Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

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

Both the size and chance of triggering avalanches increase as you move up in elevation. To reduce your risk, you can avoid steep slopes with stiff, wind-drifted snow at the surface. On slopes sheltered from the wind, triggering a large avalanche is much less likely.

Problemi valanghivi

  • Wind Slabs

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

    What and where is it? Sunday night's storm formed 1-3 foot thick slabs and drifts in upper and some middle elevation locations. Slabs are largest, stiffer, and most widespread in alpine and exposed upper elevation terrain. The wind slabs overlie a variety of old surfaces:

    • Upper elevations: Mostly rock-hard wind hammered snow and hard crusts
    • Middle elevations: Weak layers (facets, crust + facet combos, and isolated surface hoar) on the northern half of the compass (NW-N-NE-E), and ice crusts on slopes that face the sun more directly (W-SW-S-SE) 

    How can we recognize and manage this problem? You can dramatically decrease the chances of finding trouble by avoiding steep, wind-loaded slopes. The character of the snow below the fresh slabs can change dramatically as you move a little bit up or down in elevation and to slightly sunnier or shadier aspects, increasing the difficulty in determining whether a given slope is safe or sketchy. Assume that weak layers exist beneath the slabs on NW-N-NE-E aspects at middle elevations and near the upper-middle elevation transition, increasing the chances of triggering a slide. To spot the fresh slabs, watch for smooth pillows, dunes, and large wind features below ridgelines, rollovers, and cornices as well as on the sides of cross-loaded gullies and couloirs. If you feel the snow getting stiffer, it's time to pay attention. Cracks shooting out from your skis or sled and snowpack collapsing are clear signs of unstable conditions. 

    Additional Discussion - Loose Snow avalanches: The recent storm deposited about a foot of snow above 8500'. On shady, wind-protected slopes facing NW-N-NE-E, loose snow avalanches could entrain faceted "recycled powder", creating a larger slide than expected. Periods of direct sunshine and calm wind could trigger natural loose snow avalanches on slopes on the southern half of the compass. Most of these slides will not be big enough to bury people, but they could knock you over and carry you where you don't want to go. Additionally, they could trigger larger slab avalanches as they move downslope.

Avalanche Discussion

Yesterday, we were able to trigger a small wind-loaded pocket near Galena Summit (see photo below and video above), we observed a small natural wind slab avalanche on a NE aspect in the same area, and we heard a couple avalanches release in the Owl Creek drainage. Thanks to several submitted Observations from you and local professionals (thank you!), we're able to provide some details on how the snowpack is reacting to the load from the recent storm:

Upper elevations: The avalanche equation is fairly straightforward: more snow + more wind = larger slabs and a good chance of triggering avalanches. The snow and wind-driven snow fell on stout crusts and wind-hardened surfaces on most slopes. Without obvious weak layers below the slabs, stability should be improving. However, assume you can still trigger slabs today in steep wind-loaded terrain, and loose snow slides are possible in more sheltered areas.  

Middle-upper elevation transition: You may find wind slabs and wind-stiffened snow. The slabs overlie widespread, pronounced weak layers (facets, crust + facet combos, and isolated surface hoar) on NW-N-NE-E aspects—don't let your guard down if you're skiing and riding on slopes just below the obvious "wind zone." The well-developed weak layers did not "heal" last night; slopes with wind slabs above weak layers remain suspect. You could also trigger loose snow sluffs in wind-sheltered terrain on these shadier aspects. On sunnier W-SW-S-SE-facing slopes, the new snow appears to be bonding well to the crusts below. 

Middle and lower elevations: The storm snow buried a hodgepodge of surfaces: wet snow, stout crusts, windboard, thin crusts, facets, surface hoar, and combinations of those layers (e.g. previously melted or wet surface hoar and facets). Before this storm, we had an unusual amount of variability over small distances; you could be skiing or riding soft, faceted "recycled powder" on a shady middle elevation slope and hit a hard "island" of wind-affected snow that did not become faceted during the extended drought. On slightly sunnier aspects (W or E), you found thin surface crusts with facets beneath. On many slopes, any weak facets and surface hoar that existed partially melted during the hot weather last week, forming razor crusts at the surface on even north facing terrain. That's a lot to digest. The good news: initial observations indicate the new snow did not form a significant slab in terrain protected from the wind and is bonding well to the crusts—for now, triggering large avalanches is unlikley on wind-protected middle and lower elevation slopes.    

 

 

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