Bulletin d’avalanche

Bitterroot

West Central Montana Avalanche Center
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Danger d’avalanche

Wed
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

The snowpack is unstable throughout the forecast area, and human-triggered avalanches are likely. Avalanches can be remotely triggered from above, below, and to the side. Choosing slopes less than 30º and staying well away from the avalanche paths and slopes can decrease your chance of being caught in an avalanche. Stay alert for signs of instability, such as whumpfing, cracking, and recent avalanches. 

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2.5

    The primary avalanche problem is persistent slab.  It is likely for skiers and riders to trigger an avalanche up to two feet in depth on buried surface hoar and facets. In some areas, such as the southern Bitterroot, there is potential for avalanches to step down to deeply buried depth hoar.

    As the new snow consolidates into a stiffer slab, conditions may become less touchy, but the size of avalanches will increase.  The problem will take time to heal as facets, and surface hoar are slow to bond, be patient.  

    Whumpfing, cracking, and recent avalanches are signs that the snowpack is unstable.

    Choosing slopes under 30º can help mitigate the problem. Avalanches can be remotely triggered from above, below, and the side, give avalanche slopes and paths a wide berth to avoid being caught. 

     

    (12/5/2022)

    Remotely triggered D1 persistent slab avalanche on a NE aspect at 7,400 ft.

    Photo: Arden Feldman

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2.5

    Wind slab is the secondary problem. However, it is intertwined with persistent slab. Wind slab that is triggered will slide on the facets and could run farther and wider than just wind slab alone.  Slabs triggered in wind slab will be denser and deeper than persistent slab alone, increasing the hazard.

    Moderate winds with strong gusts and abundant fresh snow have built wind slab on easterly slopes. Today's wind will add to the problem. Travel advice remains the same as persistent slab; avoid slopes over 30º. Stay well back from ridges and wind lips; they may be triggered remotely and break farther back than anticipated, catching travelers unawares. 

Avalanche Discussion

It’s been a snowy start to December. Up to 2” of snow water equivalent (SWE) has fallen since November 30th, adding close to two feet of new snow in some areas. In the last 24 hours, 2” to 4” of snow have fallen. Winds overnight were moderate with strong gusts out of the west, and temperatures were in the mid 20s above 6000’.

Today we expect an additional 2“-5” of new snow in the Rattlesnake and Seeley Zones and 0”-1” in the Bitterroot. Temperatures will be in the upper 20s, and winds to 25 mph out of the west.

The new snow has loaded a weak base. The lower half of the snowpack consists mainly of facets. In addition, at least one layer of surface hoar, formed mid-November, is reactive throughout the forecast area. The southern Bitterroot has a well-developed depth hoar layer at the base of the snowpack. Stability is poor.

Throughout the weekend, we received reports of natural and human-triggered avalanches, widespread whumping, shooting cracks, and easy failures in tests. Lolo Pass had multiple reports of touchy conditions. 

Yesterday, we found near-constant whumpfing and shooting cracks as we traveled in the Rattlesnake. Test results showed touchy conditions with very easy initiation and propagation in the Extended Column Test (ECT) and Propagation Saw Test (PST) on surface hoar above facets. A remotely triggered avalanche from ridge top corroborated test results.

In the Southern Bitterroot, widespread whumpfing was also experienced. Tests revealed easy failures on surface hoar above the facets. Over the weekend, snowmobilers triggered multiple small avalanches on road cuts indicating a likelihood of larger slides on larger slopes.

Given the similarities in snowpack throughout the forecast area, it is safe to say that conditions are touchy, and human-triggered avalanches are likely. The persistent slab problem will be slow to stabilize. Continued snowfall and wind transport will make the problem touchier in short term. The added depth will help the problem heal in the long term, but it will take time. Be patient. 

Shooting cracks, whumpfing, collapsing, recent avalanches, and wind loading are all red flags. These signs are overabundant currently, making it clear that we should be careful.  Reduce your likelihood of triggering an avalanche by sticking to slopes under 30º.  Slopes may be remotely triggered from above, below, and beside. Give avalanche paths a wide berth to avoid triggering a slide and being caught. There is great skiing and riding on low-angle slopes in fresh powder. 

To everyone who submitted observations over the past few days, a huge thanks. Observations are especially helpful this time of year as we are cataloging conditions. Please let us know if you are finding signs of instability in the backcountry.

Ski and ride safe

Régions couvertes