Forecasters say heavy rain, wind next for Washington

The Cascade Mountains and foothills will be drenched by 10 to 15 inches of rain and wet snow over the next two days, with major flooding forecast for many rivers in Western Washington.

It's possible Interstate 5 could be under water near Centralia and Chehalis — though the freeway flooding may not be as extensive as last year's, said National Weather Service meteorologist Ted Buehner.

Flooding is expected to begin by tomorrow, with the hardest-hit areas including Concrete, Snohomish, Monroe, Snoqualmie Falls, Centralia, Randle and Willapa.

"This is probably going to be one of our major events in the past 10 to 15 years," Buehner said. "We have a lot of water headed in our direction."

University of Washington meteorologist Cliff Mass is slightly more sanguine.

"It's not going to be as bad as November '06, but it's going to be bad enough," he said. "It's going to rain all night tonight, through Wednesday and into Thursday morning."

The urban Puget Sound corridor, especially from Seattle north to Everett, will likely be spared the heaviest of the rainfall and the probability of flooding there is not high, Buehner said.

But rain on mountain passes and slopes will melt some of the snowpack that has accumulated over the past couple of weeks. Snowmelt will be aggravated as freezing levels climb to 6,000 to 8,000 feet.

Much of the precipitation will fall as rain in the mountains, adding a heavy, soggy layer to a snow pack already primed for avalanches.

"What we're looking at is potato chips on the bottom of the snowpack and we're loading it with bricks," said Mark Moore, director of the Northwest Weather and Avalanche Center. "It's a very dangerous situation now."

The heavy rain forecast for Western Washington will increase the risk of mudslides, according to the Weather Service.

Between this afternoon and Thursday morning, about 3 inches of rain are expected in the Seattle/Everett area, with more in the eastern suburbs. Olympia will get about 5-6 inches of rain, with up to 8 inches in the Willapa Hills and west slopes of the Olympic Mountains. The Weather Service says Mason County could see up to 15 inches of precipitation.

With all the snow on the ground in Eastern Washington when the rain arrives, forecasters say urban and small stream flooding is possible.

Forecasters say soil moisture is moderate after precipitation the past two weeks. Several inches of rain expected by Thursday will put extra pressure on areas prone to landslides.

The Weather Service also has issued a wind advisory for the Seattle-Tacoma-Bremerton area until this morning.

Forecasters say the storm blowing through Western Washington will have winds of more than 30 mph with gusts of 45 or more.

That could bring down some trees and cause power outages.

A warm plume of moisture from the subtropics will begin moving over Western Washington this evening. The rains will pick up then. But the strong westerly flow will be blocked by the Olympic Mountains, creating a rain shadow from Seattle to Everett.

And there is a bright spot ahead for weather-weary Washingtonians, Mass said. A high pressure ridge appears to be developing, which is likely to bring dry, warmer weather by the weekend.

"We're going to go from snow and cold, to rain and flood, to warmer and dry," he said.

Sandi Doughton, Seattle Times science reporter; sdoughton@seattletimes.com.

Material from The Associated Press is included in this report