BOISE, Idaho (AP) — As a massive winter storm dumped snow across much of the western U.S., winter sport enthusiasts headed to ski resorts and backcountry slopes ahead of the long Martin Luther King Jr. Day weekend.

But in many areas, the storm brought a high risk of avalanche conditions along with the wind and snow, with fatal results.

In northern Idaho, two men were rescued and a third was presumed dead after they were caught in an avalanche Thursday afternoon. Later that night the occupants of two vehicles escaped without injury after an avalanche buried their cars in another part of the state.

The first reported U.S. avalanche death of the season happened Wednesday morning in California, on a section of expert trails at the Palisades Tahoe ski resort near Lake Tahoe. Four people were trapped and one was killed in that slide. A second avalanche struck the same area the following day, with no reported casualties.

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Here’s a look at why, when and how avalanches occur, and some tips on how to stay safe from slides:

Why do avalanches occur?

It generally takes just two ingredients to create avalanche conditions: A slope of 30 degrees or more, and layers of snow.

“Avalanches are really tied into having layers within the snowpack, and those layers are caused by weather conditions,” said Ben Bernall, an avalanche forecaster with the U.S. Forest Service Panhandle Avalanche Center. “It’s kind of like a cake: You might have a nice thick layer that’s good and cohesive, and then a thinner layer of frosting. Then put another cake on top of that, and throw in the factor of a slope angle or steep terrain.”

Extra pressure on top of that snowpack “cake,” from wind, rain, heavy snow or motion can cause some of the layers to shear off and slide down the slope, Bernall said.

Sometimes that slide happens in the form of loose snow, called a “sluff.” Sluffs account for only a small percentage of deaths and property damage from avalanches, according to the Sierra Avalanche Center.

But many avalanches are made up of slabs, where a large layer of snow breaks away and slides down the mountainside. Those account for most fatalities.

Another kind of avalanche occurs when wind creates a cornice of snow that hangs over a ridge or the edge of a steep slope. An overhanging mass of snow can fall suddenly, catching anyone standing underneath or on top of the cornice by surprise, according to the Colorado Avalanche Information Center.

What triggers an avalanche?

Movement, rapidly changing weather, wind — any one of these things can cause an avalanche. But experts say 90% of the avalanches that cause injuries or deaths are triggered by the victim or someone with them.

That means that people who ski, snowmobile, snowshoe or enjoy other activities in the snowy backcountry should check the avalanche forecast before they head out, and make sure they have the right safety gear.

Most ski resorts have avalanche protocols or mitigation systems. Often that means checking snowpack stability. Avalanche experts and ski resorts also sometimes use remote detonations to trigger slides manually, removing the riskier layers of snow, before skiers are allowed on the slopes.

What doesn’t trigger an avalanche?

Loud noises, generally. Despite what cartoons and movies might have you believe, the sound waves created by someone yelling aren’t enough to trigger an avalanche, according to the Sierra Avalanche Center.

In particularly unstable snow conditions, however, an extremely loud noise, such as from a nearby explosion, could do the trick.

How common are avalanches?

An average of about 28 people die in avalanches every year in the U.S. according to the Federal Emergency Management Agency. Last winter, 30 people died in avalanches in the United States. All of them were skiers, snowboarders, snowmobilers, snowshoers, climbers or hikers, according to the Colorado Avalanche Information Center.

Last February, three members of a mountain climbing club from New York perished in an avalanche on a remote peak in the Cascade Mountains of Washington state.

Three climbers in Alaska’s Denali National Park died in May in two separate incidents on the same day. One triggered an avalanche while skiing in the park’s backcountry. Two others were swept away as they prepared to climb a peak known as Moose’s Tooth. Their bodies were never found.

How can someone avoid an avalanche?

There are several ways to avoid an avalanche, but running isn’t one of them. Dry slab avalanches typically reach speeds of between 60 and 80 miles an hour (between 96 and 129 kph) within seconds of starting, according to the Sierra Avalanche Center. Wet avalanches usually travel around 20 miles an hour (32 kph).

By comparison, eight-time Olympic gold medalist Usain Bolt reached a record running speed of just under 28 miles per hour (45 kph) in the 100-meter dash in 2009 — and that was on a flat track, not a churning, snow-covered slope. Experts say most people who are caught in avalanches are on the slope when the slide occurs.

A better plan is to avoid the avalanche before it happens. Recreationists can check the avalanche forecast at their regional avalanche center or at www.avalanche.org. Experts say backcountry users should also learn and watch for the signs of avalanche terrain and unstable snow, and avoid cornices and risky areas.

Bernall said three pieces of gear are essential for anyone in in avalanche country: an avalanche transceiver, which sends a location signal to other people in the group if one person gets buried; a shovel to test the snowpack and dig out any buried companions; and a thin, folding avalanche pole that can be used to poke into the snow to find someone who has been buried by the snow.

It’s also a good idea to use the buddy system, Bernall said, and to be versed in wilderness first aid and rescue skills.

“Good decision-making in the backcountry is the biggest piece of the puzzle,” he said. “With the right decision-making, everything else is kind of obsolete.”

This weekend the best bet in central Idaho’s avalanche country is to stay on lower elevations and away from steep slopes, said Boise County Sheriff Scott Turner.

“People have been waiting for the snow to recreate, but the snow conditions right now aren’t good. We’re encouraging people to stay on lower ground,” Turner said.