Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
Vedi sulla mappa

Pericolo valanghe

Wed
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
Open full map
Non aggiornato — questo bollettino è scaduto 2 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Choose your destinations and play spots with an eye to avoiding today's trifecta of avalanche hazards. The most serious is triggering large, serious avalanches that break on weak snow buried 2 to 4 feet deep. Secondary hazards today include the potential for sluffing in the new snow, particularly if the sun comes out, and the possibility of triggering dense slabs of drifted snow. Careful attention to terrain will help you find a Goldilocks band of quality, low-risk riding.

Problemi valanghivi

  • Loose Dry

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    The most obvious hazard today will be the easiest to avoid. If the sun comes out this afternoon, expect sluffs of fresh snow to run on very steep slopes, triggered by snow falling off trees or rocks. Most will be harmless. The exceptions will be:

    • Slopes with more than 8 inches or so of recent snow that remains loose and unaffected by the wind. 
    • Chutes and other confined terrain where you can't escape once the surface snow starts moving.

    If you're in or below this terrain and you see rollerballs fanning downslope, move on. If you want to ride steep, confined slopes, get down and out from under them before the sun hits.

    (12/23/2023) The kind of terrain where triggered or natural dry loose avalanches could be dangerous today. Photo: David Kerner

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    The most serious hazard today is the one to account for when choosing destinations, routes, and places to play. The lowest risk choices don't require travel under steep, shady start zones or riding up or down convex slopes where snow depths are likely to be variable. The potential for triggering avalanches from below lingers, and thin spots in the snow cover are potential trigger points for releasing hard slabs that would be very difficult to escape.

    A thin layer of weak, faceted snow is buried 2 to 4 feet deep, a few inches above a stout ice layer and/ or melt-freeze crust. While this layer is subtle in snowpits, the avalanches releasing on it are anything but. Multiple large to very large avalanches failed on this layer during the end of February storm and avalanche cycle. Many occurred on very steep, leeward slopes below ridges, where wind-transported snow built thicker, harder slabs over the facets. This upper elevation terrain remains the place where you're most likely to trigger an avalanche on the faceted layer. 

    At mid elevations, we've had almost no reports of avalanches, whumpfing collapses, or snowpit tests that show instability in the past week. However, fatal avalanche accidents to the west and north of our region likely involved this layer and occurred near 6500 feet, the boundary between our mid and upper elevations. That tells me that it's important to minimize exposure to this hazard in terrain approaching that elevation, to leave a margin for safety. 

    So pick destinations that offer options to ride sunny slopes where the weak layer is less common, especially because the riding on these slopes will deteriorate with the sun and warming temperatures headed our way. In shady terrain, stick to lower angled slopes that are planar or concave in profile. Park or stop well out of the runouts of steep start zones, even on what look like small slopes. 

     

    A hard slab avalanche that released naturally on an upper elevation, leeward slope. It likely failed on the facets above the 2/3 ice/ crust. There were at least 16 tracks on this slope prior to storms at the end of February.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Today's third hazard lies in between the other two - apparent if you're looking, with the potential to bury or injure a person. The region has seen bouts of moderate to strong winds the past week that have built multiple generations of wind slabs. If winds pick up today as forecast, we may see another, as the low-density snow from Monday gets drifted.

    Patches of dense but soft snow near ridges will be the most sensitive to the weight of a person or snow machine. They may be covering older hard slabs of wind-packed snow that are harder for a person to trigger, but produce larger, more dangerous avalanches. These can step down and release very dangerous avalanches that break on the weak facets above the crust. 

    Look for dunes, drifts, and whale-shaped piles of hard snow below ridge crests and on the downwind sides of terrain features of rocks and gully walls. More subtle are drifts cross-loaded onto steep slopes, as shown by this unintentionally-triggered slide Monday east of Whitefish Mountain Resort. Hard, hollow-feeling snow isn't "bomber," it's a slab. 

    (3/4/2024)

    A rider triggered this soft slab east of WMR on Monday, 3/4. The slab involved snow cross-loaded onto the mid-elevation, east-facing slope by southerly winds. It's a good example of the hazard posed by the most recent generation of wind slabs.

    Photo: Flathead Avalanche Center

Avalanche Discussion

Problems, problems, problems. We've listed three avalanche hazards today. Recent obs describe a rain crust that extends to about 5000 feet and sometimes 5300 feet. Others, a sun crust on solar aspects, particularly southwest.

The Goldilocks band is above that rain crust, in shaded, wind-sheltered terrain that's not too steep and free of the lingering danger of triggering slides that break deep in the snowpack. Whew. It's half-caff semi-dry cappucino with oat milk. But being specific is the key to low risk, fun riding today. Check the problem descriptions for ideas how to maximize your your riding and safety today.

The general trend is decreasing danger, as weak layers slowly strengthen and adjust to the load delivered by this past weekend's storm. A ridge building in at the end of this week will interrupt that trend on sunny slopes. Clear skies and direct sun may produce a wet snow avalanche cycle on sunny slopes. Shaded slopes, however, will continue to stabilize so long as temperatures remain below freezing overnight. It's hard to say yet whether the ridge will bring a diurnal melt-freeze cycle strong enough to produce supportable crusts and corn on sunny slopes early in the day. Stay tuned. 

Regioni coperte