Avalanche forecast

Whitefish Range

Flathead Avalanche Center
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Avalanche danger

Thu
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Dangerous avalanche conditions exist. Careful terrain selection is key to enjoying the fresh snow with no nasty surprises. Stick to slopes less than 30 degrees that aren't connected to steeper slopes. Steer clear of fresh deposits of wind-drifted snow. Ride one at a time and keep your partners in sight.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    On shaded slopes, you can trigger soft slab avalanches that break 1 to 3 feet deep on thin weak layers that have developed above a hard, slick crust deposited in early March. You can trigger these slabs remotely and they can break in unexpected ways. Don't wait to be surprised. Pick routes that are not below steeper slopes. Ride lines that avoid convex rollovers and aren't above terrain traps. You may not get much warning before you trigger one of these slabs, though the whumpfing sound of a collapse, shooting cracks, and fresh avalanches are clear signs of this danger.. You can trigger these slabs well below ridgelines, on convex rolls, and in openings on otherwise forested slopes. 

    (3/15/2022)

    Remotely triggered soft slab. The rider also reported roughly 10 collapses over the course of the day. 

    Photo: Erich Peitzsch

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2.5

    Despite the rain in the valley, Tuesday's storm left plenty of dry snow to blow around up high. Expect stiff and even hard slabs of drifted snow on the lee sides of ridges, at the tops of chutes and couloirs, and downwind of saddles and other terrain that funnel winds. These slabs will feel dense and hollow compared to snow on sheltered slopes. Avoid steep slopes where you find these conditions, or terrain below steep slopes where you see blowing snow. Compounding the danger, moving debris can trigger slides that can step down and break on weak layers well below the surface. 

Avalanche Discussion

Tuesday's southwet flow - warm, wet air from the southwest - brought 6-10 inches of snow to mid elevations, 0.7 inches of rain to the valleys, and strong to extreme winds to upper elevations. The snow line rose during the day to about 5000 feet before the precipitation wound down near the Flathead Valley. Snowfall did continue overnight in the Swan and Flathead Ranges. That, coupled with strong westerly and southwesterly winds, makes for larger potential slides in those ranges and a higher danger there.

The people who did brave Tuesday's rain reported obvious signs of dangerous conditions. Most reports were from east of Whitefish Mountain; riders there reported small natural and triggered soft slabs, collapses, and more alarming, a remotely-triggered slab. All that is evidence of a Persistent Slab avalanche problem. That is, slabs that break on weak faceted layers buried well below the interface between new and old snow. Snow profiles and tests show multiple potential failure planes above a hard, slick crust deposited during the early March Atmospheric River event. These layers may be anywhere from 6 to 36 inches below the snow surface, depending on elevation, aspect, and mountain range.

However, regardless of where you ride, these layers are close enough to the surface that they can be collapsed by a rider or snowmachine. And in the past few seasons here, many close calls and accidents have involved near-surface faceted layers like these. They may fail on slopes that appear anchored by trees, or on lower angled slopes. You can trigger them from below or from adjacent slopes. They can connect across terrain features that might otherwise stop a fracture, like small ridges and stands of trees. Unless you're into high-stakes gambling, be diligent about choosing slopes less than 30 degrees that aren't below steeper slopes. 

Complicating matters are freshly-deposited slabs of drifted snow that remain unstable today. These are widespread on leeward and cross-loaded slopes at upper elevations; they're more isolated at mid elevations. Many upper elevation start zones and tracks were well-filled before this storm, so these slabs of drifted snow can produce large, long-running avalanches. 

The Persistent Slab and Wind Slab problems overlap in shaded terrain. Windward, sunny slopes likely lack the layers of faceted snow found on shaded slopes, and sun crusts formed at the surface during this weekend's sunny periods. Terrain like this where you find soft snow undisturbed by winds will offer the least risky riding today.

Regions covered