Storm system to unload late-season snow in Northwest

·3 min read

A big change in the weather pattern will unfold across the northwestern United States this week, with areas of rain, thunderstorms and even heavy snow forecast for the region.

Temperatures in the Northwest have been generally above average for about a week. Seattle, which usually stays in the mid-60s throughout May, has had high temperatures in the 70s from last Tuesday to Sunday. Temperatures even reached into the 80s over the weekend in Portland, Oregon.

Following this mainly dry and mild start to the week, a potent storm will slam into the Northwest at midweek, according to AccuWeather long-range meteorologists.

Wednesday is when substantial precipitation is forecast to arrive in the Northwest. Dramatically cooler air will pour into the region as the jet stream dips southward.

By Wednesday, temperatures will fall 15-30 degrees lower than early week highs, according to long-range meteorologists.

"A late-season, long-duration snowstorm will develop along the northern Rockies and the Bitterroot Range Wednesday night," said AccuWeather Storm Warning Meteorologist Michaela Heeren.

Showers and perhaps a few thunderstorms will target the Pacific Northwest as the storm moves inland.

"By Wednesday night, this storm will be able to pull deep moisture all the way from the Gulf of Mexico into the northern Rockies," said AccuWeather Meteorologist Mary Gilbert.

"This abundance of moisture means there will be plenty of fuel for lower elevation showers and thunderstorms, in addition to higher elevation snow," said Gilbert.

Showers are expected to spread from Seattle and Spokane, Washington, to Portland and Eugene, Oregon, Wednesday and Wednesday night.

"As markedly colder air sinks southward out of Canada on Wednesday night, snow levels will begin to drop across the northern Rockies," Gilbert said.

"Snow levels look to bottom out around 4,000 to 5,000 feet on Thursday and Friday, meaning many of the major passes in the area, including Donner Summit, may be at risk for slushy or snowy road conditions," said Gilbert.

Heavy, wet snow can even affect areas down to 3,500 feet, with the highest accumulations above 5,000 feet, according to Heeren.

At 3,700, feet, Great Falls, Montana, has the potential to receive several inches of snow from the storm from mid- to late week. Meanwhile, Marias Pass, Montana, at an elevation of 5,200 feet, may receive more than 2 feet of snow. An AccuWeather Local StormMax™ of 48 inches is foreseen over the higher elevations of western Montana from the storm.

"While wet weather could be perceived as an annoyance to some residents, any drop of rain or flake of snow will help in the fight against drought in the Northwest," Gilbert said.

"Any precipitation in this area would be helpful to the drought situation, and suppress wildfire concerns for at least a little bit," added AccuWeather Senior Meteorologist Courtney Travis.

Over 25% of Oregon is in extreme drought, according to the United States Drought Monitor, while about 91% of the state is in moderate drought. Portland has reported only 5% of its average precipitation amount so far in May.

Washington state is also in dire shape, with over 20% of the state in severe drought. Spokane has received only a trace of rainfall so far in May, putting it at a fraction of a percent of the normal 0.80 of an inch that is typical at this point in the month.

This storm will taper off by Sunday, according to Heeren.

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With most of the western U.S. expected to reside under an expanding dome of high pressure next week, the general storm track will likely shift north into the Pacific Northwest and Pacific Canada, according to long-range meteorologists.

This may result in near-average rainfall across western Oregon and Washington, with above-average precipitation being limited to areas north of the Canadian border.

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