How the forecast works

Day length and leaf color

As days shorten and temperatures cool, deciduous leaves stop producing chlorophyll. The fading green reveals yellow and orange pigments already in the leaf. The National Park Service explains this process in its fall color guide.

Typical seasonal dates give a starting point for planning. Weather and tree species can shift the timing, so a local report may be more useful close to your trip.

Latitude and elevation estimates

We anchor at roughly October 20 for 40°N at sea level, then shift:

• about 2.5 days earlier per degree of latitude north
• about 3.5 days earlier per 1,000 feet of elevation

The model gives different dates for high ridges and nearby valleys. These are estimates for reference locations; they do not describe every tree in a state.

Newfound Gap and Cades Cove share a county but have different elevations and typical peak dates. Each location page uses its own elevation.

How the national terrain map works

The national map applies the latitude and elevation model to a terrain grid from Mapzen Terrain Tiles, using USGS 3DEP, SRTM and GMTED2010 elevation data. This separates ridges from valleys within a state. The source tiles have kilometer-scale spacing at the zoom level we use; enlarging the map does not add finer observations.

The date slider shows typical seasonal progression. Actual timing can vary by weeks. The layer has no forest-cover, tree-species or current-weather input, so its colors describe timing wherever deciduous trees grow, including within otherwise dry or evergreen regions. Areas outside the model's 30–49°N range are neutral. Alaska uses its separate regional timing estimate. The state table uses reference elevations, so local terrain can show a different stage on the same date.

How the county maps are built

Each county is sampled at up to twelve points inside its boundary to avoid including water or neighboring counties. Every sample gets an elevation from the USGS 3DEP 10-meter model, or from SRTM at 30 meters where 3DEP has a gap. The county is shaded using the median of those samples, run through the same latitude and elevation model as everything else on the site.

The 10th and 90th percentiles of the same samples give the number next to each county in the table: how many days separate its high ground from its low ground. Summit County, Colorado spans more than a week in the model. The county table shows this spread alongside its central estimate.

A dozen samples can miss a narrow canyon or isolated ridge. Shapes with fewer than six samples, such as Kodiak's islands, are sampled again on a finer grid. Counties outside the model's calibrated latitude band have no estimate and are drawn hatched.

Alaska's boroughs use regional windows published by Travel Alaska and the US Forest Service: mid-August to mid-September in the Brooks Range, September in Southcentral, and late September into early October in the Inside Passage. Elevation adjusts dates within each region. The Interior uses September 7 as its reference peak date.

Bethel and Kusilvak have no estimate because the source windows available to us describe other parts of Southwest Alaska. We do not extend those dates to the Yukon-Kuskokwim delta.

County shapes come from US Census Bureau boundary files. Connecticut is drawn as its eight historic counties rather than the nine planning regions that replaced them for statistical purposes in 2022, because Litchfield County is still what people call it. Alaska is drawn with Valdez-Cordova intact rather than split into the Chugach and Copper River census areas it became in 2019; the boundary file still carries the older shape.

Weather and color

Weather affects the timing and intensity of fall color. Bright days and cool nights generally favor colorful displays, while wet, cloudy weather or early freezes can mute them.

Local moisture, sunlight and tree species also matter. The National Park Service describes these interacting factors in its Blue Ridge Parkway color guide.

Sunrise and sunset calculations

Location pages calculate sunrise, sunset and sun direction using NOAA solar equations and the location's coordinates. The displayed date identifies which day's light is shown.

Model limitations

• The map estimates where peak usually falls. It does not use live satellite greenness or feed community reports into its timing calculation. Reports are shown separately.
• The timing model does not adjust for this year's weather. Drought and temperature anomalies can shift peak dates and change color intensity.
• Species vary within a state. Oaks can hold color weeks after maples have dropped. Larch and tamarack can turn gold after nearby deciduous trees are bare. Our per-state species lists say what to expect, but the dates describe the dominant canopy.