Slabs of new and drifted snow can have harsh consequences if they drag you into rocks, trees, stumps, or gullies. Warming and rain can trigger avalanches when they affect cold dry snow. The hazards are most widespread at upper elevations, where there is enough snow covering hard, icy layers near the ground. Shooting cracks and collapses, or wet snow and roller balls are warning signs. Carry rescue gear and keep your partner in sight.
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Above 6500 ft.
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5000-6500 ft.
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3500-5000 ft.
Recent storms and winds have deposited slabs of new and drifted snow 1 to 2 feet thick near ridgelines, in gullies, and behind prominent terrain features. Stormy weather throughout the week will add more layers to the early season snowpack. Cracking in the surface snow or whumpfing collapses are signs that the slabs are sensitve to the weight of a rider or snowmachine, and that you can trigger slides on adjacent slopes. You can test for these slabs by punching your hand below the snow surface when you stop you snowmachine or by probing with a ski pole, being alert for hard-over-soft or dense-over-loose structures. Hidden obstacles can increase the consequences of getting caught in an early season slide, so look downslope for these obstacles before committing to steep terrain.
Sunshine, warming, and rain that thaws dry surface snow can create a loose wet avalanche hazard. Watch for soft, dry, snow transitioning to wet glop. Roller balls coming down from rocks and shallow areas are warning signs that wet avalanches are increasingly possible.
Since last Wednesday morning, 6-17" of new snow and 0.8-2" of SWE has accumulated at mountain weather stations. The series of storms looks to have favored the Whitefish and Flathead Ranges. The new snow is reported to be a little upside down (denser snow above lower-density snow). Some sites may also have picked up a thin ice crust near the surface, thanks to a brief freezing rain event Friday evening. Wind speeds were generally light to moderate during the snowfall, but have increased as the snowfall has tapered off. They are expected to increase as more winter weather comes in throughout the week.
After this combination of snowfall and wind, you can trigger slabs of new and drifted snow 1-2 feet thick on slopes steeper than about 35 degrees. These will break at density changes in the storm snow or where slabs of wind-stiffened snow are sitting on soft snow. Below about 6500 feet, there's few spots with older layers beneath the storm snow. Above that elevation, a very hard, icy layer of refrozen snow exists at the ground. This basal ice is, however, not continuously distributed below about 7500 feet, thanks to the warm winds and rain that preceded this week's snowfall. The top of that icy layer is also a potential failure plane for triggered slides, particularly if it harbors a thin layer of faceted snow.
The upshot is that assessing the hazard will be more straightforward at mid elevations, where drifts, dunes, and scoured patches can show where winds have created or removed slabs. At upper elevations, the spotty distribution of the basal melt-freeze layer makes things trickier. Because the snowpack is shallow, you can test for the presence of that layer by probing - ski pole or proble - before committing to steep slopes.
Freezing levels rise into the upper elevations on Tuesday. Spring-like avalanches will become possible where loose dry snow turns wet on steep slopes. Shallow, rocky areas on sunny aspecst will warm fist and most. Loose wet avalanches are an overhead hazard as you climb to higher elevations looking for deeper snow.
With more light snow forecast for tonight and into the weekend, expect these wet conditions to be brief. Freezing levels lower beginning Tuesday night with more moisture and wind.
FAC staff will continue to monitor conditions and post updates as conditions warrant. We anticipate daily forecasts and full operations to begin in December. Your observations would be very helpful in the meantime. Stay in touch using our handy Snowpack Tracker.
We continue to get weather stations back up and running for the season.
Now is the time to tune up your avalanche knowledge. The last session of The 2020 Northern Rockies Snow and Avalanche Workshop is tonight (Saturday). You can still register!and buy raffle tickets
Check out this winter’s avalanche safety course schedule and sign up for a class that fits your needs.
Recent low pressure veers north bringing clearing skies and very warm temperatures on Tuesday before another cold front drapes itself across the region bringing more snow and wind. The northern Rockies will be under westerly flow for the rest of the week, bringing a healthy dose of snow to the mountains.
| Tuesday | Tuesday Night | Wednesday | |
|---|---|---|---|
| Cloud Cover: | Partly Cloudy | Mostly Cloudy | Mostly Cloudy |
| Temperatures: | 36 to 48 deg. F. | 24 to 33 deg. F. | 28 to 41 deg. F. |
| Wind Direction: | South | Southwest | Southwest |
| Wind Speed: | 10 to 15, gusting to 30 | 10 to 15, gusting to 35 | 10 to 15, gusting to 30 |
| Snowfall: | o in. | 2 to 4" in. | 0" to 1" in. |
| Snow Line: | 7500' | 5500' | 4000' |
This forecast applies only to backcountry areas outside established ski area boundaries. The forecast describes general avalanche conditions and local variations always occur. This forecast expires at midnight on the posted day unless otherwise noted. The information in this forecast is provided by the USDA Forest Service who is solely responsible for its content.
