Chilly air outbreak to bring heavy snow to mountains of the West

Snowflakes will fly and chilly winds will blow over parts of the western United States from late this week to this weekend, sweeping away lingering summer-like heat.

The coldest air of the season so far will sweep across the western United States into Friday and is forecast to take root over the Rockies and Great Basin through the weekend. AccuWeather meteorologists expect snowflakes to fly over many of the higher elevations, with locally heavy snow in store for some of the high country.

The sweep of chilly air is marking an end to what has been a long, hot summer for much of the interior West. Temperatures from June through August were 3-8 degrees Fahrenheit above the historical average. The temperature departures increased from September to the first week of October, even more extreme at 5-10 degrees above the historical average.

The evolving sweep of chillier air will send temperatures 5-10 degrees below the historical average in many areas and 15 degrees lower than average in some locations. At this level, it will feel more like the middle of November rather than the middle of October.

For example, in Las Vegas, a high near 90 at midweek will be swapped with a high near 70 on Friday.

Because the air will be even colder in the middle and upper parts of the atmosphere and a large and slow-moving storm will grab some Pacific moisture before heading inland, rain showers will proliferate over the valleys and intermediate elevations of the interior. The colder air will allow some snowflakes to mix in at some intermediate elevations as well and mostly snow to fall over the high country.

"Freezing levels will be low enough over the Cascades for accumulating snow down to some of the passes, but the track of this storm will mean little, if any, rain and snow for the Sierra Nevada," AccuWeather Senior Meteorologist Heather Zehr said.

More substantial snow will fall over the higher elevations of the Intermountain West, including in areas all the way down into Arizona and New Mexico.

"Flagstaff, Arizona, will likely see its first snowflakes of the year, with up to an inch of snow possible from Friday into Saturday," Zehr said.

Snow levels will be such that from 8,500 to 9,000 feet in elevation, several inches of snow to a foot or more can pile up, leaving many of the mountains in Colorado, Utah and Wyoming capped in snow by the end of the weekend. Snow of varying intensity may fall on some of the high country through the weekend.

"In the Denver area, it probably will be just a touch too mild for snow to fall downtown and at the airport, but in the foothills, there can be a mixture of snow and rain from Friday night to Saturday," AccuWeather Senior Meteorologist Dave Houk said, "Farther north, the first snowflakes of the season are likely in Cheyenne, Wyoming."

Motorists venturing through the higher passes in Colorado should be prepared for slushy and slippery conditions for a time this weekend.

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Gusty winds will precede, accompany and follow the change to chilly conditions in the West. Winds can get strong enough to cause any wildfire that breaks out to spread quickly.

"In California, westerly winds will pick up south of the storm on Thursday through the Antelope Valley, then strong northerly winds in the wake of the storm will affect the mountains from Thursday night through Friday night before diminishing on Saturday," Zehr said.

Zehr warned that gusty winds, accompanied by low relative humidity and dry fuels over much of the West, will create an elevated fire risk.

Because the winds will be more northerly and such a large pool of chilly air will be involved, this will not create a hot Santa Ana situation for Southern California. Highs will be mainly in the 70s in coastal Southern California into Friday then will trend upward into the 80s this weekend as the core of the cool air pivots farther inland over the West.

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