Silver Valley & Bitterroot Mountains

Strong radiational cooling overnight is allowing for a firm melt freeze crust to form on the surface. As the sun heats up southerly aspects during the day and breaks down the crust, plan your route to favor more shaded aspects with a supportable surface in the afternoon. If you find yourself sinking past your boot top in wet snow, limit exposure to steep terrain and overhead hazards.

On Thursday, February 5th, many of our weather stations reported some of the warmest temperatures we’ve seen so far this season, with most areas across the mountains reaching the upper 40s F and low 50s F. If you were out and about, it probably felt like more of a late spring day as opposed to what we would expect (or hope for) in February.

Today will feature another sunny day with warm temperatures, so long as you climb above the inversion and thick valley fog. Temperatures in the mountains will be slightly cooler than yesterday, but still well above freezing. Clear skies at night have promoted some strong radiational freezing, which is why you might be surprised to find a firm and refrozen surface despite sustained above freezing temperatures at night.

Despite the warm spring weather, the snowpack remains complex, with lurking worrisome weak layers. This is most prevalent and widespread in the northern mountains, and for the next paragraph we will focus on conditions that exist in the Selkirk and West Cabinet Mountains. Down south in the Silver Valley-Bitterroots, things have evolved slightly differently.

Do not be fooled by the lack of recent avalanche activity or potential lack of feedback from surface instability clues. Recent snowpack tests have produced continuous propagating results on the buried surface hoar and facet layers that developed widespread and are now buried about 6 to 12 inches below the surface. These test results really speak to the fragile nature of these weak layers. Natural avalanche activity has been on a downward trend, but human triggering potential remains heightened. This means we land at the “spicy” end of the moderate rating- essentially the upper end of the danger rating. Due to the nature of the weak grains, avalanches could fail from remote trigger or propagate widely across a slope and connect terrain features. The sun working to soften the surface crust will increase the possibility of a skier or rider having an influence on the buried weak layers and triggering an avalanche.

Now in the Silver Valley Bitterroots, we are still finding remnants of the buried surface hoar and facets, but the slab depth above the weak layers is significantly smaller (average 2”-4”) than that in the northern mountains. Because of this, the warm temperatures have had a greater influence on the weak snow in the southern end of the forecast area and is helping to destroy the continuity in those layers. Open areas at higher elevations on north facing aspects with a softer surface are the most likely places to find these weaker layers still intact.

No matter what zone you are in, these weak layers are close enough to the surface that taking a moment to dig down and investigate the snowpack structure doesn’t take a lot of time. If you can identify a distinct grey line, then you are probably looking at a layer of buried surface hoar with weak facets below it, signaling that the persistent slab problem has a good chance of being present. This can offer a quick and helpful snapshot of what’s happening directly below you but should be an ongoing process as you move through the mountains before committing to a slope.  

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