Series of storms, including an atmospheric river, to blast Northwest US
A parade of storms will march into the Northwest from the Pacific with drenching rain, heavy mountain snow, and gusty winds into next week, raising the risk for flooding, travel disruptions, and power outages.
AccuWeather’s Joe Lundberg looks ahead to next week where the Northwest region of the U.S. can expect stormy weather. In the Northeast, cool weather will hit the region with the risk of a nor’easter.
As many as a half a dozen storms are expected to roll onshore in the northwestern United States through next week, bringing periods of drenching rain to lower elevations and accumulating snow to the higher terrain, AccuWeather meteorologists warn.
"Rainfall from this pattern may help ease long-term drought conditions, but it may fall at a heavy enough rate to lead to travel disruptions and ponding in urban areas," AccuWeather Senior Meteorologist Chad Merrill said.
Quick rises on some of the small streams and short-run rivers in the region will become more likely with each successive storm. "There will be an increasing risk of mudslides and other debris flows in recent wildfire burn scar locations," AccuWeather Meteorologist Kia Kerkow said.
Around midweek, satellite images showed a long plume of moisture extending from Japan and the Philippines to just off the coast of the northwest coast of the United States.
This long conveyor belt of moisture, or atmospheric river, will advance into the western U.S. coast this weekend as a storm moves ashore. Additional storms may follow, bringing repeated rounds of an atmospheric river activity through next week.
"From 1 to 2 inches of rain fell along the Interstate 5 corridor of Washington and Oregon in waves from Thursday night to Friday night and ended up being the biggest rainfall since mid-August," Merrill said.
Greater amounts of rain, averaging 2-4 inches, will fall on the lower western and southwestern facing slopes of the Olympics and the lower western slopes of the Cascades. The AccuWeather Local StormMax™ rainfall total is 6 inches through Saturday night, representing the highest localized amount expected during this storm period.
Locally gusty thunderstorms with hail will accompany the rounds of rain through this weekend.
Additional storm systems will roll in through much of next week, with spacing in between each precipitation event ranging from several hours to a day or two. The next storm in the series will begin on Saturday and continue into Sunday.
"Road impacts are not expected over the mountain passes early in the weekend, but conditions may deteriorate by Sunday," Merrill said. "Snow levels will stay above pass level until later Saturday night, when they may dip to 3,000 feet." Some roads may become slushy and snow-covered over the passes from late Saturday night to Sunday in the Cascades.
The storm during the weekend will bring 3-6 inches of snow over the passes, with less on the roads.
A brief lull in precipitation is possible on Monday, but additional rounds of rain and mountain snow are forecast to return Tuesday. Snow levels will vary from storm to storm and during each individual storm. Where snow is not creating slushy or slick conditions over the passes, fog and gusty winds may still cause travel delays and hazards.
Cumulative rainfall totals Wednesday through next weekend may average 3-6 inches in lower elevations along the I-5 corridor and as much as 6-12 inches along some of the west- and southwest-facing lower slopes of the Cascades and Olympics. Cumulative snowfall over the high country is likely to range from 1 to 3 feet.
In addition to the rain and snow, gusty winds will accompany some of the more potent storms, particularly the pair arriving from Thursday night to Sunday. Wind gusts of 40-50 mph are expected in most areas, with an AccuWeather Local StormMax™ of 60 mph along the coast and through mountain passes. These winds may be strong enough to break tree limbs and cause sporadic power outages.
Gusty Santa Ana winds may develop next week
South of the storm track, which will occasionally dip into Northern California and spread inland across the Northwest, dry winds may develop in Southern California.
"As high pressure builds into the Great Basin, a pattern of stiff breezes to gusty winds, known as a Santa Ana, will develop through the canyons of Southern California from Tuesday to Wednesday," Merrill said.
While the dry winds will raise wildfire concerns, the fire risk is not expected to be as high as usual for this time of year. That’s due to recent vegetation growth following multiple rain events in recent weeks.
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