Selkirk Mountains

New snowfall and increasing winds will form a denser slab on top of an array of different layers of weaker snow at mid and upper elevations. Avalanches triggered below ridgelines and in wind-drifted areas have the potential to step down and fail deeper within the snowpack, where weak snow rests on a firm crust. Expect the likelihood of triggering an avalanche to rise by the end of the day as wind speeds increase. Cracking that extends beyond your skis or boards is an indicator that you should reroute your travel plans to lower-angle terrain that avoids traveling on or below steep slopes over 30 degrees.

Happy first forecast day of the season! Since December 1st, most areas across the forecast region have picked up 9 to 13 inches of new snow. The snowy regime keeps rolling today, with another 3 to 6 inches expected by sundown. Longer-range models hint at a steady train of storms that will impact the area for the foreseeable future.

As the snowpack builds and evolves with each storm, new layers are forming while the old ones continue to get tested. We see a generally similar snowpack structure across the region. Weak snow layers of near surface facets and hidden surface hoar can be found in the upper snowpack.  The distribution of old snow is where our zones begin to diverge. Throughout the forecast area, the snowpack essentially falls into two categories: slopes with old snow from October through early November, and slopes that are starting fresh without any of that early season base.

The distribution of this old snow is the first place where the zones start to differ. The northern mountains—the Selkirks, East Cabinets, and West Cabinets—have old snow more broadly distributed across mid and upper elevations. These ranges received more snow early on, which helped them weather the subsequent rain and warm spells better than areas that started out shallower. North and east facing upper elevation slopes likely offer the most continuous old-snow coverage.

Tracking this early season snow is crucial because steep terrain that holds the old snow is where avalanches are most likely to break deepest and widest. The firm refrozen snow is creating a bed surface, while the weakened snow above is creating a failure plane, and the accumulating new snow is forming a growing slab. Put together, these create the ingredients needed for an avalanche.

The southern end of the forecast zone—the Silver Valley and Bitterroots—received less early season snow, and many south-facing slopes melted back to the ground. Recent observations show that old snow persists mainly on sheltered north and east aspects, primarily above tree line but dipping into near tree line terrain. While our forecast staff hasn’t yet reached the Purcells for observations, snow totals and weather data suggest that their old snow distribution is likely similar to what we’re seeing in the Bitterroots.

Increasing winds throughout the day will only heighten the potential for triggering avalanches. With low-density snow already on the surface and more snow falling today, slabs will continue to grow and spread across the terrain. Expect the most sensitive slabs just downwind of ridges and in or near gullies. Triggering a slab avalanche has the potential to step down deeper into the snowpack, producing a larger and more dangerous slide.

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