Denver and Boulder could see snow the day after hitting 90 degrees

For some, accumulating snow is possible Tuesday into Wednesday, just a week into September.

A temperature change of 60 degrees in 36 hours is possible, the heat of summer abruptly giving rise to a flash winter.

“This will be an interesting potential weather event,” said David Barjenbruch, a meteorologist at the National Weather Service office in Boulder. “Monday we could be 100 in a few spots. We could go from record hot to near-record cold.”

A rare event

The sudden seasonal reversal would be extreme just about anywhere else, but in Denver, strong cold fronts aren’t terribly unusual. On October 9, 2019, Denver hit a high of 83 degrees. Eight hours later, it was snowing.

Temperatures that day plummeted some 40 degrees in four hours, the meteorological caprice wrangling both a record high and low for the date.

“Going from a high of 90 or greater to accumulating snow the next day is extremely rare at either Denver or Boulder, but not unprecedented,” wrote Paul Schlatter, the science and operations officer at the National Weather Service in Boulder.

“On September 12, 1993, the high at Denver was 92, then 5.4 inches fell at Denver on September 13th before midnight,” recalled Schlatter in an email. “I was a junior in high school in Boulder and remember that one well!”

Schlatter also noted an event in Boulder on Sept. 11, 1974, when the city hit 91 degrees a day before 2.7 inches of snow fell. Denver peaked at 93 degrees, but only saw a trace of snow out of that event.

The forecast

In Denver, Sunday is predicted to approach the century mark, with lower 90s on Monday. Tuesday could peak in the lower 50s before tumbling into the 30s. Some accumulating snow cannot be ruled out during Tuesday’s second half.

To the northwest in Boulder, highs near 90 are likely on Monday, with overnight showers and thunderstorms along the cold front. Tuesday will feel like late fall or winter, with temperatures eventually falling below freezing and even a higher chance of snow than Denver.

In Cheyenne, Wyo., the transition could be squeezed entirely into Labor Day, with highs in the 80s and lows near freezing.

“Cold Arctic air mass in the wake of the low will turn the area thermostat down quite considerably with record low temps possible Wednesday morning and widespread frost potential,” wrote the National Weather Service in Cheyenne.

Temperatures Wednesday morning in Cheyenne could drop into the teens.

High Plains temperature magic

The terrain of the High Plains and Front Range is the secret to its temperature swings. Stationary fronts can get caught up in the mountains, leading to large temperature changes over very small distances. Other times, the forcing of air up or down slopes can compress or expand pockets of air, their temperatures warming or cooling, respectively.

“Right along the front range, one main reason is that, before the front comes in, you get downslope warming and compressional heating,” said Barjenbruch. Downsloping, which occurs when air moves down a hill or mountain, can result in warming as its air pressure increases near the earth’s surface. For every 1,000 feet a pocket of air sinks, it warms 5.5 degrees Fahrenheit.

“Then we switch to this big upslope and cooling. … It’s a double whammy,” Barjenbruch said. “There’s not much moderation with these air masses. That’s the big difference from something that moves into the eastern U.S.”

The start of fall in the High Plains

The change of the seasons doesn’t come like a gradual downhill slope in Denver; it’s more like being pushed down a flight of stairs. In most years, the strongest cold fronts come between October and December, though it’s not unusual to see them as early as September.

“It’s generally early to mid-September we’ll have our first strong cold front move through for the season,” Barjenbruch said. “But this one is really packing a big punch.”

An analysis of same-day temperature fluctuations in Denver shows that temperature swings in September and October are typical, along with in the spring as the seasons shift.

The greatest calendar day temperature change in Denver came on Jan. 25, 1872, when a morning high of 46 degrees crashed to minus-20 in the evening.

It’s not unusual for the Denver region to have a 40-plus degree temperature change across it as the winds change and scour out one air mass for another.

“Parts of west side of Denver may be 60 degrees with downslope warming, yet the east side of Denver might be 10 or 20 degrees,” said Barjenbruch, referring to air that pours down the lip of the Rockies and warms. “We call that the Chinook wind.”

Pacific typhoons playing a role?

As crazy as it may sound, part of the meteorological roller coaster over the High Plains can be partially traced back to a pair of typhoons in the northwest Pacific.

As the typhoons, named Maysak and Haishen, move northward to the midlatitudes, they modify and supercharge the jet stream by colliding with it. That influences places downstream, realigning where the crests and troughs in the serpentine river of air become established. Warm air from the south builds in beneath the ridges, while the troughs are occupied by cool air and storminess.

Picture the ripples that form downwind of a stone in a river. If you move that stone or add another atop it, the shape and position of those downstream ripples changes accordingly.

Whatever lies in store early next week is bound to keep Coloradans on their toes.

Wild temperature swings of the past decade in Denver

  • Feb. 24, 2014: In two hours, the temperature jumped from 26 degrees to 57 degrees in the midafternoon. The temperature stayed at 63 degrees until 10:30 p.m. By midnight, it had fallen to 29.
  • Jan. 5, 2015: 27.9-degree jump, from 12 degrees to nearly 40 degrees in one hour. The same day featured a 15-degree drop in one hour, thanks to a stalled stationary front. After a morning low of minus-4, the thermometer stood at 55 degrees just before midnight, marking the largest one-day temperature swing of past decade. Wednesday’s cold front just missed the mark.
  • Dec. 9, 2016: 11 degrees at 3 p.m., 53 degrees at 8 p.m.
  • Dec. 27, 2017: 2 degrees at 2 p.m., 45 degrees by 6 p.m. Two days later, on Dec. 29, the temperature swung from 63 at 8 p.m. to 30 degrees by midnight. The next morning’s low was 9 degrees.
  • Oct. 9-10, 2019: 82 degrees at 3 p.m., 32 degrees at 10 p.m. Winds gusted to 55 mph as the cold front rushed through, with snow falling shortly thereafter.

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