• Mon. Oct 26th, 2020

From summer heat to snow in 18 hours: Weather whiplash strikes Rocky Mountain states – The Washington Post

The sudden cold snap and September snow come just days after anomalously high temperatures set records across the region. And although the cold won’t last for long, the ferocious appetizer from Old Man Winter could bring upward of a foot of snow for some.

Winter weather advisories and winter storm warnings blanket the Rockies, the alerts Monday overlapping with warnings advertising high fire danger as wildfires continue to rage across the West. Exceptionally hot, dry weather this week left many asking “is this a joke?” when bulletins were issued for the sudden anticipated snowfall.

The dramatic drop

On Saturday, Denver hit 101 degrees, its latest-ever triple-digit reading and a record for the month of September. Sunday snagged a record high, too, at 97 degrees. Even Labor Day was unusually toasty with highs in the 90s. And as daylight dawned Tuesday, parts of the city were reporting snow.

It’s not just the Mile High City that has borne the brunt of a dramatic seasonal shift. Pocatello, Idaho, fell from the 80s at 5 p.m. Monday to the 40s by 7:15 p.m., headed off by a 14-degree temperature drop in 15 minutes. Cheyenne, Wyo., dropped from 86 degrees at noon to a snowy 37 at 10:30 p.m.

Several roadways were affected by toppled tractor-trailers, while power lines were reported “snapped” in the strong winds.

Salt Lake City International Airport saw gusts to 70 mph late Tuesday morning, with “rotor clouds” visible, a sign of strong waves of wind that develop in the lee of mountains in response to fluctuating density surfaces.

The setup

Driving the atmosphere’s haywire thermostat is a combination of low pressure in western Colorado and unusually strong high pressure for the season parked over Montana. The low-pressure zone blasted south, along with cold air, nestled in a dip in the jet stream that had been energized and shaped by two earlier typhoons in the Pacific. That led to an erratic wind shift that flip-flopped the seasons.

In Denver, for example, winds on Sunday and Monday had a westerly and southerly component. That meant air was forced down the Rockies, compressed by increasing atmospheric temperatures while being warmed and dried. The result? A hot air mass that toasted the region for days.

Residents could be seen riding scooters downtown Monday, clad in tank tops and short sleeves. Diners frequented outdoor restaurants, with a reddish tinge to the air beneath wildfire smoke streaming in from the north and west. Many of those same individuals were dressed in winter jackets and gloves Tuesday morning.

When winds switched around out of the east and northeast late Monday, air was forced upward by the high terrain instead, where it expanded and cooled. And with an already chillier air mass screaming in from the north, that effect was amplified dramatically.

The dramatic change in pressure over the intermountain west was responsible for the fierce winds in Utah which expanded into the Pacific Northwest and parts of California, where they were fanning extreme fire conditions.

The forecast

Denver residents were preparing for 3 to 5 inches of snow across the metro area, with more to the northwest. Accumulations were expected mainly on trees and grassy surfaces, since roadways are still so warm from heat earlier in the week.

But in the higher elevations to the west, the snow was falling and stacking up fast. Meeteetse, Wyo., at about the same elevation of Denver, had picked up 11 inches by 9:30 a.m. Tuesday. Casper Mountain, its 8,130 foot peak overlooking Casper, recorded 17 inches at the summit. Downtown Casper had more than a half-foot by sunrise.

Snow will continue across the area and shift south and east during the afternoon hours Tuesday, overspreading the High Plains and even stretching into northern New Mexico during the evening and overnight. Isolated instances of thundersnow can’t be ruled out as the atmosphere becomes acutely unstable ahead of a sharpening cold front from the west.

The cold front was even surging southeast into the central and southern Plains on Tuesday, albeit in a weakened state. A rope cloud could be seen on satellite across Oklahoma and Missouri ahead of the encroaching cold air.

How rare is this?

That said, the magnitude of the ongoing event sits in a class of its own. Neither Denver nor Boulder has ever seen a four-day stretch of 90-degree weather followed by snow before, according to the local National Weather Service office.

Denver has gone from 90s one day to snow the next twice since weather records began — once in 1974 and again in 1993.

There is good news for those weary of the early season wintry weather. It is September, after all, and warmth is still going to win out. Highs in the 70s and 80s are likely across much of the Front Range by late this week.