Selkirk Mountains

Shallow, dense slabs of snow sitting on top of weaker snow or a firm crust may be susceptible to failure from added weight of a rider or machine. These slabs will be more sensitive directly below ridgelines and in steeper gullies where winds have drifted the recent snowfall into a thicker and stiffer slab. Cracking that extends beyond your skis or machine, along with a lens-shaped or textured snow surface, are clues these slabs are present and should steer you toward sheltered, lower-angled terrain.

During the extended dry spell in January, mid and lower snowpack layers became more well-bonded and are less of a bigger picture concern now. However, upper snowpack layers on the majority of slopes got weaker below a widespread crust, and developed a layer of surface hoar above. The sugary facets below range 8 to 12 inches deep and are mostly Fist to 4F hard. The surface hoar grew 6 to 10 mm in size and was generally widespread across all elevation bands up to almost 7,000 feet. One of the main avalanche ingredients lacking was a slab, and we got one…well, kind of.

Snowfall over the last 3 days ranges 4 to 6 inches in the Selkirk and Cabinet Mountains and 1 to 3 inches in the Bitterroot Mountains. Steady winds have likely added a little more depth and stiffness to fresh slabs on leeward slopes, but the denser storm snow and mild temperatures has helped limit the volume of drifting. This may change later Sunday and into Monday as winds are expected to increase into the 30s mph along with another round of snow and lower snow densities. This may enhance drifting along ridges and in more open, exposed terrain and nudge slabs closer to a more critical depth and reactive state. 

Recent observations report that the fresh slab may be a bit shallow to provide us with widespread signs of unstable snow, but the lack of instability could lure you into a spot you don’t want to be. Even with few signs of cracking, audible collapses, or fresh avalanches, snowpack tests suggest the new snow is willing to fail and propagate where the right slab properties exist. This is a function of how weak the snow is below. Achieving the right slab property may come from additional settlement making the slab stiffer, or more snow and drifting increasing slab depth.

In the Silver Valley area, less snow and more rain along with warmer temperatures recently has helped disturb the surface hoar layer around 6,000 feet and below making it less prevalent. Upper elevations and northerly-facing slopes are the best bets for it to be lingering below the very modest snowfall totals there. Regardless of whether the surface hoar is present or not, there is still weaker snow below the crust and should not be discounted if you travel on slopes with around 6 inches or more of wind-drifted snow.

No matter what zone or area of a zone you are recreating in, dig down to investigate what’s below you before stepping into steeper terrain. A thin and distinct grey line below the new snow is a likely indicator that a layer of surface hoar is present. If you find the sugary snow below the new snow easily falls into your snowpit, you also know you have a stiff over soft combination and another indicator of looming avalanche potential.

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