You can trigger slabs of new and drifted snow on steep terrain features. Use small test slopes to track how well new snowfall and wind drifts have bonded to the old surface. Cracking in thick, drifted snow means that slabs are unstable. You’re most likely to get into trouble below corniced ridgelines, on the sidewalls of gullies, and near convexities where more than 6 inches of new snow has accumulated. Avoid slopes with terrain traps below.
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Above 6500 ft.
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5000-6500 ft.
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3500-5000 ft.
3 to 6 inches of new snow and gusty winds have likely built small slabs on steep terrain features. They can be larger and break further below ridgelines, in the tops of chutes, and near convexities. Stiffer new snow can crack above the softer, lower density stuff below. Use hand pits and test slopes to track whether slabs are forming and how well they are bonded to the underlying surface. Cracking, especially on wind-loaded features, means you should avoid bigger, steeper terrain.
Persistent weak layers are buried under 1 to 4 feet of snow. The last report of an avalanche failing on one of these old, faceted crusts was on a southeast aspect around 7000 feet some time around Christmas. Feedback from these layers has been inconsistent. Snowpack tests sometimes propagate above and sometimes below the crusts 2+ feet below the surface, especially where the snowpack is thin near rocky outcrops and convex rolls. Those are the most suspect trigger points for dangerously large slabs. Plan to limit your exposure to that kind of terrain as new and drifted snow continues to add an additional load to the snowpack.
Observers in the Swan Range yesterday reported increasing snowfall and wind in the afternoon. In the past 24 hours, the Noisy Basin SNOTEL has been gut punched with 1.7 inches of new water. Snow fell at rates of 1 to 3 inches per hour overnight. The Mt. Aeneas weather station recorded periods of moderate to strong winds during the heaviest period of loading. We expect another half inch of water and more gusty winds this morning. So, very dangerous avalanche conditions have developed and slabs of new and drifted snow will only be larger and more sensitive to your weight as you gain elevation. All that new load is further burdening persistent weak layers buried 2+ feet deep in the snowpack. Avalanches that fail on faceted crusts can be very large and run down low in their tracks. Storm slab avalanches can be enough to trigger these deeper problem layers.
Elsewhere across the region, 2 to 5 inches of new snow have fallen and another 1 to 3 are expected today. While winds in the Whitefish and Flathead Ranges have gusted at snow-moving speeds, they've only averaged in the 5 to 10 mph. Storm slabs will be smaller where less new and drifted snow have accumulated. I expect that you can find trouble, if you’re look for it, below favored ridgelines and near breakovers. Using small test slopes on the same aspect and elevation as you want to ride are a good way to see how new slabs are bonding. Use caution and dial back your terrain choices if you see cracking in 6 inches or more of denser new snow.
The last persistent slab in the Flathead Range failed around Christmas. Above about 6000 feet, there is the off-chance of triggering a large slab that breaks deep in the snowpack. Facets around crusts buried earlier this month have been found on just about every aspect. Snowpack tests continue to show propagating failures where the snowpack is shallow. The most suspect terrain for persistent slab avalanches is on southerly and easterly slopes, near rocky outcrops, and where the snowpack is stretched over convexities. Reduce your exposure to that kind of terrain as more precipitation reloads buried weak layers.
Heavy snowfall overnight will increase again this morning, especially in the Swan Range and near the Montana/Idaho border. Showers will linger this afternoon and into Friday morning. Heavy mountain snow and strong winds are still on tap for the weekend.
| Thursday | Thursday Night | Friday | |
|---|---|---|---|
| Cloud Cover: | Mostly Cloudy | Mostly Cloudy | Mostly Cloudy |
| Temperatures: | 23 to 28 deg. F. | 21 to 24 deg. F. | 25 to 31 deg. F. |
| Wind Direction: | Southwest | Southwest | Southwest |
| Wind Speed: | 13 | 15G26 | 16G29 |
| Snowfall: | 1" to 2" in. | 0 to 1" in. | 0" in. |
| Snow Line: | 3000' | 3000' | 3500' |
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.
