The coldest air of the season so far will sweep across the western United States by Friday and is expected to gain a foothold over the Rocky Mountains and Great Basin by the weekend. AccuWeather meteorologists expect snowflakes to fly over many of the higher elevations, with locally heavy snowfall in some parts of the highlands.

The cool air signals the end of a long, hot summer for much of the inner West. Temperatures from June to August were 3 to 8 degrees Fahrenheit above the historical average. Temperature fluctuations increased from September to the first week of October and were even more extreme at 5-10 degrees above the historical average.

The developing momentum of cooler air will result in temperatures 5 to 10 degrees below historical average in many areas and 15 degrees below average in some locations. At this level it will feel more like mid-November than mid-October.

For example, in Las Vegas, a midweek high near 90 will be replaced by 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, slow-moving storm will pick up some Pacific moisture before moving inland, showers will spread over the inland valleys and mid-elevations. Due to the colder air, some snowflakes will also mix at some medium altitudes and mostly snow will fall over the highlands.

“Over the Cascades, freezing temperatures will be low enough to accumulate snow up to a few 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 snow will fall over the higher elevations of the Intermountain West, including areas down to Arizona and New Mexico.

“Flagstaff, Arizona will likely see the first snowflakes of the year, with up to an inch of snow possible Friday through Saturday,” Zehr said.

Snow levels will be high enough to allow several inches of snow or more to accumulate at elevations of 8,500 to 9,000 feet, leaving many mountains in Colorado, Utah and Wyoming covered in snow by the end of the weekend. There may be varying degrees of snowfall in parts of the highlands over the course of the weekend.

“It will likely be a little too mild in the Denver area for snow in the downtown area and airport, but the foothills may see a mix of snow and rain Friday night into Saturday,” said Dave, senior meteorologist at AccuWeather Houk said, “Further north, the first snowflakes of the season are likely in Cheyenne, Wyoming.”

Motorists venturing over Colorado’s higher passes should prepare for muddy and slippery conditions at times this weekend.

Gusty winds will precede, accompany and follow the change to cool conditions to the west. The wind can become so strong that a forest fire that breaks out spreads quickly.

Get the free Accuweather app

“In California, westerly winds south of the storm will increase through the Antelope Valley on Thursday, then strong northerly winds in the wake of the storm will impact the mountains Thursday night into Friday night before easing on Saturday,” Zehr said.

Zehr warned that gusty winds accompanied by low relative humidity and dry fuels across much of the West would create an increased fire risk.

Since the winds will be blowing more northerly and there will be such a large pool of cool air, this will not result in a hot Santa Ana situation for Southern California. Highs will be mostly in the 70s along the Southern California coast through Friday, then trend upward into the 80s this weekend as the core of cool air moves further inland across the West.

Do you want security at the highest level, without ads? Unlock enhanced, hyperlocal severe weather alerts when you subscribe to Premium+ in the AccuWeather app. AccuWeather Alerts™ are triggered by our experienced meteorologists who monitor and analyze dangerous weather risks around the clock to keep you and your family safer.