Boletín de aludes

Banner Summit

Sawtooth Avalanche Center
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Peligro de aludes

Sun
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 4 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Human-triggered avalanches involving layers of weak snow buried 1.5-2.5 feet down are possible. You can significantly reduce your risk by steering clear of recently wind-loaded slopes steeper than about 35 degrees. Small loose snow avalanches will become possible in the heat of the day on slopes that face the sun. 

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1.5–2

    Several weak layers are buried in the upper 1.5-2.5 feet of the snowpack. On slopes that face away from the sun, you'll find some surface hoar and facets at these interfaces. On slopes that face the sun, you'll find crusts with some small facets here. These weak layers have had some time to get comfortable with the new load that was added last week and the chances of triggering a slide on them is gradually decreasing but hasn't disappeared completely.

    You are most likely to encounter a sensitive slab in terrain steeper than about 35 degrees, particularly where the wind has been building slabs on top of these weak layers.  Avalanches failing on persistent weak layers can break well above you on the slope and may break wider than you expect. Many of the avalanches that were triggered in mid-January on the prominent layer of surface hoar failed in terrain sheltered from the wind (photo at right/above). Today, avalanches failing in sheltered terrain are less likely but remain a possibility.

    ADDITIONAL DISCUSSION: If you head into exposed upper elevation terrain you may find a batch of small, recent wind slabs (photo here). Look for these along ridgelines and near mid-slope terrain features like ribs and gullies. Cracks shooting out from your machine or skis are clear indicators of an unstable slab. Small slabs that knock you off your feet and send you downslope can have high consequences in the wrong terrain. Smaller triggered wind slabs can be effective triggers for the buried weak layers described above.

    (1/23/2023) Persistent slab avalanche apparently triggered by a skier on Banner Summit. Slab depth and terrain characteristics are consistent with early January surface hoar layer. 7250', NNW. The date of this avalanche is uncertain, but likely occurred since Jan 18th. Photo taken Jan 23rd.

Avalanche Discussion

Snowpack depths in most portions of this zone exceed 6 feet. Weak snow that developed early in the season has become deeply buried and has had time to gain strength. Our focus has shifted to the upper few feet of the snowpack, where several layers of weak snow buried in the past month exist. Read on.

A prominent layer of surface hoar produced a handful of human-triggered avalanches in shaded, sheltered terrain in late January. Snowpack tests and a lack of avalanche activity indicate that this layer is gaining strength, but we still haven't written it off. On slopes that face the sun you'll find crusts with some small facets at this interface. This combination produced unstable snowpack test scores in January but has not produced any avalanches that we've observed.

Closer to the surface lies another weak layer. Storms in February favored this area and this weak interface is now buried roughly 1 foot below the surface. You'll likely need the added weight and stiffening from wind-loading in order to have troubles involving this newer weak layer. Observations from other zones, including a few small, triggered slabs, indicate that this layer is capable of producing slides and may stick around for a bit.

Much of the snow at the surface has been re-worked by winds and cooked by the sun this week. Today will bring a mix of clouds and sun, along with temperatures starting out significantly warmer than in the past few days. If you're playing in steep, rocky, sun-soaked terrain during the heat of the day, you may be able to trigger some small loose snow avalanches. These start at a point and pick up snow and speed as they travel downslope. If you're feeling the snow get heavier and wetter, triggering these types of slides will be possible.

For more info on what happened last week check out Scott's latest Weekend Update.

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