Next up: Heavy rain, floods across W. Wash.

Cities across Western Washington are bracing for another slap from a season that has already dealt the region a series of nasty blows.

This time, the pain will come in the form of drenching rain followed by floods and the threat of landslides and avalanches, forecasters warned Tuesday. Some rivers, particularly in Lewis County, could reach record levels, and it's possible Interstate 5 near Centralia could be submerged again — as it was for several days in December 2007.

Major flooding also was forecast on the Skagit River near Concrete, the Snohomish River near Monroe, the Tolt River near Carnation, and the Snoqualmie River near Carnation and Snoqualmie Falls.

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

The flooding is expected to start today and continue through Friday in some areas.

The steady rain should begin to sputter out by Thursday.

Seattle and much of the urban Puget Sound corridor will be protected from the heaviest downpours by an Olympic rain shadow, said University of Washington meteorologist Cliff Mass. "I don't think we're going to see a lot of flooding here in the city."

But the Cascades and foothills could see 10 to 15 inches of precipitation through Thursday, much of which will fall as rain. At the same time, an influx of warm air will raise the snow level to 5,000 to 8,000 feet. That means much of the low-lying snow pack will melt, adding to the flood danger.

The risk of avalanches has already reached red alert and will only get worse as heavy, wet layers accumulate on top of an unstable base, said Mark Moore, director of the Northwest Weather and Avalanche Center. "What we're looking at is potato chips on the bottom of the snowpack and we're loading it with bricks."

Highway 2 across Stevens Pass, one of the state's principal east-west routes over the Cascade Mountains, was closed Tuesday afternoon by an avalanche, and Stevens Pass ski area was evacuated after authorities cut a car-width path through the snow.

State Patrol troopers said they didn't think any cars were caught in the slide. High winds also blew through the region Tuesday, causing more than 30 small power outages, according to Puget Sound Energy.

The Chehalis, Newaukum and Skookumchuck rivers in Lewis County were all predicted to crest at or near record levels by Thursday.

"What are we doing?" asked Cyndi Ticknor, the county's maintenance and operations superintendent. "We're praying."

But not waiting for divine assistance. Crews were standing by with sand bags ready to surround the courthouse and jail, which flooded in the 2007 storm that closed I-5.

"It's really a mess for us when I-5 closes down," Ticknor said. Freeway traffic, including semi-trucks, pours onto local roads, causing congestion that could be even worse if more rivers overflow, she said.

The town of Randle in Lewis County, nestled between mounts Rainier and St. Helens, is facing the possibility of a Cowlitz River 3 feet above record levels, according to the National Weather Service. Tuesday evening, Fire Chief Jeff Jaques was trying to decide whether to evacuate his own offices.

"The last record flood, we had 8 inches of water in the building," he said.

The town still has up to a foot of snow on the ground — all of which could be turned to water, if the forecasts prove accurate, Jaques pointed out.

One saving grace is that this Pacific storm is relatively fast-moving, and not likely to park in one place for too long, Mass, the UW meteorologist, said. "One area is not going to get hammered for the whole period."

Precipitation across the region will vary widely through Thursday: Up to 15 inches at Mount Rainier and other high points; 8-10 inches in foothill areas; 5-6 inches in Olympia; 8 inches in Mason County; and 3 inches from Seattle to Everett.

Flood levels will become clearer today, as rainfall totals are tallied and fresh computer model runs predict the storm's details.

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.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com