Winter storm to bring snow, avalanches to western Montana

A month of relatively little snowfall in the mountains of western Montana took snow depth in the region from above average a month ago to below average for late January. But now a weekend storm is set to deliver a foot or more of fresh powder at high elevations, and possibly a few inches at middle elevations. Valleys are expected to see less than 1 inch.

The incoming weather system, expected to begin impacting the area Friday afternoon, is likely to snarl vehicle travel over mountain passes and elevate avalanche danger in the backcountry as fresh snow builds up on top of a snowpack that contains weak layers unable to support the extra weight. 

On Friday morning, the Idaho Panhandle Avalanche Center issued an avalanche watch that warned of “very dangerous avalanche conditions.” 

“Heavy snowfall and strong winds may be sufficient to overload very weak snow layers and cause a widespread cycle of avalanches large enough to bury, injure, or kill a person,” the watch stated. “Some of these avalanches could release naturally, and it will become very easy for backcountry travelers to trigger them.”

The Flathead Avalanche Center in northwest Montana also issued an avalanche watch on Friday, cautioning on Facebook that “we have all the necessary ingredients to develop dangerous avalanche conditions in the next 24 hours.”

The West Central Montana Avalanche Center, based in Missoula, tracks, analyzes and advises on snowpack conditions and avalanche danger across a broad forecast area stretching from Lost Trail Pass on the south to the Seeley area on the north. The nonprofit has not yet been able to issue forecasts or avalanche watches and warnings so far this winter because it lost about half its annual budget, and liability protection, when the U.S. Forest Service cut its funding to the organization for this season. 

Its operations limited, avalanche center urges public observations

However, forecasters will issue an updated general information product online Saturday morning, advising backcountry recreators on what they are likely to encounter as the storm adds weight to the snowpack. That information will be available at missoulaavalanche.org.

As of midday Friday, the National Weather Service predicted 10-20 inches of snow would accumulate around Lolo Pass, on U.S. Highway 12 southwest of Missoula, from Friday afternoon through Saturday evening. Another 2-4 inches could accumulate Sunday night into Monday. 

Lookout Pass, on Interstate 90 west of Missoula, was predicted to get 14-25 inches by Saturday evening. Lost Trail Pass, on U.S. Highway 93 south of Missoula and Hamilton, was predicted to get 5-11 inches in the same timeframe, but with an additional 3-7 inches coming Sunday into Monday. 

Mountain temperatures were predicted to be in the mid-20 degrees Fahrenheit overnight from Friday into Saturday, with predicted highs on Saturday in the upper 20s and low 30s, just below freezing. Beginning Saturday evening, an incursion of arctic air is expected to drop overnight lows into the low to mid-teens. Sunday’s highs were predicted to be in the low 20s, with overnight lows into Monday in the low teens or single digits. 

From a snowpack-loading standpoint, that means the storm is likely to come through “right-side-up,” said Avalanche Forecaster Cam Mackenzie, with heavier, higher-density snow falling at the beginning of the storm and lighter, drier snow falling toward the end. 

Regardless, he said, “We have potential for avalanche hazard to increase.” 

Avalanches are possible on any slope 30 degrees or steeper, and can be triggered remotely and without warning from above, below or adjacent to a slope that steep. Avalanches are most likely to begin on slopes between 35 and 45 degrees.

Avalanches are the leading cause of death by any natural hazard on national forests, killing an average of 25-30 people in the U.S. each year, according to the National Avalanche Center, a program of the U.S. Forest Service. Almost all of the deaths occur on lands in the National Forest System, where nearly all backcountry winter travel like ski touring, snowmobiling, mountaineering and ice climbing takes place.

Missoula avalanche center faces existential threat as Forest Service cuts funding

As of the past week, public observations and information from the West Central Montana Avalanche Center indicate layers of crusts and loose, angular, faceted snow crystals exist near the top of the snowpack in many locations across the region. There was also widespread surface hoar — vertical, leaf-like ice formations atop the snow. Any of those things could become a dangerous weak layer within the snowpack once buried Friday night into Saturday, although wind could blow the surface hoar down as the storm rolls in. 

Mackenzie said his mindset when new snow accumulates is to consider what it’s falling on. In this case it’s potential weak layers. 

“If you come across 6 inches of more of new snow,” he said, “or you’re seeing signs that snow is being transported by the wind and developing slabs, then avalanches are possible.”