Cohesive slabs of snow are stacked on top of various old snow surfaces such as a crust, surface hoar, or lower-density snow. Slopes with around 8 inches of new snow are the most concerning and could produce an avalanche large enough to push you somewhere you don’t want to go. At upper elevations, triggering an avalanche on a heavily drifted slopes may provoke a deeper failure in weaker snow below a thin crust.
Snow totals range 8 to 10 inches around 6,000 feet. You may experience varying sensitivity of fresh slabs depending on what the new snow formed above. Continually assess new snow depths as you move up in elevation and make conservative terrain choices while the storm snow settles. If you see cracking in the snow surface or hear audible collapses, stick to slopes less than about 35 degrees.
Incremental snowfall is slowly adding more stress to weak snow layers and faceted crust in the upper snowpack. The hardness and thickness of the early December rain crust varies across our forecast zones and has been found above 6,500 feet. Where the crust is around 2 inches thick or more, weak grains below have been more stubborn to fail in snowpack tests. Where the crust is thinner and more breakable, those same faceted grains have failed with less force in tests. Avoid shallow areas near exposed rocks or below cliffs where triggering a shallow slide may provoke a deeper, more dangerous slide.
Snowfall over the last couple of days has favored the Selkirk and Cabinet Mountains. Accumulations are inching closer to a foot of new snow and making broader storm snow hazards the soup de jour. Be mindful of how steep a slope is regardless of which direction the slope faces. Fresh slabs are deep enough that even a slide confined to the storm snow could knock you over a take you for a ride. Monitor how conditions change as you move through the mountains and recognize that potential avalanche size increases alongside increasing new snow amounts.
In the Bitterroot Mountains and Silver Valley, snow totals range 4 to 6 inches and the new snow has increased avalanche conditions. However, you likely need a wind-drifted slope to find trouble. Leeward-facing areas below ridgelines and in cross-loaded terrain features are where hard slabs of snow lurk waiting for the extra push from a rider or machine before avalanching. Fresh Wind Slab avalanches are typically more reactive while forming, so avoid steeper terrain where you see the wind actively loading the slope below. Use small test slopes to gauge sensitivity of the new or drifted snow before venturing into larger openings at higher elevations.
On Saturday, snow safety teams in different mountain regions were able to trigger larger avalanches on heavily drifted slopes. Some broke deeper into weaker layers below previous rain crust. While explosive triggered slides may be associated with a bigger load, it does demonstrate the lingering sensitivity of persistent weak grains below. Finding the shallow part of the slope or where the overlying slab tapers could mimic a similar impact on weak snow layers buried below like placing a larger trigger in a fatter part of the slope where the slab above is either deeper or stiffer. Additionally, slopes with a hard slab or thicker crust above weaker snow, it may take several people descending the slope before someone finds the “not-so-sweet” spot.
