(WJET/WFXP/YourErie.com) — As the first official day of fall approaches this week, you may be wondering when the leaves in your part of the state/country might be changing.

Thursday, Sept. 22 marks the first day of fall this year.

The Smoky Mountain Fall Foliage Prediction Map for 2022 is interactive, allowing users to view when their area will see its fall foliage peak. This map is not fully accurate, it’s meant to help travelers better time their trips to have the best opportunity of catching peak color each year.

According to the prediction map, during the week of Sept. 19, Erie County can expect to see leaves beginning to change. During the week of Sept. 26, the map moves Erie County closer to “near peak.”

Photo: A look at the week of Sept. 26 on the 2022 Smoky Mountain Fall Foliage Prediction Map

Erie County’s fall foliage peak, according to the map, should be around the week of Oct. 3. This is also when the rest of the state should start seeing some changing leaves.

By Oct. 10, Erie County should start moving past its peak, but the rest of the state will see its peak fall foliage around Oct. 17.

By Halloween (Oct. 31), the entire state of Pennsylvania, and northern border of the U.S. will be past its peak.

By Nov. 21, the entire U.S. is expected to be past its peak, aside from the southernmost part of the U.S. along the Gulf of Mexico. That is when parts of Texas, Louisiana, and Florida will be at or near peak.

So why do leaves change color?

The Smithsonian reports as the season changes, temperatures drop and days get shorter so trees get less direct sunlight, and the chlorophyll in the leaves breaks down.

The lack of chlorophyll reveals the yellow and orange pigments that were already in the leaves but masked during the warmer months. Darker red leaves are the result of a chemical change: Sugars that can get trapped in the leaves produce new pigments (called anthocyanins) that weren’t part of the leaf in the growing season. 

The Pennsylvania DCNR reports Pennsylvania has a longer and more varied fall foliage season than any other state in the nation — or anywhere in the world. Only three regions of the world support deciduous forests that display fall autumn color:

  • Eastern North America
  • The British Isles and parts of northwestern Europe
  • Northeastern China and northern Japan

Forests in other regions are either tropical or dominated by conifers.

The best fall colors are produced when the weather is dry, sunny and cool. Places that are cloudy, damp or warm won’t see the same degree of changing color.