Silver Valley & Bitterroot Mountains

Moisture working into the snowpack from rain and warm temperatures may penetrate into weaker layers and cause the snow to lose cohesion. The surface snow may not present with typical signs of instabilities like shooting cracks, so heed warning signs like snow easily sheeting under your skis or boards, and seek sheltered lower-angle slopes if you experience collapses. If heavy snowfall arrives sooner than forecast and snow totals exceed 6 inches, the danger could be closer to MODERATE (2of5) above 5,500 feet.

Snow totals range 2 to 5 inches in the Selkirk Mountains, 3 to 6 inches in the Cabinet Mountains, and 1 to 2 inches in the Purcell and Bitterroot Mountains. Snow densities were generally 16 to 20 percent. The Hidden Lake and Bear Mountain SNOTELs report 0.8 inches and 1.1 inches of SWE respectively. Most other SNOTELs report around half an inch of SWE.

The new snow and water amounts are not expected to increase the natural avalanche threat today but it does move the triggerability needle to a little higher potential. Not all the way to CONSIDERABLE (3of5) danger but definitely in the “spicey” MODERATE (2of5) realm. That is, a somewhat stubborn slab-weak layer problem broadly speaking, but one that carries big consequences because of the widespread distribution of the weaknesses below.

The tricky part about our current Persistent Slab avalanche problem is not the lack of weak layers in the upper snowpack. It’s the slab depth variable, which appears to be a primary driver of whether or not you could trigger a slide. Slab properties that produced propagating results in snowpack tests don't fit the same criteria. It's been a combination of slab depth and stiffness, and the flavor of persistent grain below (surface hoar versus facets below a crust) that need to line up to complete the avalanche recipe. If those variables align together and you travel on a steep enough slope, odds are you can trigger a dangerous avalanche.

Outside of propagating ECTs and sudden planar CTs, other signs of unstable snow like long shooting cracks or audible collapses waned during the most recent warm-up. This has only masked the danger below but by no means eliminated the hazard. Some had hoped that Saturday’s rain would be enough to wet or disturb the buried surface hoar, but rain totals appear too light to drive a pulse of water deep enough. Even slab settling over the last week did not effectively squash the surface hoar on top of the crust or compress the facets below the crust to be non-issue. They merely persist waiting for the extra nudge from the wrong spot.

Continue to travel with a healthy dose of suspicion on the majority of aspects at mid and upper elevations. We chose to pull the Persistent Slab avalanche problem from south and southwest aspects at middle elevations on February 8. Those aspects warmed and consolidated the most during the last warm spell and we anticipate the return to colder weather will help take the bite out of our snowpack concerns there. Those same aspects above 6,500 feet should be treated differently as the more recent weather may not have provided the same snowpack settling benefits as lower in elevation.

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IPAC is now on the Avy app! View, submit, and share observations on the go using the AvyApp. Your backcountry observations are critical for building our avalanche forecasts. Avy provides a centralized location for core trip planning, and you can view streamlined, up-to-date avalanche and weather forecasts for your selected zone, using offline caching. Download Avy today - App Store and Google Play.