MOUNT PILCHUCK — Experts are warning that record snowfall this season has built up the potential for huge spring avalanches with the power to cause extensive damage to mountain hillsides, roads and bridges.

Nearly 10 feet of winter snow has accumulated already in parts of the Cascades, including near Stevens Pass. If a massive slab of snow were to break off — and conditions are in place for this type of avalanche to occur — it could ravage the landscape, destroying timber stands, homes, roads and whatever else is in the avalanche’s path.

“There’s the potential for something really big,” said Mark Moore, director of the Northwest Weather and Avalanche Center.

This winter already is the worst avalanche season in Washington in modern history. Since December, three Snohomish County residents are among the nine people from Washington who have died in avalanches.

Now, officials fear the death toll could rise and the potential for other damage from rushing walls of snow is high.

“We have the potential for some pretty unusual events,” Moore said. “It really depends on how the rest of the winter evolves. There’s a lot we don’t know yet.”

Changing temperatures, rainstorms, winds and heavy snowfall have created dangerous instability in the snowpack, Moore said.

Deep in the snow, there are slick layers of ice and layers of unconsolidated snow. When heavier, dense snow stacks above weaker layers, it’s like piling bricks on top of potato chips, experts said.

The unstable layers developed in early December when rain fell on top of heavy snow, said Paul Baugher, director of the Northwest Avalanche Institute and the ski patrol at Crystal Mountain.

Rapidly warming temperatures or a warm, tropical rainstorm could load the snowpack, and those deep layers could suddenly cause serious problems, he said.

“You have to realize that those layers are at this point dormant and not gone,” Baugher said.

Late into the spring, large slab avalanches could come barreling down hillsides onto unsuspecting people who believe that warmer temperatures and melting snow have eliminated avalanche risk, he said.

Avalanche deaths in Washington can occur late into the season, sometimes as late as May, Baugher said.

Anyone going into the backcountry should have some avalanche knowledge, be properly equipped and know how to assess the risks, officials said.

Weather, snowpack and terrain are the best predictors of avalanche safety, Baugher said.

People venturing onto the snow should wear avalanche beacons, electronic transmitters that can help rescuers find someone buried in the snow. They also should carry avalanche probes and snow shovels, said Eddie Espinosa, an Everett Mountain Rescue volunteer.

Volunteers from Everett Mountain Rescue, a unit of Snohomish County Volunteer Search and Rescue, are called when people around the region get stuck in the mountains, trapped on cliffs or buried in avalanches. They are a group of trained men and women who regularly give up weekends and evenings to rush into the woods.

It was an Everett Mountain Rescue team that found Emily Swanson, the 13-year-old Mukilteo girl who died when she was buried in an avalanche on Mount Pilchuck earlier this month.

Last week, three Everett Mountain Rescue volunteers and Snohomish County Sheriff’s Office search-and-rescue deputies flew in a helicopter onto the snow-covered slopes of Mount Pilchuck. There, they dug a trench 6 feet deep in the snow to demonstrate some of the important steps in evaluating avalanche danger.

Before snowshoeing, skiing, hiking or snowmobiling, experts recommend people dig a hole to look at the layers in the snow and to test the snowpack’s strength.

“During the course of a trip, I might do this one, two, three times,” Espinosa said. “Similar to rings of a tree, you can tell the history of the snowpack.”

Two snowballs from the trench told two different stories.

One snowball was heavy, and could be tossed about while still holding its shape. The next, the same size but made from snow found in a different layer, was barely strong enough to hold together and crumbled into pieces when it landed.

“Good snowball snow makes for a good avalanche,” Espinosa said.

Espinosa examined the layers and then pounded on the top of the snow to see if and where it crumbled. He was mimicking the forces made by a skier, snowshoer or snowmobile that might trigger a larger avalanche.

Crumbling snow would mean that the risk of a slide is high. Even if the snow tested stable, it might not be the same someplace else, Everett Mountain Rescue’s Robert DeWolf said.

Snow on different slopes facing different directions can be very different, he said. Each should be tested.

Last week’s test showed the snow was in pretty good shape on the west face of Pilchuck, around 3,200 feet elevation, said Jon Wilson, 61, an Everett Mountain Rescue volunteer since 1974.

“At this elevation, I’m surprised to see how wet this is,” he said, testing the snow in his fingers.

Colder, more stable weather conditions have helped the snowpack to settle some recently, and avalanche risks have diminished slightly, experts said.

Still, the La Nina weather pattern that already has dropped deeper-than-usual snow this year is likely to continue, said Clifford Mass, a University of Washington atmospheric sciences professor.

Statistical models show that the weather pattern — colder temperatures and heavier-than-normal precipitation — tends to occur more in the second half of the winter, he said.

“This is a historical year, and it’s not over yet,” Mass said.

Reporter Jackson Holtz: 425-339-3437 or jholtz@heraldnet.com.