Rain tapered off across Washington and British Columbia on Thursday after a potent storm pushed rivers beyond their banks and left roads, towns and farmland submerged in floodwaters that aren’t expected to recede for days.
But more rain is on the way, forecasters said, with another storm set to approach the region as early as Sunday night and last into early next week.
The details are still uncertain, but forecasters said the most likely scenario was that the next storm would be less intense than the one that led to mass evacuations in western Washington this week.
At this point, though, even a small amount of rain can have a bigger-than-usual impact.
“The risk is that we are already pretty saturated,” said Harrison Rademacher, a meteorologist with the National Weather Service office in Seattle. “That is going to add more rain to already swollen rivers.”
A ‘storm highway’ sets up.
The West Coast of the United States and Canada has been locked into a weather pattern in recent weeks as one area of high pressure parked off the coast of California and another over northwestern Canada. Think of the high pressure as invisible walls in the atmosphere — the storms are moving between them, pulling moisture off the Pacific Ocean and shooting it directly toward the western Washington and southwestern British Columbia.
Ken Dosanjh, a meteorologist with Canada’s federal forecasting agency, called the setup a “storm highway,” as the high pressure areas shift around, bringing dry conditions to one area and saturating another. (This same pattern has kept much of California unusually dry for the last three weeks.)
By Sunday night, the next major storm will find its way through, taking aim at some of the same areas that have already endured significant rain this week.
Western Washington is expected to receive two waves of precipitation. The first is forecast to arrive Sunday night into Tuesday, bringing up to two inches of rain to the valleys close to the Pacific Coast, up to three inches of rain in the Cascade Range and up to five inches in the Olympic Mountains. This will be a warmer weather pattern, with snow falling only at the highest elevations.
More precipitation is expected Tuesday night into Wednesday, and it is likely to be colder, with snow falling at elevations as low as 3,000 feet. Low-lying valleys could receive an further one to two inches of rain.
“We’re talking about transitioning from mostly rain in the Cascades to mostly snow, and then there’s less of a threat of rivers flooding when it’s snowing,” said Jeff Michalski, a meteorologist with the Weather Service in Seattle.
These projected rainfall totals are small compared to the amounts from this week’s storm, which dropped two to fives inches of rain in the valleys and over 15 inches in the mountains. But the additional precipitation will fall over watersheds where rivers have been spilling over their banks, spreading across farmland and into communities.
“These rains could prolong flooding or renew flooding,” Mr. Michalski said, “but we’re not expecting as widespread flooding.”
Some of the most severe flooding this week has occurred on the Skagit River, which is expected to crest at a record level on Friday morning in the town of Mount Vernon, Wash. The river is predicted to recede over the weekend and then rise again next week, when it is like to bring additional, but more moderate, flooding.
Across the Canadian border, there is likely to be less precipitation in British Columbia than in western Washington, and Brian Proctor, a meteorologist with Environment Canada, said rain was being forecast for Monday into Tuesday.
“We do think it’s going to have some impact,” Mr. Proctor said. “It doesn’t look to be as severe as what we had this week.
“We’re still trying to get a feel for how it’s going to behave and what’s going to happen.”
Amy Graff is a Times reporter covering weather, wildfires and earthquakes.
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