Lawinenlagebericht

Northwest Mountains

Crested Butte Avalanche Center
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Lawinengefahr

Thu
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Kernpunkte

The softest snow and safest riding is in wind-sheltered terrain, where your primary concerns are shallow sluffing and the isolated threat of a soft slab avalanche breaking just over a foot deep on buried surface hoar in very steep terrain.   The bitter north wind event left us with hard wind slabs scattered across leeward terrain features, mostly at higher elevations.  Although wind slabs are generally difficult to trigger, they could break above you unexpectedly: consider terrain consequences before crossing a recent drift.   

Lawinenprobleme

  • Wind Slabs

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

    Gusty northerly winds over the past two days formed dense slabs of varying thickness on leeward terrain features.  Although slabs are now stubborn to triggers, they are stiff enough to break above you if you trigger one.  Identify the problem by looking for smooth, hard deposits below ridges, in crossloaded gullies, or below rollovers.   Evaluate the slab size and consequences of getting carried before crossing a wind drift.  You can avoid the problem by traveling in wind-sheltered or wind-eroded terrain, where the snow is softer.

    12/25:  A pair of skier triggered wind slabs on the south side of Ruby Peak. 

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Unlikely
    Größe
    1–2.5

    In wind protected terrain on the northern half of the compass, there is isolated potential to trigger a shallow soft slab on a layer of buried surface hoar up to 20" deep.  Look for cracking in the snow and be wary of unsupported rollovers in high consequence terrain.  After below-average snowfall for the last three weeks, depth hoar layers near the ground are generally dormant on most slopes, with the potential for isolated exceptions on recently wind drifted slopes on westerly, northerly, or easterly facing terrain. You can reduce your risk by traveling on south-facing slopes that have thick crusts, or by traveling on smaller terrain features without terrain traps.  You can avoid the problem on slopes consistently less than 30 degrees. 

    This large persistent slab was triggered by skiers descending the ridgeline on the Aspen side of the Elk Mountains on 12/17.  

Avalanche Discussion

The wind event on Monday and Tuesday spurred numerous natural and triggered wind slabs on east to south aspects at higher elevations, mostly on Monday.  So far we've only observed D1 to D1.5 slides, but we expect potential for a few D2s in heavily drifted terrain with larger fetches, especially in the Northwest Mountains which picked up the most snow out of the weekend storm.  Even though most slabs aren't particularly large, they can be difficult to manage because they are often hard and won't break reliably at your ski tips.  Most slabs will be quite stubborn and difficult to trigger today, although in a few areas, wind slabs formed over weak facets and may still pop on you as you cross from the thick to thin part of the slab.  In contrast to these thick, hard deposits that pose a lingering hazard, look for softer snow or sastrugi for safer and higher quality riding today.  

There are several weak layers in the snowpack that we are monitoring for the potential for larger releases deeper in the snowpack.  We have a layer of surface hoar (the 12/7 interface) that is now buried by over a foot of snow in the Northwest Mountains, and less in the Southeast Mountains.  With small and gradual loading events since burial, signs of instability and avalanche activity have been both small and isolated (Ex: Axtell, Schuykill Ridge), but could surprise someone on a very steep or unsupported slope in wind-protected terrain on the north half of the compass.   Look for cracking in the snow or do a quick test to see if it is reactive where you are. We're highlighting the potential for this problem more in the Northwest Mountains, while the Southeast Mountains typically lack a sufficient slab in wind-protected terrain. Exceptions may exist along the zone borders.  Our deeper weak layers (primarily the 11/16 depth hoar layer) have been inactive since December 17th, due to a combination of lack of significant loading, extensive previous activity, wind erosion of slabs, and faceting of slabs.  We remain wary of isolated exceptions, particularly in wind-drifted terrain at higher elevations where there's more variability and more uncertainty when assessing the problem.  The recent wind event has further eroded slabs from some suspect aspects (generally northerly) and cross-loaded other potentially worrisome aspects (westerly and easterly).  

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