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Taste of fall in Northwest to bring first snowfall for some

AccuWeather logo AccuWeather 9/15/2021 Alex Sosnowski

A large storm with drenching rain is brewing over the Pacific Ocean, and it has eyes for the northwestern United States, which is in need of rain, AccuWeather meteorologists say. Much lower temperatures will follow, and some residents in the West may see their first snow of the season.

Many large wildfires continue to burn in the western U.S., where more than 5 million acres have been turned into cinders since the start of the year, according to the National Interagency Fire Center.

There has been little help from Mother Nature this summer, although some pockets of cool air have taken the edge off the heat over the past couple of weeks in the Northwest.

Seattle has experienced its driest spring and summer in 77 years, according to the National Weather Service, and a blistering and unprecedented heat wave with triple-digit readings in late June set the pace for the summer for the Northwest. Temperatures climbed as much as 2-6 degrees Fahrenheit above normal across the region through the duration of meteorological summer, which spans June, July and August.

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A storm earlier in September helped firefighting efforts in some locations with the aid of rain, lower temperatures and higher humidity levels, but many places in the Northwest did not get lasting benefits from that system. While Redding, California, managed to pick up 0.38 of an inch of rain from that storm, most areas received less than 0.10 of an inch of rain. Seattle picked up only 0.02 of an inch.

There is some good news on the way as the calendar flips to mid-September, a time when the storm track over the northern Pacific Ocean and across Canada typically becomes more active, AccuWeather Senior Meteorologist Brett Anderson said.

"We start to look for storms to swing in from the Pacific and bring much-needed moisture and lower temperatures this time of the year for the Northwest," Anderson stated.

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Thanks to a southward plunge in the jet stream, a storm over the Bering Sea at midweek will drop across the Gulf of Alaska on Thursday then pivot into the northwestern United States later Thursday night and Friday.

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AccuWeather's water vapor satellite shows a storm system moving into the Bering Sea on Sept. 15, 2021. (AccuWeather)

"The storm should have enough moisture and energy with it to bring drenching rain to western Washington, Oregon and southwestern British Columbia, but some beneficial rain is also likely to spill over the Cascades and reach the eastern portions of Washington, Oregon, northern Idaho and western Montana spanning Friday to Sunday," Anderson said.

This storm is highly likely to bring the most significant rainfall for the Northwest since early in spring or last winter.

"This amount of rain will be very beneficial for the Northwest in general, even though the heaviest rain will avoid the worst areas battling wildfires," Anderson said.

A general 1-2 inches of rain is forecast in the zone in between the Coast Ranges and the Cascades in Washington and Oregon, with 0.25 to 1.00 inch of rain possible east of the Cascades in Washington, Oregon and northern Idaho. Heavier rain is forecast along the western slopes of the Olympics, Coast Ranges and Cascades, where 2-4 inches of rain are expected with an AccuWeather Local StormMax™ of 6 inches.

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A second burst of moisture may swing in and soak northernmost California later Saturday and Sunday. There can be enough rain around San Francisco to wet the ground, make puddles and create slick roadways.

With the burst of rain forecast to fall in coastal areas of Washington and Oregon, quick runoff can lead to flooding in urban and poor drainage areas. The National Weather Service's Seattle office also recommended that residents make sure to remove any debris that is blocking storm drains and gutters.

"While there is some concern that too much rain may fall too fast and lead to debris flows in burn scar locations, less intense rain is forecast east of the Cascades, where many of the large fires have been burning," Anderson said, adding that these more inland areas are likely to pick up just enough rain for it to be more beneficial rather than problematic.

But it may not be all good news. Erratic wildfire behavior can occur ahead of the storm and south of areas that receive rainfall in its wake.

"Much of California and farther inland over the Great Basin could have an enhanced fire risk with stronger winds and still mainly dry conditions," AccuWeather Storm Warning Meteorologist Joseph Bauer said.

It would take a more drastic southward plunge in the jet stream and much more moisture from the Pacific to bring widespread relief to the Southwest. AccuWeather forecasters say there's no sign of that happening yet.

First snow of the season

Summer warmth will be replaced by cooler air as the storm sweeps through, and the air will be chilly enough to allow wintry precipitation to make a comeback in some areas.

"There will be some high-country snow in British Columbia and the Washington Cascades with the storm from Friday to Sunday," Anderson said. Snow is likely to be restricted to areas above 7,000 feet in Washington this weekend.

After that, the jet stream pattern may amplify to the point to allow enough colder air and moisture to bring the first snow of the season to portions of the Rocky Mountains from Idaho and Montana into Wyoming, northern Utah and northern and central Colorado by early next week.

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"If things pan out the way we believe from Sunday night to Tuesday, then snow levels could dip to 6,000 feet from Montana to Colorado," Anderson said. This means there could be a mixture of rain and snow over the passes, with accumulating snow over the ridges and peaks.

That turnaround will be the first taste of fall, and quite extreme for the region, following highs in the 80s and 90s over the weekend.

As that jet stream dip moves in, temperatures will plummet from northwest to southeast across the Northwest and central and northern Rockies, Anderson said.

"Temperatures are likely to dip to 10-20 degrees below average, and widespread rain showers are likely where it doesn't snow," Anderson stated.

This means that highs in the 80s and 90s will be replaced with highs in the 60s and 70s and nighttime lows will dip into the 30s and 40s. Following a high near the record of 93 set in 1895 in Denver on Saturday, temperatures in the Mile High City will drop into the 40s on Tuesday morning.

The cooler conditions and moisture should bring an improvement for fighting wildfires beginning in the Northwest this weekend and spreading over a large part of the interior West beyond that.

For the latest weather news check back on AccuWeather.com. Watch AccuWeather Network on DIRECTV, DIRECTVstream, Frontier, Spectrum, fuboTV, Philo, and Verizon Fios. AccuWeatherNOW is streaming on Roku and XUMO.

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