Persistent storms to elevate flood, avalanche risk in northwestern US

By Renee Duff, AccuWeather meteorologist

January 27,2018, 3:22:01AM,EST

As storms tap into tropical moisture and snow levels rise, the risk of flash flooding and avalanches will increase across the northwestern United States into early next week.

Warm, dry conditions will dominate the Southwest this weekend, but it will be a different story for residents farther north, where umbrellas, rain jackets and windshield wipers will get extensive use.

Storms with significant precipitation will track across the Pacific Ocean and take aim at the Northwest and southwestern Canada into early next week.

The risk of flash, urban and river flooding will increase with each storm.

The next wave of moisture will slam into Washington and Oregon through Saturday afternoon. 

Despite rising during the event, snow levels will remain at or below pass level in Washington. Travelers over Interstate 90’s Snoqualmie Pass will once again need to be prepared for lane closures and treacherous driving conditions following the numerous delays from snow this week.

In the Oregon Cascades, warmer air pressing in will bring a changeover from snow to rain for elevations generally below 5,000 feet.

“Gusty winds could also be locally damaging at the coasts and in the mountains,” AccuWeather Meteorologist Jordan Root said. Tree damage and sporadic power outages are possible.

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A second, warmer storm will arrive by Sunday.

“This [storm] will bring an even deeper river of tropical moisture into the region,” Root said.

Root anticipates the heaviest rain to fall over Vancouver Island in British Columbia and the Olympic Mountains and northern Cascades in Washington.

The west-facing slopes of Washington’s Olympic and Cascade ranges, where the snowpack is feet deep, will be at highest risk of flooding and landslides due to the combination of rising snow levels, rainfall and melting snow. 

After dropping below flood stage on Thursday night, the Skokomish River in northwestern Washington is projected to again reach flood stage by the end of the weekend.

Other small streams and rivers throughout the region will be in jeopardy of overflowing their banks.

Parts of Washington may get a break from the deluge on Sunday night as the heaviest precipitation lifts northward for a time into southwestern Canada. However, the damp weather, as well as much colder air, will sag southward at the start of the new week.

After being soaked by rain for part of the weekend, snow will again fall to pass level in Washington by Tuesday. The dramatic fluctuation in snow levels over the next several days may cause the existing snowpack to become unstable enough to trigger avalanches.

Showery weather is likely to persist across the region into the middle of next week.