The days are shortening, the nights are cooling and come the beginning of September, Colorado’s high country starts experiencing its annual leaf change among the forests, meadows and riverbanks.

The golds, reds and oranges of Colorado’s famous aspen trees popping across mountain sides from Steamboat to Silverton are a sight to see.

Typically, colors peak in Colorado’s northern mountains — the Park, Mummy, Never Summer and Rabbit Ears ranges — throughout the month of September. The central mountains — the Flattops, Gore, Sawatch, and Front ranges — peak the last week of the month. The southern mountains — San Juans, Sangre de Cristo, Wet and Spanish Peak ranges — peak the first and second weeks of October.

Leaves change color each autumn when they stop photosynthesis. As the green chlorophyll fades, yellow pigments become visible, leaving behind the colors we see on aspens, cottonwoods, and other deciduous trees.

How does weather affect autumn color?

According to the US Forest Service, the amount and brilliance of the colors developing in any particular autumn season are related to multiple weather conditions.

Occurring before and during the time the chlorophyll in the leaves dwindles, temperature, moisture and health of the trees are the main influences.

“A succession of warm, sunny days and cool, crisp, but not freezing nights, seems to bring about the most spectacular color displays,” the Forest Service said.

Rainy and windy weather can prematurely knock down leaves and shorten the display window. Furthermore, early-fall snow storms can do similar damage to the trees’ leaves by weighing them down, making them more prone to fall early.

What does September 2024’s monthly outlook look like for the fall foliage start?

The National Weather Service’s monthly temperature outlook for September shows most of western Colorado, including the state’s mountainous regions, is forecast to have a 60-70% chance of above average temperatures.

Concurrently, the monthly precipitation outlook for September has approximately 75% of Colorado, including nearly all mountainous regions, having a 40-50% chance of below-average precipitation.

Following the Forest Service’s ‘loose rule’, above-average temperatures with below-average precipitation could bring about a good fall foliage showing in 2024.