Bulletin d’avalanche

Seeley Lake

West Central Montana Avalanche Center
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Danger d’avalanche

Fri
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 6 months ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

New snow and wind are overloading weak layers in the snowpack. Stay off of slopes over 30 degrees where you find strong snow over weak snow, or see wind-deposited snow. The largest and most dangerous avalanches are most likely on northerly aspects.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    In the past few days, we have begun to receive reports of human-triggered persistent slab avalanches. In Seeley Lake, a snowmobiler triggered a large persistent slab avalanche on a north-facing slope (Click here for initial information).  While this avalanche problem exists on all aspects, avalanches will be the largest and easiest to trigger on northwest through northeast aspects.

    This is because the snowpack has two distinct weak layers. The one responsible for the large avalanche in Seeley is surface hoar buried about two feet deep, which exists on northerly aspects. The other weak layer is surface hoar buried more recently, and is about a foot deep. We’ve found this shallower layer on most aspects at middle and upper elevations.

    Dig down and look for weak layers before approaching any steep slope. Surface hoar will look like a thin grey line in the snowpack. In some places, the deeper weak layer may be facets rather than surface hoar. Facets look like weak sugary snow. If you find either of these weak layer types, avoid slopes over 30 degrees.

    You may not see red flags or warning signs before triggering a persistent slab avalanche. It is possible to trigger persistent slab avalanches remotely from flat areas, so give suspect slopes a wide berth. (Click here for a recent remotely triggered avalanche.)

    A snowpit on a North aspect at 6000 feet.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    We’ve had almost a foot of new snow this week, all of which is available for transport as winds pick up. While winds are strongest in valleys this morning, ridgetop wind speeds are forecast to increase later today.

    With two airmasses meeting over western Montana, today’s wind direction forecast contains uncertainty. Look for evidence of wind slabs on all aspects, and remember that wind direction can change based on terrain.

    Wind slabs are most likely to exist at ridgetops, roll-overs, and on the downwind sides of gulleys. You can recognize them by their pillowy shape and by looking for locally deeper and harder snow. Avoid steep slopes where you see these indicators. Cracking, collapsing, and recent avalanches are all signs to stay off of slopes over 30 degrees.

Avalanche Discussion

New snowfall means improved skiing and riding conditions, but it also means increased avalanche danger. As winter weather returns to the mountains, snowfall and wind will test recently buried weak layers.

These weak layers are spatially variable in character and sensitivity to triggers. This is particularly true for the deeper layer, which formed during the January drought. In sheltered places it is surface hoar, but elsewhere it is facets or a facet-crust combination.

Spatial variability means that conditions can change quickly, even on the same slope. Careful snowpack investigation is critical. Use the forecast as a starting point, but make your own ongoing assessments as you travel.

Often, surface hoar gets destroyed up high by the wind, but survives to get buried in sheltered areas. A common trap is to dig at ridgetop and not find surface hoar, but then trigger a surface hoar avalanche partway down your line. Don’t let yourself get caught unaware by this situation.

With persistent weak layers, remote triggering is always a possibility. Tracks are not necessarily indicators of stability. It could be the second or fifth rider that triggers an avalanche.

Even in locations without weak layers, the foot or so of new snow presents a hazard in itself. Look for “upside down” storm snow, where a harder layer exists above a softer layer. You will most likely find these conditions in wind-loaded areas.

Remember that the elevation bands are not concrete, and conditions don’t change abruptly exactly at the elevation cut-off. The Bitterroot in particular is a large forecast region with large-scale spatial variability. For example, middle elevations at Lolo Pass receive more snow than middle elevations in the rest of the range and may have avalanche conditions more comparable to the high elevation rating. The eastern side of the Bitterroot, where most of the skiing is, receives less snow than the Bitterroot crest to the west, and so the hazard where you're skiing may sometimes be lower than forecast.

We are still collecting data on the sensitivity and distribution of the surface hoar layer buried this past weekend. If you dig in this snow, please send us a picture of your pit wall, and let us know your elevation and aspect. If you see avalanches, cracking, or collapsing, please let us know. You can submit an observation here.

Storms throughout the western US have created dangerous conditions everywhere. Our condolences go out to all involved in the tragic accident in California. Click here for a preliminary accident report.

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