Pacific Northwest bracing for flooding rain, feet of snow as December storms line up
An increasingly stormy Pacific pattern is expected to deliver many inches of rain and feet of mountain snow across the Northwest, raising the potential for flooding, mudslides and avalanche dangers.
AccuWeather’s Long-Range Expert Joe Lundberg looks ahead to next week, highlighting a period of persistent cold from the Dakotas to the Northeast and the potential for snow and ice from the Midwest into the Northeast.
A wintry setup is expected to intensify across the northwestern United States and southern British Columbia as a series of storms line up. A storm that closed out this week only offered a small indication of what the setup may deliver through much of December, including an elevated risk of flooding, mudslides and avalanches, AccuWeather meteorologists said.
This storm is forecast to push inland and produce significant snowfall from Montana to the northern and central parts of the Plains during the weekend. Snow is expected to fall over many of the same areas that received substantial snowfall late last week.
As the weekend arrives, so too will another storm, delivering another round of rain to the lower elevations and a mix of wintry precipitation across the Washington Cascades.
However, a much more serious situation is forecast to unfold in the western parts of Washington, Oregon and southwestern British Columbia as inches of rain pour down next week and beyond.
Brace for high-impact Pacific storms starting next week
The stormy pattern will be far from over beyond this weekend in the Northwest. A storm will roll ashore every one to two days next week, disrupting travel and daily routines for people across the region.
A storm from Monday to Tuesday may be associated with a strong atmospheric river. "That storm could produce heavy rain and a heightened risk of flooding across western Washington and Oregon," AccuWeather Storm Warning Meteorologist Kyle Lavery said.
Storm activity from Monday to Tuesday night alone will bring a general 1-4 inches of rain in much of the Interstate 5 corridor, with 6-12 inches in the west-facing lower slopes of the mountains in western Washington, Oregon and British Columbia. The AccuWeather Local StormMax™ for these two days is 16 inches.
The runoff generated by the atmospheric river from Monday to Tuesday alone will bring flooding along small streams and the short-run rivers that flow out of the Cascades and Olympics. A major flooding event may unfold.
"A storm later next week has the potential to signal a broader pattern change," AccuWeather Chief On-Air Meteorologist Bernie Rayno said.
That storm late next week is forecast to dip south from the Gulf of Alaska along the coasts of British Columbia, Washington and Oregon. The setup will allow lower snow levels in the Cascades and could easily bring accumulating snow to the passes, intermediate elevations and hills in the region.
"A series of systems during Dec. 12 to 16 may be especially potent along the Northwest coast and could extend south to Northern California," AccuWeather Senior Long-Range Meteorologist Joe Lundberg said. "Along with varying snow levels, a couple of the storms could bring snow down close to sea level."
Copious amounts of moisture will be thrown at the Pacific Northwest over the next couple of weeks. The historical average monthly rainfall at lower elevations may be far exceeded. Seattle typically receives 5.72 inches of rain during December. There is the potential for twice that amount to fall along portions of the Interstate 5 corridor. Some of the west-facing lower slopes of the Olympics, Coast Ranges and low elevations of the Cascades may pick up 1-2 feet of rain in the weather conditions through the third week of December.
This rainfall will significantly increase the risk of flash flooding and mudslides across the region.
Over the highest ridges and peaks in the region, there is the potential for 10-20 feet of snow to fall over the next couple of weeks. As the snow piles up on the steep slopes and temperatures fluctuate, the risk of avalanches increases, along with the likelihood of flooding and mudslides down below.
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