Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Fri
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

In the northern Bitterroot triggering persistent slab avalanche on surface hoar buried up to 12" deep is possible. Stay alert for signs of instability, such as whumpfing and shooting cracks. Avoid slopes over 35º and  convexities.

In the southern Bitterroot, isolated persistent slab could still be triggered on upper elevation north aspects.  Avoid slopes over 35º and areas of shallow snow. 

Problemi valanghivi

  • Persistent Slabs

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

    In the northern Bitterroot, surface hoar buried up to 12" deep under the new snow was propagating in stability tests on all aspects yesterday. The buried surface hoar we observed yesterday was lying flat. As a result, we were not seeing obvious signs of instability like shooting cracks, so it feels stable despite the lurking weak layer. The depth of the surface hoar will vary throughout the zone due to the localized nature of Monday's snowfall. The best way to manage it is to avoid slopes and convex rolls over 35º. 

    Isolated pockets of surface hoar exist throughout the central Bitterroot, and they may catch you by surprise. Despite a week of observations indicating stable conditions throughout the central Bitterroot, skiers triggered a D1 slide yesterday, mid-slope on a 37º rollover. 

    If you choose to ski steeper terrain, carefully assess the snowpack for a lower-density stripe of buried surface hoar. Use small, safe, and representative convex rolls to test the reactivity of the layer to skier travel. Avoid traveling in or above terrain traps. Whumpfing or cracking are obvious signs to stay in low angle terrain.

    (1/18/2023)

    Skier triggered release on 37 degree rollover, west aspect, 7,300', central Bitterroots

  • Deep Persistent Slabs

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

    I've transitioned the  basal  facets in the southern Bitterrrot to deep persistent slab to differentiate them from the buried surface hoar problem . The recent warm temperatures are helping basal facets bond. However, weak sugary facets can still be found below a cohesive slab in shallow north-facing terrain. It is unlikely to trigger this layer, but it could happen with just the right shallow trigger spot. Sledders high marking a slope may be able to trench down and trigger this layer. Avoid shallow areas in upper elevation north aspects, such as around wind-swept ridges.

  • Loose Dry

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

    A public observation yesterday noted sluffing on Murphy in the Rattlesnake that rapidly entrained more snow. Fresh snow in the Bitterroot is capable of similar loose dry avalanches. If you choose to enter very steep terrain, greater than about 40º, practice sluff management by skiing out of the sluff and allowing it to run. Plan your descent carefully and avoid terrain traps like tight chutes where loose snow can become funneled and powerful.

    (1/18/2023)

    The debris from a loose dry avalanche is visible on the apron below Mt. Murphy.

    Photo: Kevin Bissett

Avalanche Discussion

Since the last forecast, the northern Bitterroot has received up to 10" of fresh snow.  The central and southern Bitterroot have received little to no new snow. Temperatures have been mild but below freezing, and winds have been light to moderate out of the west. These are good conditions for the snowpack that will help it gain strength. Basal facets that have been plaguing us for over a month have been removed from the problems in the Swans and Rattlesnake. They are gaining strength in the southern Bitterroots, but there is still too much uncertainty to remove them yet.   New snow from Monday, up to 10" in the northern Bitterroot, is bonding well to old snow surfaces and melt-freeze crusts. However, a buried surface hoar layer is causing issues throughout the forecast area. This one will linger for a bit and may become more reactive as new snow consolidates into a cohesive slab. 

Crusts extending to 7000' have been covered by new snow, and the riding has improved considerably since earlier in the week. The possibility of avalanches at mid-elevations remains low in the Bitterroot and Rattlesnake. However, exceptions exist, and avalanche conditions could be found. Stay alert to changing snow conditions and red flags as you travel.

The lower elevation snowpack has shrunk considerably, and many early-season hazards are re-exposed. In the central Bitterroot, slopes are almost bare to 6000' on the south aspects and 5300' on the north. Today temperatures and cloud cover should keep snow surfaces frozen. However, wet loose may be possible if temperatures warm enough in areas with fresh low-elevation snow. These would be small but could push a skier into hazards. 

The high-quality observations that continue to be submitted greatly help the backcountry community and forecasters. Thank you for submitting them. You can submit further observations here.

Ski and ride safe.

 

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