Denver endured a month of record-breaking temperature swings, from summer to winter and back again

Yet, only eight hours later, it was snowing. The temperature had plummeted to freezing, with 55 mph winds whipping through the city. More than an inch of snow fell in spots, requiring residents to scrape off their windshields just hours after sporting flip-flops. The National Weather Service confirmed it was the city’s single greatest one-day October temperature fluctuation on record, with the mercury falling from 81 to 41 in four hours.

By midnight, the temperature bottomed out at 28 degrees.

Four days after going into the deep freeze, Denver had climbed back into the mid-70s, though nights still flirted with the freezing mark. On Oct. 16 and 17, Denver again made it into the 80s. Much of the following week remained in the 50s and 60s.

Then the polar plunge struck again. On Oct. 26, Denver saw yet another “summer-day snowfall.” The temperature hit 76 around lunchtime but dropped to 29 by midnight, with a trace of snow observed. It didn’t get any warmer: The next day’s high was 29, with a low of 17 degrees.

Denver hit 83 degrees on Wednesday afternoon. Eight hours later, it was snowing.

After that, a deep freeze set in, with a record cold low temperature of 3 degrees occurring on Wednesday. For Halloween, temperatures were back up in the comparatively balmy 40s.

Quantifying the wild swings

We all tend to think the weather where we live is weird. But we crunched the numbers and found Denver’s weather this year has been extra wild.

We can look at this two ways. For starters, in terms of rapid temperature drops — namely from powerful cold fronts. In an analysis conducted earlier in October, our reporting showed that top-tier temperature crashes like the ones that occurred twice this month are much more common in December and January than in October.

The fact that not one, but two, summer-busting fronts rolled through with a 40-degree plus drop shows how tumultuous this month has been.

Moreover, we can look at the diurnal range of days (both this October and year-round) to show how much daily fluctuation occurs between high and low temperatures.

Typically, Denver’s greatest daily temperature range comes, believe it or not, during the summertime. This is a running average of daily temperature variability over the year. Around day 181, which corresponds to June 30, the 23-year average temperature difference between the daily high and low is about 24.3 degrees.

Once more moisture builds in for July and August, that wild variability declines, since the more humid atmosphere is tougher to nudge up or down, temperature-wise.

We noticed other peaks in April and again in October; these are likely associated with the transitional seasons of spring and fall, when strong fronts of opposing summer and winter can slosh the temperature back and forth significantly. In addition, it tends to be drier during those seasons, making big daily temperature swings easier.

The volatility winds down in the winter when it’s just cold all-around. In fact, December’s average daily range tends to be about 25 percent less than typical summer fluctuations.

Now, we’ll overlay with a red line a moving average of this year’s daily temperature swings; individual days are plotted in red. Notice a trend?

This year, and especially this fall, has featured much crazier daily swings than average. In fact, more than half of days this October had a daily range of 30 degrees or more. Six of these days saw daily high/low temperature differences topping 40 degrees.

Throughout the year so far, 2019 has featured roughly a 29 percent greater range in daily temperatures, compared with a two-decade average. More impressively, October has been 35 percent wilder than normal — the average October daily temperature range is 22.4 degrees, but this year’s was closer to 30.2 degrees.

Most impressive is that the greatest daily ranges in 2019 came in the autumn, rather than summer, as would be expected. This may be thanks to dry air over the western United States that has accompanied a stubborn and deep dip in the jet stream as of late.

The snowfall

Even for Denver’s notoriously wacky climate, this October was unusually snowy and cold.

Denver’s official observation site at Denver International Airport — 20 miles northeast of downtown — officially saw 12.5 inches of snow in October. That’s about triple the monthly average of 4.2 inches, and it also made this October the city’s snowiest in a decade.

In Denver’s highly localized climate, however, the city’s official observation site doesn’t necessarily accurately reflect the majority of the metro area’s weather. At the city’s more centrally located Stapleton Airport site (which is no longer an actual airport), 15.7 inches of snow fell in October, a more accurate number for the majority of the metro area’s snowy October. That was the snowiest October at that observation site since 1997 and the second-snowiest overall in the last 50 years.

For Denver, the snow and cold also meant that the city finished with its fourth-coldest October on record. Nearby Colorado Springs tied its third-coldest October. In southern Colorado, the city of Pueblo had its second-coldest October ever recorded.

Denver stayed below the freezing mark for an astonishing 107 consecutive hours during the last week of October, with the mercury sinking as low as 3 degrees on Wednesday morning. Temperatures finally bounced back above freezing after a nearly five-day hiatus — Denver’s average low for October 30 is exactly 32 degrees — late Thursday morning. All of Sunday, Monday, Tuesday and Wednesday were spent below freezing.

(Talk about weather whiplash. Denver had finished with its second-hottest September on record the month prior, including its latest 100-degree reading on Sept. 2.)

Statewide, dozens of monthly record low and low maximum temperatures were broken last month, according to the Colorado Climate Center.

Statewide, the fresh snow and cold has Colorado’s statewide snowpack at 212 percent of average, as of Friday and according to official data from the Natural Resources Conservation Service. One key high-elevation mountain pass was forced to close for the season far earlier than usual.

For Colorado skiers and snowboarders, however, all of the snow means an extra early start to the season. Even though it’s not opening for another three weeks, the Steamboat Ski Resort finished with at least 58 inches of October snowfall, by far the resort’s snowiest October on record. Winter Park Resort will open on Saturday for its earliest opening date in 80 years, and Keystone Resort opened its doors in early October for its earliest open since 1995. More resorts are expected to open their doors this weekend, boosted by the extra snow.