Avalanche danger rises as snow falls

By Joshua Murdock and Zoe Buchli

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    MISSOULA, Montana (Missoulian) — Snow depths in the mountains across western Montana are becoming deep enough to ski and snowboard, which means deadly avalanches are possible.

As much as 2 feet of new snow accumulated at high elevations over the weekend, offering early season powder turns at backcountry hot-spots like Lost Trail and Lolo passes.

In many places, the new snow piled up on an existing snowpack base that was likely to be weak. And the new snow was accompanied by high winds that could pile the snow into dense, sensitive slabs above a weak base. All of that creates dangerous avalanche conditions for backcountry travel and recreation.

The coming arctic front also fouled vehicle traffic in and around Missoula. Wind chills as low as 30 below zero are expected in the Glacier Park region and as low as 25 below zero are expected in the Flathead Valley, along US-2 through Tuesday morning, a National Weather Service forecast bulletin said on Monday. Low temperatures at Missoula were expected to drop to 13 Monday evening, and stay in that nighttime range through the rest of the week.

“The new snow and wind are loading the minimal base of snow that exists,” according to general avalanche information from the West-Central Montana Avalanche Center. “In areas where the base is deep enough to cover surface features and anchors, it could be a sliding layer. This is most likely in wind-drifted areas such as the leeward side of ridges and gullies. There is a high likelihood the base is faceted and weak, especially on shaded north and east aspects. Strong winds will create sizable drifts of new snow and load leeward (downwind) slopes. Avalanches large enough to bury, injure, or kill a person are likely in specific areas above 6,000 feet.”

The Avalanche Center, based in Missoula, won’t begin regular avalanche forecasts until early December, according to Friday’s general information. The center’s director, Jeff Carty, wrote Friday’s update, which is valid through 11 p.m. Thursday. The bottom line, he wrote: “North and easterly wind-loaded slopes will have the highest likelihood of avalanches that could bury, injure, or kill a person. Early season hazards, such as rocks and trees, increase the likelihood of trauma. Avoid wind-loaded slopes and stick to (slope) angles under 30 degrees. Do not travel under steep slopes and avalanche paths.”

On Sunday night, the Twin Lakes weather station at 6,400 feet in the Bitterroots reported 32 inches of snow. That was an increase of 20 inches from early Friday morning, when the station measured 12 inches of snow depth. In the Rattlesnake Mountains, the Stuart Mountain station at 7,400 feet reported 11 inches of snow on Friday morning. The recent storm increased snow depth there to 21 inches by Sunday evening.

“My main concern right now is we’re getting a rapid load on the snow … and we’re getting quite a bit of wind,” Carty said in a phone call Monday. “We’re going to see a lot of snow coming in, a lot of drifting, and those wind-loaded areas are going to be the riskiest. They’re also going to be deepest. The areas that are going to be the best to ride right now are also going to be the most likely to slide.”

Avalanches are possible once snow is deep enough to cover ground debris and form a coherent layer across a slope. The risk is particularly high in places where new snow fell on top of existing snow, which degraded in strength as it sat through frigid nights.

When shallow, early-season snow sits through frigid conditions, the snow crystals can transform into angular, roughly pyramidal facets that do not bond to each other. When that happens, and the faceted snow is then buried by later storms, the facets become a weak layer in the snowpack — like a pile of marbles as the base beneath a more solid slab of snow above.

Strong winds that blow snow across ridgelines often create dense and touchy wind slabs that raise avalanche risk. Snow crystals become small and round as they tumble along in the wind, allowing them to pack together tightly once they are blown over a ridge or other terrain feature and settle on the other side. That phenomenon can produce a localized wind slab on the downwind, or leeward, side of terrain features.

A slab over a weak layer is a recipe for avalanches. Avalanches can occur on slopes 30 degrees or steeper, and even small avalanches can sweep a person through trees or over rocks. Avalanches, even small ones, can also pile up deep if they accumulate in a gully, stream or road-cut — burying a person under many feet of debris with little chance of survival. Avalanches can be triggered from on, above, below or adjacent to where they actually occur on a slope.

Four people died in three separate avalanches in Montana last winter: three snowmobile riders and one rider on a snowbike (a dirt bike-snowmobile combination). One snowmobile rider and one backcountry snowboarder died in avalanches in Montana in the 2020–21 winter.

Backcountry travelers — that is, anyone traveling around snow deep enough to slide and outside of an operating ski area with avalanche control — should carry avalanche rescue equipment and practice how to use it. A modern avalanche beacon, shovel and probe are the three essential pieces of equipment for backcountry travel.

Public observations are vital to understanding and documenting avalanche hazard, he said. With a staff of only a few forecasters, observations from the public can greatly magnify understanding of how snowpack around the center’s vast forecast area is behaving. Observations, which can be submit online at missoulaavalanche.org, don’t need to be limited to spotting actual avalanches — although that does help. Observations about snowpack collapsing or “whumphing,” shooting cracks through the snow, rapid warming, wind transport and other snowpack phenomenon can help the center track the snowpack in various locations through the season.

“If they’re out in the field,” Carty said, “any information they can give us about what they saw is helpful.”

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