Selkirk Mountains

Assess steep freshly wind-drifted slopes before committing where a small reactive slab may be lurking near ridgelines, on rollovers, and in gullies. Strong westerly winds recently may have formed slabs further down the slope than you expect and in unusual places. Seek out sheltered terrain where the snow hasn’t been stiffened by the wind if your snowpack assessments of drifted slab sensitivity are inconclusive.

**On January 6, a very-large (D3) glide avalanche released in St Regis Basin in the Bitterroot Mountains. Additionally, the forecast staff has observed several other glide cracks in the Silver Valley and areas north that warrant attention. Avoid traveling on or below slopes where a glide crack has opened up. These types of avalanches are difficult to forecast when they will release.**

The last storm came and went with more blow than snow – at least for most of our forecast zones. The Bitterroot Mountains were closer to the exact track of the low-pressure system and main vorticity, and therefore ended up with slighter higher snowfall totals. Across the north, lesser snowfall means concerning Wind Slab avalanche hazards are likely smaller and easier to manage or avoid. In the St Joe River drainage just south of our Bitterroot Mountains-Silver Valley forecast zone, recent snowfall is around a foot deep and drifting hazards may be more widespread or problematic and linger in sensitivity longer.

The wind blew from various directions, but mostly along the trajectory from southwest to west to northwest. Expect slab formation to extend their reach and possibly exist lower on the slope than you usually find them. Broad openings on the lee sides of terrain are more likely to harbor drifts deep enough to warrant a second snowpack opinion before diving in. Whereas areas closer to the treeline where thicker vegetation reduces snow transport, those same slabs may be more isolated and shallower.

The upper snowpack consists of a variety of layers like a thin rime crust, sun crust, or a mishmash of differing hardness layers from individual storm moments. None of these interfaces has readily produced concerning results in snowpack tests recently, but adding a freshly drifted slab on top could make conditions slightly different from the last time you were out. If gauging consequence of triggering a slide, the potential size of avalanche will likely hinge on how thick the drifted slab is. While deeper drifts could produce a slightly larger slide, we do not expect slides to break deeper where the mid and lower snowpack is generally strong and well-bonded.

The lack of recent natural avalanches and rider-triggered slides failing in one of several faceted rain crusts two to three feet deep, we decided to remove the Persistent Slab avalanche problem. Snowpack tests have mostly stopped producing failures in these deeper layers, but isolated locations still produce propagating results - albeit with hard force or many loading steps. The grains immediately above and below both rain crust are showing signs of rounding, and those interfaces are getting squashed by the overburden weight of the slab above them.

During periods of waning persistent weak layer concerns, solid terrain management is crucial to offset uncertainty. With limited recent field observations above 7,000 feet, be aware that traveling to higher elevations could push you into terrain where faceted grains are slightly more preserved. Avoid steep, rocky areas with numerous trigger spots, and choose slopes free of vegetated runouts or terrain traps to minimize your risk around the "what-if" scenario. Glide avalanches are unlikely to be triggered by a person, but natural failures are very challenging to predict. Open glide cracks are a significant indicator, especially those that have opened recently, that those slopes may be imminent to avalanche.