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A look ahead at September and the rest of fall season in Colorado

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Posted at 2:00 PM, Sep 10, 2021

DENVER – Autumn is an enigma in Colorado.

One of the most pleasant and certainly most beautiful times of the year, it's also a time when the weather can't seem to make up its mind. Summer is over and winter hasn't begun, but we still see a little of both.

Thunderstorms still occur in September, but so does snow. October offers some of the most pleasant weather of the entire year, but you'd better have the snow blower ready to go just in case! By November, it can feel like January if subzero air settles in after a big, early snow. Autumn, of all the seasons, seems to have the biggest identity crisis.

As the days grow shorter, we see the end of the thunderstorm season. The lack of intense daytime heating doesn't allow for the strong convective lifting that helps to brew up afternoon storms.

Additionally, the mid and upper levels of the atmosphere are comparatively warm after the long summer, so the air is relatively stable. Although the official end of summer is not until the third week of September, the psychological end of summer is Labor Day weekend. We begin to think less about thunderstorms and more about the first snow of the season.

Usually, those early snows first occur in the high country. One morning a thin veil of fresh snow will suddenly show up on the high peaks and usually disappear by midday. Once in a while, an early snow will visit the Front Range cities.

The earliest snowstorm on record for Denver was September 3, 1961, when four inches fell over the western suburbs of the city. One year ago, snow fell in the mountains and foothills and even Denver had a dusting on Sept. 8 and 9, with lows dipping to the freezing mark.

Just a few years further, on September 21, 1995, Denver was bombed by a record snowfall of nearly ten inches. This soggy, slushy preview of the upcoming season was very unwelcome - it damaged thousands of trees and power lines. The wet, heavy snow settled on trees still covered with leaves, and the overloaded branches crashed down on nearby wires.

But even a major storm such as this does not last long in September. Within a day or so, the snow had disappeared under the steady gaze of the warm, early-fall sun. Golfers and gardeners were quickly back on the job under a brilliant blue sky.

By mid-to-late September, the jet stream winds are beginning to flex their muscles once again.

The jet stream usually gets pretty quiet in late summer as the strongest winds aloft are found in central Canada in late July through August. In September, the increasing chill over the northern latitudes helps to force the jet stream farther south once more. As the winds aloft increase over the Rockies, the potential for stronger storm systems and major fluctuations in temperature increases as well. Cold fronts start to roil down from the north with more vigor, and the result can often be a gorgeous summer-like day with a few inches of snow coming right on its heels.

The average date for the first frost in Denver is Oct. 7, with the first measurable snow on Oct. 19. Most years stay snow-free in Denver through September, but October is a different story.

In October, the first significant low pressure storm systems of the season begin to form over Colorado. These storms bring easterly upslope winds along the Front Range and often the first opportunity for shovels and snow tires.

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Autumn is not just a waiting period for the inevitable onslaught of winter. Every Coloradan looks forward to the delightful stretch of warm, sunny days known as "Indian Summer" – perhaps the nicest weather of the entire year: Brilliant blue skies, highs in the perfectly comfortable seventy-degree range, light breezes – in other words, ideal! Indian Summer is loosely defined as the warm, dry, quiet period of weather that follows the first killing frost. This weather pattern usually occurs during late September or the first two weeks of October and is especially delightful because it coincides with the peak of the aspen leaves.

There really is no meteorological significance to Indian Summer – it doesn't portend anything about the upcoming winter or reflect on the summer just past. Indian Summer is simply a time to try to slow down and savor the good fortune of living in Colorado.