Silver Valley & Bitterroot Mountains

Continued light snowfall and steady southwest winds have built thin wind-drifted slabs on slopes facing northwest through north to southeast. Deeper, more reactive drifts likely lurk below ridgelines and in cross-loaded gullies, and some may grow large enough to knock you over or push you around. These same slabs are also adding more load to weaker snow around a crust on similar aspects at mid and upper elevations. Back of slopes around 35 degrees if you encounter signs of instability and stick to wind-sheltered slopes for safer travel options.

Incremental snowfall can sometimes be exactly what the snowpack doctor ordered, but it can also sneak up on us. One scenario continues to refresh the snow surface and improve riding conditions, while the other slowly inches slabs thicker to the point of failure. There’s an old saying in the snow and avalanche game, never underestimate the ol’ angry inch. Either way, the current drib and drab pattern of snowfall combined with pesky winds has created fresh surface slabs that warrant our attention and evaluation to help avoid a potentially bigger, higher consequence avalanche event.

In the last couple of days, the Selkirk and Cabinet Mountains have received 5 to 9 inches, while the Purcell and Bitterroot Mountains land closer to the 2-to-6-inch mark. Steady winds have efficiently built deeper slabs at higher elevations, and they may be broadening their existence to near 6,000 feet along exposed ridges and in broad openings. While the growing wind-drifted hazard is likely confined to areas below ridges and in cross-loaded gullies, snow safety teams report drifting in unusual places from stronger, erratic gusts. Pay attention to how conditions change below your feet or machine as you move through the mountains and recognize potential avalanche size increases alongside increasing slab depth.

Small surface hazards are often managed with terrain-specific evaluations, but these newly formed slabs overlap on similar slopes and elevations as our more concerning Persistent Slab avalanche hazard. The stormy period on December 17 and 18 delivered various precipitation flavors and added another thin rain crust before low-density snow fell above. In the aftermath, the forecast staff observed natural avalanches that likely failed in deeper faceted grains around the December 7 rain crust. These were not widespread throughout our forecast zones, but did provide a glimpse of our slab, weak layer behavior. Most slides failed on heavily drifted slopes around 6,000 feet or higher, broke wide across adjacent terrain features, and mostly pushed size 2.

Since the storm ended, we have no reports of rider or machine-triggered slides. The lack of a big, rapid snow load is helping the upper snowpack find a bit of equilibrium – the overlying slab is also settling into a stiffer slab. Regardless, we still have a hard over soft snowpack combination and we need to ask ourselves why did some steeper slopes not slide when others did? Uncertainty during a low frequency, high consequence regime warrants heightened snowpack evaluations, minimizing group exposure, and simply avoiding places where a shallow slide could provoke a deeper failure. At the end of the day – when stability is the question, terrain is always the number one answer.

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Avalanche classes are filling up fast! IPAC has a few openings left in a couple of our classes. With the snowpack becoming more complex, it's the perfect time to give a loved one the holiday gift that shows how much you care! Learn more here.