Silver Valley & Bitterroot Mountains

The take home message today is that avalanche conditions involve a complex distribution of weak persistent layers buried under the abundant new snow from the past week. Trying to outsmart the avalanche problems is a game of roulette. You can have a fantastic day in the mountains today by sticking to slopes right around 30 degrees or less.

In the past week our region has received abundant mountain snowfall: Selkirks 23 to 46", West Cabs 32 to 46", East Cabs 25 to 32", Silver Valley Bitterroots 13 to 21" and Purcells 20-30". These impressive snow totals rest on a variety of layers that evolved during the January drought: deep layers of weak well-developed facets (1-1.5 mm), buried surface hoar (15 to 25 mm), sun crusts and wind boards, making for complex spatial distribution of avalanche hazards. To this we add strong winds today. Even though we are entering a period with little expected new snowfall, we need to be cautious and remain patient as the possibility of triggering a large, dangerous avalanche on slopes steeper than about 30 degrees remains.

Recent observations show reactive conditions – natural avalanches triggered by falling cornice, snowpack tests failing and/ or propagating during isolation and with easy force, and folks triggering avalanches with long-running cracks from a distance. Most of which is occurring at a well-preserved and upright layer of surface hoar with feathers ranging 15 to 25 mm in size in the north, or on a buried weak breakable crust in the south, which sit atop of a foot of large, faceted grains. Today we will experience moderate sustained winds out of the northeast, opposite our typical prevailing wind, with gusts into the 20 to 30mph range for most of our region. The Selkirks and parts of the Cabinet ranges will likely see higher wind speeds: sustained 20 to 30mph range, gusts to 40mph. With plenty of light low-density surface snow available for transport, expect wind slabs to build on atypical leeward sides of ridges and terrain features. These are to be taken seriously as even a small wind slab avalanche could step-down to deeper buried persistent weak layers to produce a large and destructive avalanche.

By their nature persistent weak layers stick around for a while. With how weak (mostly F to 4F-) these persistent grains are we will likely be stuck at an elevated danger level for some time. We may notice a subtle increase in sensitivity of the slab-weak layer combination as the snow settles under its own weight, or the stiffening of the slab may help mask the danger looming below. Sledders may be more likely to initiate an avalanche as they dig in and effect a stronger slab on persistent weak layers buried deeper in the snowpack then skier/riders who float higher in the snowpack. As you move through the mountains, carefully evaluate not only the terrain you’re traveling in, but how close you put yourself to steeper slopes. With the patchwork of old, wind-affected layers and crust, surface hoar and faceted snow now buried, avoiding slopes and convex rolls steeper than 30 degrees might be better than trying to outsmart the sleeping monsters. Triggering a large avalanche from below is a distinct possibility, so avoid travel above terrain traps like gullies or depressions with steeper hazards directly above – especially where you have no fast exit option.

The take home message today is that the avalanche problems are complex and not likely to go away anytime soon. The skiing and riding conditions are great out there, but trying to outsmart the avalanche problems is a game of roulette. You can have a fantastic day in the mountains today by sticking to slopes right around 30 degrees or less. The usual risk management strategies should not be relied on during times like this. The overall size and weakness of buried persistent grains is nerve-racking and somewhat atypical for what we usually deal with this time of year. Don’t rely on stability tests as a go or no-go decision, and heed early warning signs such as fresh avalanches, cracking in the snow, and loud audible collapses.

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**We continue to receive intermittent reports of opening glide cracks and some failing naturally producing large to very large glide avalanches. Avoid traveling on or below slopes where a glide crack has opened. These types of avalanches are difficult to forecast when they will release.**