Silver Valley & Bitterroot Mountains

Strong winds from the west and southwest built sensitive wind slabs yesterday. Today be very cautious approaching slopes over 30 degrees and be particularly mindful of wind loaded start zones on the lee side of ridges and terrain features. If you see signs of instability, like shooting cracks and density changes, back off and head for low angle, non-consequential terrain.

After a taste of first spring, winter is now making a second appearance with the long-term forecast looking promising for more snowfall. Incremental loads of new snow continues to improve the skiing and riding conditions. However, with the new snow comes another breed of avalanche problems.

Sunday night delivered a few inches of lighter density snow, which landed on a variety of snow surfaces. Some solar aspects had developed a crust, while shaded and sheltered pockets of terrain held on to softer snow. Now, warmer temperatures are driving heavier and denser snowfall. This combination has created an inverted slab, which will continue to grow with the continuation of snowfall. Winds are forecasted to increase in the next 24 hours, which will create even larger and more sensitive slabs below ridgelines, and cross loading terrain features such as gullies and sides of steep rock bands. Stay vigilant in observing snow conditions and be ready to retreat to slopes that are less than 30 degrees if you feel drifts becoming larger or experience the telltale signs of slabby snow and cracks underfoot.

Although snow totals are much less impressive at the lower elevation band, temperatures are forecasted to be above freezing. Conditions are expected to be different at the trailhead vs. the peak and will have a noticeable change as you gain elevation. Look for clues on the surface as you work your way through the mountains. If you see large pinwheels gaining in size or gouging the surface at lower elevations, avoid traversing on or below steep slopes.

Looking deeper into the snowpack, we haven’t forgotten about the widely discussed January Drought Layer (JDL). After a long run of listing persistent slab as a problem, we are transitioning to focusing on new snow related avalanche problems. Here’s a quick breakdown of how we arrived here:

• The month of January consisted mostly of a drought period, where we experienced very little snowfall and a prolonged spell of cold temperatures. This combination “rotted” the snowpack, weakening the snow grains and creating a well-established persistent slab problem.
• The story changed in February, when the faucet turned back on, and the forecast area saw snowfall once again. The new snow successfully overloaded the weak JDL grains and observations reported a natural avalanche cycle.
• Towards the end of February, snowfall changed to rain and in the long term this helped stabilize the snowpack. That rain event helped develop a thick and impenetrable crust at mid and upper elevations. Lower elevations saw warmer temperatures and more rain, which penetrated deep into the snowpack and helped wash out most of the weaker JDL layer.

It has now been over 2 weeks since an avalanche has been observed failing on this previously problematic interface. Fast forward to now, and the JDL is buried over a meter deep with a hard slab over top - concealing the buried grains from being influenced by a skier or snowmobiler. The exception to this being specific areas on the slope where the snowpack is significantly shallower, such as around rocky cliff bands and wind scoured pieces of terrain. However, the JDL layer hasn’t completely gone to bed yet. Observations continue to report that even though the drought layer is getting further and further away from the surface, the insulating snowpack is preserving some of the weak grains. This means that if a drastic warm up occurs, this could come back in to play.

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