Lawinenlagebericht

Banner Summit

Sawtooth Avalanche Center
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Lawinengefahr

Tue
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 6 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Snowfall and wind this weekend built wind slabs in exposed, upper elevation terrain. These slabs may be sensitive to your weight today, particularly where they rest on weak surfaces such as facets and crusts. Look for signs of sensitive slabs, such as cracks shooting from your skis or board. Bright sunshine between periods of clouds will melt the snow surface and increase the hazard of loose snow avalanches. Pay attention to the snow surface as the day progresses. If it is getting wet, be alert to the increasing likelihood of these slides, particularly in very steep, rocky terrain. 

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1

    The weekend's storm brought 2-4 inches of snow to this zone. During the storm, winds blew from a variety of directions and built fresh slabs in exposed terrain. The new snow (and the fresh slabs) were deposited on a variety of surfaces that formed during last week's clear weather. Over the weekend, skiers on Galena Summit found that the recent wind drifts were reactive to their weight. The wind slabs you encounter today are most likely to be reactive where they were deposited on weak, faceted snow. This slab+weak layer combination is most likely on slopes on the northern half of the compass, particularly on the eastern and western margins, where these facets are accompanied by a fragile crust. 

    While the slabs you encounter today are unlikely to be large, they may produce slides large enough to knock you off your feet and send you downslope. Look for evidence of wind drifting such as dune-like drifts and ripples on the snow surface. Cracks shooting out from your skis or board are a sure sign that you've found a sensitive wind drift. 

    ADDITIONAL DISCUSSIONDirect sunshine on fresh snow is a recipe for loose snow avalanches. Variable cloud cover and winds blowing out of the SE will both help to keep the snow surface cool, but the intense March sunshine will quickly melt the snow when it breaks free from the clouds. If you are seeing prolonged periods of sunshine and the snow that you are traveling in is getting wet and sticky, these types of avalanches won't be far behind. These slides won't be large, but will pick up speed quickly as they travel down the firm surfaces underneath. You're most likely to encounter this problem on very steep, rocky slopes that directly face the sun. 

Avalanche Discussion

Let's think about spatial variability this morning. The storm this weekend and the avalanche activity that followed provides a great lens to look at the way things change over a distance:

  1. For the better part of the season, snowfall has heavily favored our northern zones. If you live and ski in the Wood River Valley, I don't have to remind you of this. That pattern was flipped this weekend, when a stream of moisture out of the south impacted our area, a pattern that we haven't seen since Christmas. The Sweede Peak SNOTEL recorded 15" of snow and 1.3" of SWE while Banner Summit and the Sawtooths picked up just a few inches of snow. 
  2. In the mountains above the Wood River Valley and the Little Wood drainage, the character of the snowpack is primarily a function of aspect. On slopes that face the sun, the snow has been slowly melting away over the past months, leaving nothing but dirt and sagebrush. On shaded slopes, the little snow on the ground has faceted significantly, resulting in a very thin and weak snowpack. Don't believe me? Drive north from Hailey to Ketchum and make a note of how much bare ground you see (lots). Then, turn around and drive back south and look at how much more snow-covered ground you can see.

Now, try to overlay these two patterns in your head. The areas where the most snow fell and the snowpack was weak are the areas where I observed large avalanches yesterday (photo). Keep these patterns in mind as the weather continues to bring snow to our southern mountains in the coming days and weeks. Unfortunately, this type of avalanche problem won't go away quickly. 

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