Friday, 11 October 2019

Denver CO: From 27C to 8C in 12 hours

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

What do you wear when you encounter every season within a 12-hour window?



Matthew Cappucci 
a person driving a car: Pedestrians struggle to cross Evans Street near the University of Denver as the season's first snow storm sweeps over the metropolitan area Thursday, Oct. 10, just one day after temperatures climbed to 83 degrees. (AP Photo/David Zalubowski)
MSN,
10 October, 2019
Residents of Denver enjoyed a beautiful Wednesday afternoon with temperatures in the lower to middle 80s. During the evening commute, a cold front swung through, bringing strong northerly winds and plummeting temperatures to freezing. Just eight hours after setting the record high for the day, it was snowing. Denver also set a record low by midnight.



That’s our largest one-day temperature change in October on record,” said Russell Danielson, a meteorologist at the National Weather Service in Boulder. “We tell people around here to dress in layers, because sometimes you get summer and winter in the same day.”

That chalks up to a 54-degree temperature drop within one day. It also marks a 40-degree plummet in four hours, the mercury diving from 81 degrees to 41 between 4 and 8 p.m. As the winds switched around from the north, gusts up to 55 miles per hour heralded the bone-chilling front’s arrival.

Temperatures were continuing to fall on Thursday, the National Weather Service forecasting a low of 14 degrees into Friday morning. “The record low is 22 degrees, set back in 1946. We should shatter that record.”

How strong can these cold fronts be?

Cold fronts on the High Plains are notorious for their dramatic and capricious shifts. The dry climate makes it easier for enormous fluctuations to occur, while the lack of water bodies or obstacles means that air masses of the opposite extreme can battle it out over relatively short distances. It’s not unusual for the temperature to fall 30 degrees or more behind a fall or winter cold front.
Our biggest temperature difference in a singled day was 66 degrees on Jan. 25, 1872,” Danielson said. “We had a high of 46 and a low of minus-20.”
On occasion, it’s possible to get both a drop and a leap, sometimes with multiple iterations, in the same day. That’s most common if a stationary front stalls, wobbling back and forth. These stationary fronts can also develop a very sharp gradient.

On Jan. 20, 1943, a stationary front got hung up in the Black Hills of South Dakota. The thermometer registered minus-4 at 7:32 a.m. Two minutes later, it had spiked nearly 50 degrees to a comfortable 45. Temperatures continued rising during until the midmorning, hitting 54 degrees before a drop to minus-4 27 minutes later. The temperature swings were reportedly so dramatic that motorists’ windshields cracked, the temperature fluctuations stressing the glass driving from a warm pocket to a cold one.
A vigorous cold front plowed through Denver on Wednesday. (Weatherbell.com)A vigorous cold front plowed through Denver on Wednesday. (Weatherbell.com)

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.
It’s worth noting that Wednesday’s temperature drop of 40 degrees appears to be the largest four-hour drop in temperature in Denver in at least the past decade. A four-hour temperature change of 40 degrees or more occurs on average once every two years.

Where is the cold front now?

The cold front has sailed well past the Colorado Rockies and has a history of producing record-shattering cold to the northwest in Montana. Great Falls tumbled to zero Thursday morning, its lowest October temperature ever recorded.

Denver will drop into the teens by Friday morning; if temperatures were to hit 7 degrees overnight, they would snag their largest two-day temperature swing. The current record of a 76-degree leap from December of 2008 stands.
The cold front showed up on satellite imagery over Oklahoma and the southern Texas Panhandle on Thursday morning, an “undular bore” marking the shock felt in the atmosphere. The encroaching cool, dense air was significant enough to send out a ripple-like wave ahead of it, triggering at its leading edge a roll cloud with a few oscillations behind.

By Thursday afternoon, the front had plowed through western Oklahoma, plunging some spots into the upper 30s and lower 40s while southeastern Oklahoma enjoyed a summery afternoon in the middle 80s. Kenton, a town in the Oklahoma Panhandle, was 45 degrees cooler Thursday afternoon than Wednesday. Kenton hit 87 yesterday.

a screenshot of a cell phone: Temperatures across Oklahoma mid-afternoon Thursday spanned a range of more than 40 degrees. Can you find the cold front? (Oklahoma Mesonet)Temperatures across Oklahoma mid-afternoon Thursday spanned a range of more than 40 degrees. Can you find the cold front? (Oklahoma Mesonet)

The front will continue to march east, losing its steam as it approaches the Mississippi Valley. And if you’re in Denver, good news! Next week will feature highs around 70.

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