Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Mon
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 7 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Triggering an avalanche is most likely on upper elevation slopes with fresh drifts of windblown snow. Avoid slopes with signs of wind loading: pillowed drifts, dune-like formations, or rippled surfaces. Avalanches breaking on buried weak layers remain possible even in sheltered terrain—these could be triggered from gentle terrain adjacent to steep slopes. 

Problemi valanghivi

  • Persistent Slabs

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

    A triggered avalanche in the Salmon Headwaters (see photo) yesterday was a clear illustration that our buried weak layers need to remain at the front of our minds. The slide was remotely-triggered by a snowmobiler riding in lower-angled terrain 30 feet away. The slab thickness is consistent with the layer of surface hoar that's buried 1.5-2 feet deep throughout most of our area. The Banner Summit zone has a very similar snowpack to where this slide was triggered.

    Other observations yesterday pointed towards improving stability. So what gives? First, persistent weak layers—and surface hoar in particular—are fickle beasts. Second, conditions are becoming increasingly variable as we get further away from our last major loading event. Avoid steep slopes where you're most likely to trigger a buried weak layer:

    • Slopes with fresh drifting from the recent northerly winds.
    • Where there is denser or stiffer snow in the top 1-2 feet of the snowpack.
    • Areas where the snowpack remains deeper than about 1.5-2 feet—primarily on shadier slopes and at upper elevations.

  • Wind Slabs

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

    Yesterday, gusty north winds were strafing alpine terrain above about 10,000 feet, and a few small, natural wind slab avalanches occurred in these areas (see photo). Due to it's lower elevation, the Banner Summit zone was likely spared the worst of this wind, but you may still find sensitive wind slabs at upper elevations. Avoid slopes with the tell-tale signs of freshly wind-loaded slopes: pillowed drifts, dune-like formations, or a rippled surface texture.

Avalanche Discussion

It was a welcome relief to arrive at the office early this morning to find that numerous local guides and recreational sliders and riders had submitted observations. A huge thanks to all of you! The observations ranged from folks finding stable conditions to a group triggering an avalanche. My relief quickly turned to head-scratching. We've been talking about how our persistent weak layers—especially the layer of surface hoar buried 1-2 feet deep—are remarkably widespread and exist on most mid and upper elevation slopes. So why don't the observations paint a more consistent picture? The short answer is that while our weak layers are fairly uniform, the properties of the slab above them are not.

As more time passes since our bigger loading events in mid-December, slab conditions have begun to diverge. In areas with less than 1.5-2 feet of snow, like some solar aspects, lower elevations, and a good chunk of the Wood River Valley mountains, the entire snowpack is weakening and the slab is losing strength. On sheltered slopes with a—cue air quotes—deeper snowpack, the snow above the persistent layers has retained some strength and slab-like properties. And finally, we have wind-exposed slopes where past and current winds have left a stiff and dense slab.

It's these last two conditions where it remains possible to trigger a persistent slab avalanche. The more dense or stiff the surface snow, the more concerned I'd be. But don't write off sheltered slopes, a rider in the group that triggered the slide in the Salmon Headwaters yesterday said, "it just felt like powder."

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