Lawinenlagebericht

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Sawtooth Avalanche Center
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Lawinengefahr

Tue
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
2 · Mässig
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Kernpunkte

Triggering large avalanches becomes likely as you climb into windy terrain. Natural avalanches are possible. Avoid steep slopes with stiff, wind-drifted snow at the surface. On slopes sheltered from the wind, triggering a large avalanche is less likely but still possible.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Likely
    Größe
    1.5–2

    What and where is it? Last night's snowfall and wind probably formed 1-3 foot thick slabs and drifts in upper and some middle elevation locations. Slabs will be largest, stiffer, and most widespread in alpine and exposed upper elevation terrain. The wind slabs overlie a variety of old surfaces: rock-hard wind hammered snow, hard crusts, and some weak layers (facets, surface hoar, and thin crust + facet combos). The freshly buried weak layers are most widespread the on the northern 2/3 of the compass (W-N-E aspects) as you drop below the most exposed, windy terrain. With the variety of strong and weak snow buried directly beneath the fresh slabs, some avalanches could behave unpredictably, like Persistent Slabs:

    • They could break wider than expected.
    • They could break above you.
    • You may be able to trigger slides remotely—from flat terrain below, above, and to the sides of steep slopes.

    How can we recognize and manage this problem? With a lot of uncertainty about how the snow buried by last night's snowfall will react to the fresh slabs above it (read the Forecast Discussion below), the safe play is to take a big step back and avoid all steep, wind-loaded slopes.The old snow surfaces 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. To spot the fresh slabs, watch for smooth pillows, dunes, and large wind features below ridgelines and cornices as well as on the sides of cross-loaded gullies and couloirs. If you feel the new 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: Around a foot of snow fell last night 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 light 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 (Wind or Storm Slabs) as they move downslope.

  • Storm Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2

    What and where is it? Last night's 8-12" of snowfall was probably dense enough to immediately form some slabs in terrain sheltered from the wind at lower and middle elevations. The Storm Slabs buried a variety of wet snow, crusts, weak facets, and large surface hoar. We expect the weak layers to behave worst on the northern 2/3 of the compass (W-N-E aspects); here, avalanches could break above you or be wider than expected. With the variety of wet, dry, strong, and weak snow buried directly beneath the fresh slabs, conditions are tricky. 

    How can we recognize and manage this problem? Watch for small cracks in the storm snow as you travel, a clue that the new snow formed a slab. Snowpack collapsing or "whumphing" noises are clear signs of unstable conditions. These avalanches are most likely on slopes steeper than about 35 degrees, and the chance of triggering them increases as the slope angle steepens. The old snow surfaces 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. 

Avalanche Discussion

In typically windy upper elevation terrain, the avalanche equation is fairly straightforward: more snow + more wind = more avalanches. The snow and wind-driven snow fell on stout crusts and wind-hardened surfaces on most slopes. Expect to trigger avalanches in steep wind-loaded terrain, and loose snow slides are possible.  

At the middle-upper elevation transition, you may find wind slabs, wind-stiffened snow, and storm slabs. The weak layers appear to be worst in this elevation band as well (see the next paragraph)—don't let you guard down if you're skiing and riding on sheltered slopes just below the "wind zone."

As you descend to middle and lower elevations, things get more complex. The storm snow was probably dense enough to form slabs in some places, and it 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 some slopes, weak facets and surface hoar 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. How will the freshly buried layers react to the new load? Sadly, we can't answer that question with precision today. We should be able to whittle away at the uncertainty in the coming days as patterns emerge.   

 

 

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