Atmospheric rivers to flow into Pacific Northwest, heighten flood and avalanche threats
Successive storms early this week and beyond will trigger flooding, landslides and high winds in the mountains, with impacts that may cascade into regional air and highway travel through the busy holiday period.
A police officer in Wawa, Ontario, jumped into a ditch to avoid a skidding vehicle on Nov. 26. Video shows the close call during a snowstorm on Highway 17. No injuries were reported from the incident.
AccuWeather meteorologists caution that numerous storms will press into the Pacific Northwest over the upcoming weeks, delivering repeated rounds of heavy rain and mountain snow across the region.
Fueled by multiple atmospheric rivers, this active setup carries the potential to unleash flooding downpours, spark mudslides, halt travel on highways and elevate the avalanche danger around the mountain peaks.
Significant rain and blustery conditions this week
A substantial storm is on track to arrive early week, marking the beginning of a period of increasingly frequent systems expected to impact the area through the middle of the month. Beginning Monday, the storm will start to unleash several inches of rain from western British Columbia, Canada, to western Washington and Oregon.
While rain and showers spread across the region through the end of the weekend, a more widespread and heavier event is in store for portions of Washington and Oregon come Monday.
“The storm early this week can bring a few inches of rain, which could fall over already saturated soils and lead to more widespread flooding near the coast, in valleys and in upslope areas that can trigger mudslides and rises on rivers and streams,” explained AccuWeather Meteorologist Alex Duffus.
The most notable rainfall amounts expected from Monday to Wednesday night are 8-12 inches for the upslope regions and higher elevations of the Cascades.
Across these mountainous zones, where significant flooding, landslides and road washouts could unfold, is where forecasters are highlighting the best chance for rainfall totals to approach the AccuWeather Local StormMax™ of 22 inches from the rain that falls from Monday to Wednesday night.
As the early-week storm pushes onshore, winds will strengthen along the coast and across the Coastal Range. Coastal gusts are expected to reach 35-45 mph during the day on Monday, while eastern Washington and Oregon may have winds of similar intensity by Monday evening.
Farther inland, the storm’s energy will set the stage for a blustery stretch across Montana, Wyoming and into the Dakotas on Tuesday into Tuesday night. Forecasters caution that residents in the Montana-Wyoming corridor may experience only a brief lull in the winds Tuesday night before the next approaching storm drives another round of gusts that could blow around holiday decorations or even cause property damage.
Rising snow levels initially
As the storm evolves, the precipitation type and the associated hazards will change as moisture advances across the Washington Cascades. Forecasters say that rising snow levels and an influx of warmer Pacific air will shape the risks in the coming days.
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"A push of warmer air from the Pacific will accompany the moisture from the atmospheric river, causing snow levels to rise later Tuesday into Wednesday in the Washington Cascades. Wet snow could slow travel along U.S. Route 2 near Stevens Pass Monday night into Tuesday, but eventually that wet snow should change to rain during the day Tuesday into Wednesday as snow levels rise," AccuWeather Senior Meteorologist Dan Pydynowski noted.
"However, the warmer air mass, heavy rain and rising snow levels will raise the risk of avalanches heading into midweek across the Cascades as heavy rain falls on top of a snowpack," warned Pydynowski.
Pydynowski added that mainly rain is expected on I-90 through Snoqualmie Pass during the duration of the event from Monday to Wednesday.
Beyond the Monday to Wednesday storm, additional storm activity will persist through the latter half of the week and potentially extend into the week before Christmas. This rampage of rain, wind and mountain snow will only compound the hazards brought by the first storm.
The risk of flooding, mudslides, travel-snarling mountain snow and avalanches will remain elevated. As cooler air filters into the region later this week, snow levels are likely to drop, potentially sinking to near sea level and broadening the areas that could experience disruptive snowfall.
Over the highest mountain peaks in the northwestern United States, there is the potential for feet of snow in the double digits to fall over the next few weeks.
In the upcoming weeks and as the winter holidays draw closer, heavy rain and reduced visibility can cause flight delays and cancellations at area airports. Impacts of this nature during the busier holiday travel season can result in a domino effect across the nation's air traffic grid.
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