vivid shades of gold, orange and crimson — but when? It depends on a range of factors, from drought to local temperatures.

This map can tell you when fall foliage is peaking in 2022

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