A look at September and the rest of the fall season in Colorado
DENVER – Fall is an enigma in Colorado.
One of the most pleasant and certainly the most beautiful times of the year, it is also a time when the weather does not seem to decide. Summer is over and winter has not started, but we can still see a bit of both.
Thunderstorms still occur in September, but so do snow. October offers one of the nicest climates of the year, but you better have the snow thrower ready to go just in case! In November, it can look like January if sub-zero air sets in after a big early snowfall. Fall, of all seasons, seems to have the biggest identity crisis.
As the days get shorter, we see the end of the thunderstorm season. The absence of intense heating during the day does not allow the strong convective uplift which helps prepare for afternoon thunderstorms.
In addition, the middle and upper levels of the atmosphere are relatively warm after the long summer, so the air is relatively stable. While the official end of summer isn’t until the third week of September, the psychological end of summer is Labor Day weekend. We are starting to think less about thunderstorms and more about the first snows of the season.
Usually, these first snows occur first in the high countries. One morning, a thin veil of fresh snow will suddenly appear over the high peaks and usually disappear around noon. Occasionally, early snow will visit the Front Range towns.
Denver’s first-ever snowstorm occurred on September 3, 1961, when four inches fell on the city’s western suburbs. A year ago, snow fell in the mountains and foothills and even Denver had a dusting on September 8 and 9, with lows reaching freezing point.
A few years later, on September 21, 1995, Denver was bombarded with a record snowfall of nearly ten inches. This soggy, muddy preview of the coming season was very unwelcome – it damaged thousands of trees and power lines. Heavy, wet snow settled on trees still covered with leaves, and overloaded branches crashed into nearby wires.
But even a big storm like this doesn’t last long in September. In a day or two, the snow was gone under the constant gaze of the hot early fall sun. Golfers and gardeners quickly got back to work under brilliant blue skies.
From mid-September to the end of September, the winds of the jet stream start to flex their muscles again.
The jet stream generally becomes fairly calm in late summer, as the strongest upper-level winds are found in central Canada from late July to August. In September, increasing cold over northern latitudes helps force the jet stream further south once again. As upper winds increase over the Rockies, the potential for stronger storm systems and major temperature fluctuations also increases. Cold fronts start to descend from the north with more vigor, and the result can often be a gorgeous summer day with a few inches of snow just on its heels.
The average date of the first frost in Denver is October 7, with the first measurable snowfall on October 19. Most years remain snowless in Denver until September, but October is another story.
In October, the first significant low pressure thunderstorm systems of the season begin to form over Colorado. These storms bring easterly upwelling winds along the Front Range and are often the first opportunity for shovels and snow tires.
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Fall is not just a time of waiting for the inevitable onslaught of winter. Every Coloradan eagerly awaits the delightful expanse of warm, sunny days known as the “Indian summer” – perhaps the most beautiful weather of all year: brilliant blue skies, perfectly comfortable highs to seventy. degrees, light breezes – in other words, ideal! Indian summer is loosely defined as the period of warm, dry, calm weather following the first deadly frost. This climatic regime usually occurs in late September or the first two weeks of October and is especially pleasant because it coincides with the peak of aspen leaves.
There really is no meteorological significance to the Indian summer – it does not bode well for the coming winter or reflect the summer that has just passed. Indian summer is simply a time to try to slow down and relish the luck of living in Colorado.