Back in the day, September snowfall used to be a regular occurrence.

But these days, September snowfall has vanished into the late summer abyss, at least along the Front Range.

Denver hasn’t measured snowfall since the year 2000, despite the fact that September has produced some major, notable snows in the not-too-distant past. In September 1971, Denver saw 17.2 inches of snow, and the city has seen over a foot of September snow three different times in recorded history. As recently as 1995, a heavy, wet September snow knocked out power to thousands while dropping 7.4 inches of snow at the old Stapleton Airport observation site.

Since 2000, however, there’s been a grand total of 0.0 inches of September snowfall in Denver, though a trace of snow has been measured a small handful of times in that same time span. While lengthier September snow droughts have happened in the past in Denver — the city didn’t measure any September snowfall between 1882 and 1894 and 1914 and 1926 — the current 18-year September snow drought is the longest in Denver’s recorded history, which dates back to the 1880s.

“There are adults living in Denver who have never experienced a snowfall in September,” said Colorado assistant state climatologist Becky Bolinger.

It’s not just Denver that’s lacking September snowfall, either. Colorado Springs hasn’t measured September snowfall since 2001, Fort Collins’ snow drought goes back to 2000 and Pueblo’s September snow drought extends back to 1996. For both Fort Collins and Pueblo, those are the two cities’ longest September snow droughts on records, and Colorado Springs’ second-longest. However you draw it up, there appears to be a trend away from early season Front Range snowfall.

Denver’s long-term official September snowfall average is 1.0 inches, though, to be fair, that doesn’t account for the traditional hit-or-miss nature of September snows along the Front Range.

So why is September snowfall potentially becoming less and less common in Colorado, and more specifically along the Front Range? The answer might be more complicated than face value. While September snowfall appears to have decreased in recent years, late spring snowfall may be on the increase, as perhaps evidenced in part by a stunning late May snowfall earlier this year. Local meteorologist Ben Castellani theorizes the lack of recent September snowfall could, simply, be due to a lack of cold air in an overall warming climate.

“Classic Front Range upslope storms (usually Four Corners lows) are fairly common in April and May, but not so much in September,” Castellani said. “It’s not surprising that in a warming climate, snow will become much harder to come by early in the season when background temperatures are already very warm. It takes a strong and very cold storm system to produce snow in September. This just hasn’t happened in the last two decades.”

Since 2000, Denver’s average September temperature is 64.7 degrees, an increase from the long-term September monthly temperature of 63.1 degrees. That has likely turned at least some would-be snow events into a cold rain. It’s also worth noting that the 1.6-degree September boost in temperatures is considerably greater than the long-term annual increase of 0.7 degrees in Denver.

And while September has only just begun, a scorching opening few days of the month make it likely that Denver will finish with an above average September for the 11th straight year. Warmer temperatures, of course, make it harder to get snowfall, particularly early in the fall or later on in the spring.

Another possible explanation for the lack of recent September snowfall could involve a weakening of the polar jet stream, which decreases the strength of incoming storms. The polar jet stream is caused by the sharp temperature differential between the Arctic and middle latitudes. But the Arctic has warmed considerably in the last few decades, decreasing the temperature contrast that fuels that jet stream, weakening it and potentially making it more prone to unpredictable and volatile movement.

There’s always the chance, of course, that this could be just a natural variation in climate. September snowfall is hit-or-miss, after all. However, recent trends point to at least a possible delay in the start of Front Range snow season, and the lack of September snowfall could be a part of it. Since 2000, Denver’s average snowfall (Oct. 26) is more than a week later than the long-term average (Oct. 18).

At the very least, there appears to be a chance that a shift in climate could be reducing the chances for September snowfall along the Front Range.

“For the delayed then extended snow season, I would say there is a perceivable trend in Denver’s data lasting at least 10 if not 20 years, though I question how significant it is considering that most months with snow reported in September or May, it was usually from just a single rogue storm,” Castellani said.