Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Mon
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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In sintesi

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. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2

    Several weak layers are buried in the upper snowpack:

    • Within the upper foot of the snow surface you'll find weak snow that was buried by late January and early February storms. In most places, they lack the necessary slab on top of them to produce avalanches. However, in terrain that has been loaded by recent winds, the picture looks different. Here, you'll find a weak layer+slab combination that may be sensitive to your weight. This combination of slab+weak layer is most likely in upper elevation terrain, though wind-loaded slopes exist at middle elevations as well.
    • Buried 2-2.5 feet beneath the snow surface you will find the layer of surface hoar that produced a handful of human-triggered avalanches in January (photo at right). On slopes that face the sun, you'll find crusts with some small facets here. Observations and snowpack tests indicate that this layer is gaining strength and that triggering a slide involving it is becoming less likely.

    You are most likely to encounter a reactive slab in terrain steeper than about 35 degrees where the wind has been building slabs on top of these weak layers. 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. 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 on the surface hoar layer failed in terrain sheltered from the wind. Today, avalanches failing in sheltered terrain are less likely but remain an outside possibility.

    ADDITIONAL DISCUSSION: Small loose snow avalanches will become possible as the sun warms up the snow surface and melts existing crusts. Pay attention to the snow you are traveling through and look for roller balls and pinwheels as indicators that the surface snow is heating up. These slides start small but may pick up speed and snow quickly as they travel downslope. 

    (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

STAYING OUT OF TROUBLE

As we get further out from snowfall we are seeing the likelihood of triggering slides gradually decreasing. As a result, many of us find ourselves drawn to larger, more complex avalanche terrain. In this terrain, the result of being wrong in your stability assessment can carry high consequences. Sometimes the best way to avoid getting in trouble in the mountains is by knowing what might get you into trouble in the first place. There are two broad scenarios that you should be on the lookout for today in this zone, read on:

Slabs+Weak Layers: Several layers of weak snow exist in the upper 1-2.5 feet of the snowpack. 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 completely. Newer weak layers buried in the upper foot of the snowpack generally lack the overlying slab necessary to produce avalanches. However, in terrain where the wind has been at work transporting snow you may find slabs on top of these weak layers. Field observations and small triggered slides in neighboring zones indicate that these layers are capable of producing avalanches when loaded with the right slab.

Loose Snow: Temperatures in the mountains remained in the 20s F last night and are already slowly climbing in the pre-dawn hours this morning. Today will be mostly clear and winds will remain light. As the February sun heats up the snow surface, loose snow avalanches will become possible. We've seen a few rounds of these failing naturally in the past few days. Monitor the snow surface as the day heats up and be ready to move to a colder, shadier aspect as it becomes moist.

Enjoy the beautiful weather and the gradually improving stability, but remember that we aren't out of the proverbial woods yet. If you are stepping out into bigger, more complex avalanche terrain you'll be well served to  think about the consequences of being caught and carried in the terrain you've chosen.

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

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