Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Tue
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Isolated wind drifts exist on upper elevation slopes, where triggering a 1-2 foot thick avalanche in steep terrain is possible today. These slabs will increase in size and sensitivity with today's additional snow and wind. And while unlikely, we can't rule out triggering an unsurvivable, 4-5 foot thick avalanche—especially on very steep, rocky, wind-affected slopes. 

Problemi valanghivi

  • Wind Slabs

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

    4-8" of light snow are expected to fall in this zone today. Light to moderate winds blowing out of the SW and then the NW will be able to transport this snow and built fresh slabs and drifts in exposed, upper elevation terrain. I expect them to remain isolated to ridgelines and exposed terrain features. While small, these slabs may be able to pick up steam as they travel downslope, thanks to the 10-12" of snow that fell here in the past week. Remember that even small slides can have high consequences if they carry you through rocky, treed terrain or push you down into a terrain trap like a gully or a creek bed.

    ADDITIONAL DISCUSSION:There are two weak layers in the snowpack have been unreactive recently but that continue to be worth paying attention to:

    1. A layer of surface hoar buried 2-3 feet deep. This layer has been unreactive in our recent snowpack tests and has not produced recent avalanches. 
    2. Depth hoar (large-grained weak snow) from December. This weak snow is buried beneath a very dense, 4-5'+ slab. You would have to be unlucky to trigger a slide on this, but its presence has continued to keep me out of the terrain where it is most likely. What type of terrain? Slopes that are very steep, rocky, and wind-affected. The most recent slide we've seen involving this snow failed on exactly such a slope, take a look at this photo.

    Chris found both of these layers in the Cape Horn area on Saturday—he describes them in this video.

    The ongoing snowfall has been accompanied by periods of moderate westerly winds, and you're likely to find fresh wind drifts and slabs along upper elevation ridgelines.

Avalanche Discussion

WEATHER FORECASTING Every morning I generate my own weather forecast before reading what the professional weather forecasters at the National Weather Service have to say. As avalanche forecasters, it is important to be proficient in our understanding of both the snow and the weather. After all, the snowpack is nothing more than a record of the weather written in snow and ice. However, there is a lot of uncertainty involved in both of these fields and our true expertise lies in the world of the snow. 

When weather models are in good agreement and the weather patterns are stable it is possible to have high confidence in one's weather forecast. However, when there is a lot of variability between models and we are dealing with shifting weather patterns it is much more challenging to be confident in a weather forecast. This morning, as I looked through numerous weather models and ran them through my own mental model I found myself dealing with a high level of uncertainty in my weather forecast. Different models were indicating different amounts and timing for snowfall and frontal passages. I turned to the weather professionals, and was greeted to this as the first three words of their Area Forecast Discussion: "Another difficult forecast..."  Well, at least I wasn't alone in my uncertainty!

When you are traveling in the backcountry today, think about the uncertainty involved in today's weather forecast. If you were only expecting to see a few inches of new snow and you are finding closer to a foot, think about how that difference may change the likelihood of encountering avalanche problems. If strong winds are blowing and slabs are rapidly building in front of your eyes remember that that observation is more important than the danger rating that you saw in the morning. The avalanche forecast is meant as a starting point for your backcountry travel but it should be a long way from the endpoint. Use the forecast to guide your understanding of the world that you are stepping out into but be prepared to update that model based on your real-world observations. 

Be sure to catch up on the happenings this past week in Ethan's latest Weekend Update.

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