Big change to autumn weather, including snow, coming to western US

Temperatures will tumble more than 30 degrees as a long overdue outbreak of chilly air will replace lingering summer heat in much of the West this week. A few areas will even see snow.

Temperatures will tumble over much of the western United States from the middle to late part of the week with the lowest temperatures since last spring. The chiller air will also be accompanied by some snow over the higher elevations of the region, AccuWeather meteorologists say.

The extended period of heat, which has featured temperatures of 4-8 degrees above the historical average in some areas of the interior West, including double-digit anomalies in recent weeks, is coming to an end.

Since August, there have been some quick bursts of cool air over parts of the West, but nothing like what is brewing for the middle and latter part of this week.

"There is likely to be a 30-degree-Fahrenheit drop in temperature in some locations of the West from Wednesday to Friday," AccuWeather Senior Meteorologist Dan Pydynowski said.

The chill will first settle into the Northwest during the middle of the week. Following highs to start the week well into the 60s in Seattle and Portland, Oregon, temperatures will trend downward. Highs will be in the 50s by Wednesday in Seattle and by Thursday in Portland. Some rain and drizzle will accompany the transition in the Northwest. Up in the Washington and Oregon passes, the air will get cold enough to allow some wet snow to mix in at times. Above the passes, snow is likely to accumulate.

Following record-challenging warmth into Tuesday and a high near 80 Wednesday, temperatures will trend sharply downward from Wednesday night to Friday farther to the southeast in Salt Lake City. The high Friday is forecast to only be within a few degrees of 50. Factoring in breezes and other conditions, the AccuWeather RealFeel® Temperature will be no higher than the 40s and can even dip into the 30s at times during the day.

Prior to the change to colder weather, strong wind gusts, combined with warm, dry air will raise the risk of wildfire ignition and rapid spread of existing fires.

The dramatic change to true autumn conditions will spur on more than just snowflakes over the mountains later this week. Snow will fall over higher terrain in the Wasatch, Big Horn, Tetons and Colorado Rockies.

"This has the potential to bring the first significant snowfall -- a few inches -- to the ski resorts in the Wasatch Range and perhaps as far to the south as northern Arizona," Pydynowski said.

Temperatures will dip to well below freezing at night later this week when it snows so that untreated roads and sidewalks can become icy. Gusty winds will make for RealFeel temperatures in the 20s and teens.

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The much cooler air will be felt in the deserts as well.

"High temperatures in Phoenix are forecast to trend downward from a high of 93 Thursday to 75 on Saturday, which is an 18-degree drop," AccuWeather Meteorologist Elizabeth Danco said, "The 75 on Saturday would be the first time since April 15th highs were in the 70s."

Similarly and perhaps even more extreme, temperatures in Las Vegas will trend downward from the low 90s Tuesday to the 60s Friday and Saturday. Flagstaff, Arizona, will transition from highs in the 70s Monday to highs in the low to mid-40s by Saturday.

Along the California coast, the chilly air will lose its sting. This, as the air flows partially downhill from the coast ranges to sea level. When air flows downhill enough it tends to warm. In this case the chilly effects of the cool air will tend to be diminished by the warming caused by downsloping. Temperatures in Los Angeles, for example, will trend upward through the 70s.

"Depending on where the same storm set to produce the snow over the interior West stalls out, could set up a heavy rain and flash flood situation this weekend for parts of eastern New Mexico, western Texas, the Oklahoma Panhandle, southeastern Colorado and western Kansas," Pydynowski said.

Several inches of rain may pour down in 24 hours in some locations from Saturday to Sunday.

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