Denver area frostbitten as winter seemingly overtakes summer in a day

Over Labor Day weekend, Denver held similar fire concerns as most of the western United States into Monday. Less than 24 hours later, temperatures plunged nearly 60 degrees, ushering in winterlike conditions mere hours after summer's unofficial end.

The city observed record heat both Saturday and Sunday, the mercury reaching 101 and 97 F, respectively. A red flag warning had even been issued across portions of the state. The heat lasted into midday Labor Day, when the high reached 93 degrees before temperatures began to slip.

On Monday, the National Weather Service (NWS) office in Boulder issued winter storm warnings and winter weather advisories across Colorado through early Wednesday. Freeze watches and warnings were issued as well.

Within hours after Denver observed a temperature of 92 degrees while experiencing an AccuWeather RealFeel® of 96, temperatures dropped by nearly 10 degrees. By 11 p.m., temperatures had reached 47 degrees, a 45-degree plunge from the day's high, though for residents it might have felt like a drop of 62 degrees.

At midnight, temperatures reached 46 degrees, resulting in a single-day change of 47 degrees in Denver for Sept. 7, 2020, according to AccuWeather Meteorologist Courtney Travis.

The NWS records temperature changes by the day -- meaning temperature changes spanning from one calendar day into another, even if it is within 24 hours, aren't recorded.

Monday's high temperature of 93 just after 2 p.m. dropped to 33 degrees at 8 a.m. Tuesday, a 60-degree change within 18 hours more or less. However, since the change spanned across midnight, it will be recorded as a two-day temperature change rather than a one-day change.

"Keeping single calendar day records is a lot less complicated than keeping track of hour by hour data," Travis said. "While it is easier now for computers and automated weather equipment to keep track of temperatures, we haven't always had the technology to record and store that data going back through history. The calendar day records are simpler because daily high and lows are almost always reported."

Since records began in 1872 -- the first year of Colorado's statehood -- Denver's most extreme single day temperature change has been holding firm. Jan. 25, 1872, recorded a change in temperatures of 66 degrees, which has yet to be broken. The high of that day had been 46 degrees, temperatures plunging subzero to minus 20 degrees.

Feb. 8, 1936, recorded a one-day temperature change of 60 degrees, when the temperature bottomed out from a high of 35, plummeting to minus 25 degrees. It currently ranks sixth in Denver's top one-day temperature changes since 1872.

Denver's largest two-day temperature change since 1872 started Dec. 14, 2008, when the city experienced a 76-degree temperature change from 58 to minus 18 degrees. To make the list of the top 20 highest 2-day temperature changes, Denver would have to reach a low of 27 degrees by midnight, resulting in a plunge of 66 degrees.

Even though plunges in temperatures aren't uncommon for Denver, they typically occur closer to the winter season -- not mere days after the end of meteorological summer, which occurred on Aug. 31.

The earliest calendar date to make the NWS's list for largest one-day temperature change was Sept. 19, 2010, when temperatures crashed by 55 degrees. Oct. 10 of last year was the earliest date to make the two-day temperature change list when temperatures nosedived by 70 degrees.

The sudden burst of cold came from a storm moving into the Rockies, even bringing early snowfall to some areas.

"A potent storm has been diving southward across the Rockies since Monday," Travis said. "Along with it, the storm has been pulling abnormally cool air southward across the region, resulting in snow for many of the higher elevations."

Extreme meteorologist Reed Timmer was on the scene in Georgetown, Colorado, on Tuesday covering the early-season snowstorm that hit the region. (AccuWeather)

By late Tuesday morning, Nederland, Colorado -- about 45 miles from Denver -- reported receiving about an inch of snow. Meanwhile, areas farther north in northeastern Wyoming to western South Dakota, have picked up even heavier snowfall. Terry Creek, a mountain and ski area in South Dakota, reported at least 15 inches of snowfall, and Douglas, Wyoming, 12 inches.

Extreme meteorologist Reed Timmer was in the Denver area reporting for the AccuWeather TV network on the early-season burst of winter weather that was enveloping the region. At one point on Tuesday, Timmer was positioned at Interstate 70 in Georgetown, Colorado, about 45 miles west of Denver.

Snow was coming down hard and an accident occurred on I-70, leading officials to temporarily close down the highway. The closure snarled traffic in the westbound lanes as lines of vehicles could be seen with brake lights illuminated, going nowhere. "The first closure due to winter weather this season, and it's incredibly early," Timmer said in his report for the network, noting the unusual nature of winter weather occurring in early September.

"It's actually peak hurricane season," Timmer marveled, "and we're out here chasing a snowstorm with real winter weather impacts."

The Cameron Peak Fire in northern Colorado near Red Feathers Lake, had reached over 100,000 acres as of Tuesday morning local time, and was only 4% contained. Cameron Peak fire officials ordered mandatory evacuations for the Lady Moon, Red Feather Highlands, and other areas in Larimer County on Monday, some of which were later downgraded to voluntary evacuations on Tuesday after photos of snow falling over the evacuation zone had surfaced over social media.

AccuWeather Senior Meteorologist Brian Wimer said that "a few snow showers" would linger into Wednesday, and added that the total snow amounts "should help to significantly contain the fire." (1 liked)

Wimer added that the wind may still provide a challenge in fighting the fires, but the stronger winds should recede by the latter half of the week.

On Saturday, NWS Cheyenne issued a red flag warning, which went into effect through Sunday due to warm, dry and breezy conditions that could aid in fueling fires. On Monday, the day following the expiration of the warning, Cheyenne, Wyoming, experienced a nearly 50-degree drop in temperatures.

Winter storm warnings and a weather advisory were issued for southeastern Wyoming to start at midnight, going into Tuesday. By Tuesday afternoon, the southeastern portion of the state had already started to see snow accumulate on roads.

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"Cheyenne has not recorded any measurable snow in the first 10 days of September since 1929," NWS Cheyenne meteorologist Tim Trudel told Wyoming News on Friday.

The earliest snow on record in the city was Sept. 8, 1929, totaling 0.8 of an inch.

After receiving a few flakes of snow on the last day of meteorological summer, Big Sky Resort in Montana saw snow on Labor Day as well. Areas above 7,000 feet, on average, from western Montana to Wyoming and Colorado, may receive from 6 to 12 inches of snow, and an AccuWeather Local StormMax™ of 18 inches is possible.

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