Drenching storm with snow to get jump on fall season in West

A storm packing drenching rain, much cooler air and even the first snowfall of the season for the high country is underway for parts of the western United States. The changes in the weather that it will usher in will help to ease wildfire concerns, but the rain will not reach everywhere and winds generated by the storm could make matters worse in some locations.

Even though the official start of the fall season is still several days away with the autumnal equinox at 12:20 p.m. PDT on Wednesday, Sept. 22, it seems Mother Nature is getting a little jump on the season in the Northwest.

Rain began to soak a portion of western Washington late Thursday night and early Friday morning. As the storm from the Pacific Ocean continued to spiral inland Saturday, rain reached eastward over the Cascades and into parts of the northern Rockies and extend southward into parts of Northern California before coming to an end by Sunday afternoon.

Radar imagery captured from AccuWeather.com Saturday afternoon, Sept. 18, 2021, showing heavy rain moving into the Pacific Northwest with the heaviest downpours in Northern California. The green colors depict light rain with the yellows and reds depict areas of steady rain that could be heavy at times.

The period from June 15 to Sept. 15 is typically the driest time of the year in the Northwest. Seattle averages a mere 2.95 inches of rain during that three-month stretch, compared to an annual rainfall of 29.34 inches. However, this year, the dry season was extra lean, and the stretch from March 1 to Sept. 8 was the driest in 77 years, with only 6.78 inches of rain or 54% of normal, according to the National Weather Service in Seattle. During this year's dry season, the three-month stretch from June 15 to Sept. 15, only 0.26 of an inch of rain fell at Seattle-Tacoma International Airport.

The storm pushed into the Northwest bringing more rain than all of the dry season and the last part of the spring in many areas.

AccuWeather meteorologists accurately predicted that rain would reach much of the states of Washington and Oregon and even portions of Northern California, northern Idaho and northwestern Montana.

"A general 1-2 inches of rain fell in the zone in between the Coast Ranges and the Cascades in Washington and Oregon, with 0.25 to 1.00 inch of rain east of the Cascades in Washington, Oregon and northern Idaho," AccuWeather Senior Meteorologist Brett Anderson said.

A map showing 24 hour rainfall accumulations across the Pacific Northwest that depicts the heaviest rain (in the yellows and reds) fell in the Washington and Oregon Cascades and along the Oregon coast. (NOAA/RFC Hourly Precip Tabbed MapSeries)

"Heavier rain fell along the western slopes of the Olympics, Coast Ranges and Cascades," Anderson stated. In these areas, a general 2-4 inches of rain fell with more than 4 inches of rain in the Cascades of southern Washington State.

Owl Mountain, Washington, on the Olympic Peninsula, has already racked up 3.03 inches of rain on Friday, with Sekiu, Washington, close behind with 2.61 inches.

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Since the heaviest rain fell in areas where there have been relatively few fires recently, flash flooding problems from rapid runoff generally were limited to urban areas. Still, significant rain spilled over the Cascades and push into the northern Rockies, where many large wildfires have burned or continue to burn. The wet conditions should provide at least some benefit for fighting fires, reducing flare-ups and limiting new fires.

Rain stopped short of drenching San Francisco, but rain did soak areas farther to the north in the Golden State. Redding, California, is among some of the major cities that have accumulated some rain from the storm.

Pendleton, Oregon, Spokane, Washington, and Boise, Idaho, are among the cities across the interior Northwest did receive some soaking rain late Saturday night into early Sunday morning.

And as temperatures drop off, rain is not the only form of precipitation on the way for the western U.S. The first snow of the season is in the offing for parts of the high country of the West.

"While a heavy snowfall is not foreseen in this case, there can be a few inches over the ridges and peaks in the mountains from Idaho and western Montana to Wyoming early this week," Anderson said.

"A rain and snow mix can occur over some of the passes in the northern Rockies with some accumulation beginning above 6,500 feet in elevation," Anderson added.

In order for there to be snow over the high country, temperatures have to tumble. A plunge of 20, 30, even 40 degrees Fahrenheit is forecast early this week. For example, high temperatures in Salt Lake City have peaked in the middle to upper 80s during the second half of last week, but even in the basin where the city is situated, highs are forecast to be in the mid-60s on Monday. Nighttime, low temperatures are predicted to be in the middle 40s during the last couple of nights of the summer.

In Aspen, Colorado, on Tuesday morning, temperatures are forecast to dip below freezing for the first time since May 25. Following high temperatures in the mid-90s in Denver during the latter part of last week, highs in the 70s and nighttime lows in the 40s will make it feel like fall just in time for the official change in seasons early week.

The early blast of autumn will come as parts of the region are already showing tell-tale signs of the seasonal transition. The Aspen trees in parts of the Colorado Rockies have already transformed with stunning hues of amber and yellow.

Accompanying the snap to cooler conditions, areas of rain and high country snow will be gusty winds.

"Winds can gust past 50 mph over some of the passes and along the eastern slopes of the mountain ranges," Anderson said.

Winds will pose a problem ahead of any rain and in areas south of the storm both in terms of spreading any existing wildfires and raising the risk of new wildfires igniting as burning embers are whisked along.

On Sunday, a zone of elevated fire risk was forecast to encompass portions of Nevada, Wyoming, South Dakota and western Nebraska, according to the National Weather Service Storm Prediction Center (SPC).

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