Avalanche forecast

Elkhorns

Wallowa Avalanche Center
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Avalanche danger

Sun
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
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Highlights

The incoming storm will build wind slabs as the day progresses. These wind slabs will be most prevalent on slopes facing North through East. Look for and avoid slopes that have fresh cornices, smooth firm snow or wind-textured snow.

This forecast is sponsored by: La Grande Sno-Drifters Snowmobile Club.  

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Moderate winds and steady snowfall can quickly build wind slabs. These slabs of wind-blown snow form on lee slopes. Today South and West winds will build these slabs predominantly on North and East facing slopes. Smooth hollow-sounding snow, textured snow, and shooting cracks are all signs of wind slabs.  If you see these signs avoid the terrain.

    These wind slabs will be forming on a variety of snow surfaces from the December drought. Slick crusts and weak sugary snow on the surface could make triggering one of these slabs easier.

    No avalanche danger rating has been applied to the below treeline band due to the lack of sufficient snow cover at that elevation.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Adding weight to the snowpack is a test for Persistent Weak Layers that exist in the snowpack. We don't know just what it will take to wake up these sleeping dragons. Your weight plus the weight of the new snow could be what it takes.

    Snow that fell in the late fall and early winter has turned into weak sugary snow. On top of this weak snow, there are several layers of more dense snow and some very dense crusts. Around some of these dense crusts, thin layers of weak snow have developed.  This snowpack structure creates a Persistent Slab avalanche problem. Persistent Slabs are just as they sound: slab avalanches with danger that persists due to the weak snowpack structure. These avalanches are difficult to predict and often give no warning signs. The best way to not get caught in an avalanche is to avoid the terrain they exist in.

    No avalanche danger rating has been applied to the below treeline band due to the lack of sufficient snow cover at that elevation.

     

Avalanche Discussion

Things will change quickly over the coming days. Significant snow and wind will impact the mountains.  It's good to consider the status of the current snow surface as we enter a period of unsettled weather.  After this prolonged drought, a plethora of snow surfaces exists:  On solar aspects, there is a melt-freeze crust, and on shady aspects- near surface facets (sugar snow) and surface hoar.  Above the treeline, the wind had stripped and scoured the snow surface in places.  All of these different surfaces will determine how well the new snow bonds, or how strong or weak the snowpack will be as we start to receive new snow.  All of this variety adds to uncertainty, and as the uncertainty increases so should our margins.  

What's going on with our snowpack? Warm temperatures since Christmas have consolidated the upper snowpack.  The warm temperatures and sunshine have melted more low-elevation snow and exposed more objects in the higher-elevation terrain. These changes to the surface snow have not affected the snowpack below. Facets and Depth Hoar are still present sandwiched between melt-freeze crusts and at the ground. You can see observations that showcase the Persistent Slab avalanche problem in the (1/4)Elkhorns,  (1/3) Northern Wallowas, and (12/29) Southern Wallowas. 

How will the incoming weather change the situation?  Anytime we add a new load to the snowpack, Mother Nature is allowing us to learn more about how strong or weak the structure of the snowpack truly is.  Think of adding new snow as weight lifting.  If we are doing the bench press and we slowly add weight it's like the snowpack receiving a few inches each day over a week.  Conversely, if we start slamming 45# plates on the bar, it won't take long before we are left gasping with the bar on our chest.  The snowpack often behaves the same, as a significant load in a short period tips the stress-to-strength ratio.  Weather forecasts are showing some decent accumulations this weekend and into next week.  We may find ourselves with a new buried weak layer (surface hoar) in some locations if the wind didn't blast it.  We'll keep you posted.

State of the snowpack: Small amounts of snow that fell during our prolonged autumn have rotted and weakened. On top of this several rain storms (12/4) or periods of above-freezing temperatures have created crusts. Snow depths range from nothing in the below treeline zone to 115cm/3ft+ in the above treeline terrain. While the upper portion of the snowpack feels sturdy, there are thin layers of weak snow around the melt-freeze and rain crusts and the base of the snowpack.  This snowpack structure creates a Persistent Slab avalanche problem.

The Persistent Slab avalanche problem is a tricky one. You can't see it by visually scanning the mountains. It isn't obvious on the surface of the snow. Other clues like cracking, collapsing, and whumpfing may not occur. Even if you dig down and conduct stability tests the results can be hard to interpret and inconclusive. This is the dreaded low likelihood, high consequence scenario. You might not trigger one of these avalanches or even know it is there, but if you do, it will break near the ground and take all the dense snow on top of it with it.

The best way to avoid getting caught in one of these slides is to avoid where they exist. That means anywhere snow has stuck around since fall, has more snow on top of it, and is steeper than 30 degrees. Given the shallow snowpack everywhere, colliding with obstacles would make taking a ride in an avalanche particularly nasty. Many rocks, trees, stumps, and down timber are on the surface or very shallowly buried.

 

If you get out to slide or ride please send us your observations! A picture or quick description of the conditions you encountered is SO helpful in creating these avalanche forecasts for you.

 

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