Bollettino valanghe

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Sawtooth Avalanche Center
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Pericolo valanghe

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

Although unlikely, you may be able to trigger large avalanches on shadier slopes that face away from the afternoon sun. As you climb in elevation to areas where the wind frequently forms drifts or hardens the snow surface, the chance of triggering slides increases. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2

    What and where is it? Dense, 2-3 foot thick slabs overlie multiple weak layers (sugary facets, surface hoar, and crust + facet combinations) on many slopes, including sunnier aspects. The weak layers are worst and remain suspect on slopes facing the northern half of the compass (NW-N-NE-E). 

    How can we recognize and manage this problem?  Determining whether a given steep slope is safe or waiting for a trigger—you—remains challenging. Due to the variable distribution and character of both the slabs and weak layers, stability can change quickly as you climb, descend, or move to slightly sunnier or shadier aspects. You may or may not observe snowpack cracking and collapsing, clear signs of unstable conditions, before triggering a slide. 

    Unfortunately, avoiding steep terrain where the problem exists is the only surefire way to take Persistent Slab problems off the table. You can greatly stack the odds in your favor by steering clear of steep, wind-affected slopes that face away from the afternoon sun (NW-N-NE-E aspects). Watch for dunes, waves, and textured surfaces to identify areas where the wind deposited snow. Wind-driven snow and slabs feel stiff. You may be able to trigger avalanches on slopes above, below, and to the sides of where you are riding or skiing. 

    Read the Forecast Discussion below for more details on this lingering problem.

    Additional Discussion - Wind Slabs: Watch for wind drifts and slabs in all exposed upper elevation and alpine terrain. Whether a weak layer exists or not, these smaller slabs may be sensitive to your weight. Feel for stiff snow overlying weaker or softer snow; this snowpack setup can feel or sound "hollow" or "drummy". Wind Slab slides can trigger the much larger Persistent Slab avalanches as they move downslope.

Avalanche Discussion

If we were hunting for avalanches, where would we go today?

  • NW-N-NE aspects above about 8300' in elevation: The weak layers are well-developed, and the snowpack continues to produce some snowpack collapsing and unstable snowpack test scores.  
  • Shady slopes lacking stout crusts in the upper foot of the snowpack: Last week's rain and hot weather formed crusts on some slopes up to about 9000' in elevation. Because of the spotty nature of the rain squalls, the distrubution of the near-surface crusts varies from drainage to drainage and even cirque to cirque. Shady slopes with thick crusts in the upper snowpack are not as scary as slopes lacking crusts. 
  • Shady slopes where the slab is closer to 18 inches thick than 3 feet: You're more likely to trigger slides on the buried facets and / or surface hoar where the slab is thinner. On slopes where the slab exceeds or approaches 3 feet thick, triggering slides is unlikely. That said, beware of windy areas where the slab thickness varies; if a slope is peppered with rocks, there's a good chance you could find spots where the slab thins, increasing the odds of triggering an avalanche. 
  • Avalanche paths where some part of the starting zone is over 35 degrees: We would be surprised if someone triggered a large avalanche where the entire slope is near 30 degrees in steepness. 

The good news: We can confidently identify the aspects, elevations, snowpack conditions, and slope angles where we're most likely to trigger large avalanches. 

The bad news:

  1. Accurately determining if a specific slope that checks all of the boxes above is stable or unstable is difficult at best: at the slope or avalanche starting zone scale, a fair amount of uncertainty exists.
  2. The best riding and skiing conditions are in the same terrain where the Persistent Slab avalanche problem is at it worst: shady aspects at middle and upper elevations that lack crusts near the snow surface. While stability is much better on slopes facing the southern half of the compass, expect to find less than stellar surface conditions. 

Regioni coperte