Boletín de aludes

Banner Summit

Sawtooth Avalanche Center
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Peligro de aludes

Thu
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
2 · Moderado
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Desactualizado: este boletín caducó 6 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

You are most likely to trigger an avalanche in steep, exposed upper and middle elevation terrain where the wind has built slabs or formed drifts. You can dramatically reduce the chances of getting caught in an avalanche by playing on sheltered slopes with soft snow on the surface. 

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–2

    Expect to find wind slabs near ridgelines or lower on slopes in exposed terrain. Tuesday near Copper Mtn, professionals reported that small wind features were reactive to ski cuts, and they avoided very steep avalanche paths and all wind-loaded starting zones. Some thin slabs may have been deposited above temperature or rain crusts that formed Monday; this setup could make it easy to trigger small avalanches. While this area typically experiences lower wind speeds than our forecast zones to the south, the wind speed increase that was measured across the region Monday and Tuesday occurred here as well.  Many upper elevation slopes and some middle elevation terrain have received at least some wind loading in the past two days. 

    Watch for dunes, scallops, and textured snow surfaces. While these features may not produce shooting cracks like classic drifts and slabs, they are clear signs the wind has transported snow. Wind slabs should be most sensitive to triggering on slopes over about 35*. 

    ADDITIONAL DISCUSSION:A number of weak layers—both prominent and subtle—exist in the top 3' or so of the snowpack. The Idaho Transportation Department HWY21 avalanche program reported several natural, large slides that released below 7500' on W-NW aspects while the road was closed Monday night. Last week, a snowmobiler near Cape Horn Summit triggered two avalanches remotely (one is pictured at right/above), and a skier triggered a slab in very steep terrain. It hasn't stopped snowing and blowing since those events. In addition to the weak layers in the upper portion of the snowpack, there is weak snow near the ground (depth hoar) in this zone. You'd have to be truly unlucky to trigger a slide this deep, but this layer has kept us out of the terrain where it is most likely to exist—namely very steep, rocky, and wind-affected upper elevation terrain.

    You can greatly reduce your chances of triggering an avalanche of any variety by avoiding "prime-time" slopes over about 35 degrees, especially in terrain exposed to the wind. 

    (2/16/2021) A snowmobiler triggered this avalanche from the bottom of the slope last Tuesday afternoon near Cape Horn Summit.

Avalanche Discussion

PUCKER FACTOR: AN AVALANCHE FORECASTER'S "TIEBREAKER"

Assigning avalanche danger ratings is not an exact science. LOW and HIGH danger days are usually obvious, but choosing between MODERATE and CONSIDERABLE can be difficult for forecasters, especially when facing certain avalanche problems. The expected avalanche size and likelihood play a large role in determining the avalanche danger, but we do not have an algorithm that spits out the correct danger rating after plugging numbers into it. If only it were that simple! When we're "on the fence" between danger ratings, we weigh the danger ratings' travel advice heavily to choose a  level; how will we be traveling through terrain today? Will we consider entering starting zones? Or crossing avalanche tracks? Or riding sleds or skinning up in runout zones? 

I have my own personal "pucker factor" chart that basically describes how scared or comfortable I would be traveling in avalanche starting zones/tracks/runout zones when facing various avalanche problems. In our two central zones (Galena Summit & Eastern Mtns and Sawtooth & Western Smoky Mtns), the nature of the persistent slab problem still has me puckered. Recent natural avalanche activity - check. Recent wind loading - check. Poor snowpack structure - check. A high degree of complexity and uncertainty - check. For today, that's enough checked boxes to continue to keep me, the other SAC forecasters, and most local avalanche professionals out of all sizeable or consequential avalanche paths over 35 degrees and all wind-loaded slopes over 35 degrees. That's my tiebreaker between CONSIDERABLE and MODERATE. 

Are you likely to trigger large avalanches on all slopes today? No. Many slopes have stabilized to the point that triggering them is unlikely. The problem: with the dramatic effects of the recent wind event, I'd need a pack full of explosives to determine which slopes are relatively stable and which are teetering on the edge. Facing that degree of uncertainty, I'm too puckered to risk my life in steep, exposed terrain for now.  

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