Avalanche forecast

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Avalanche danger

Tue
Alpine
1 · Low
Treeline
1 · Low
Below
1 · Low
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Highlights

Large avalanches are unlikely, but there is still a slight chance of triggering a large slide on very steep, rocky, wind-affected slopes. As the day warms up, the threat of wet snow avalanches will increase on very steep slopes that face the sun. Be alert to changing snow conditions and seek out colder snow or low angle slopes as the surface snow gets wet.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2–3

    Layers of weak snow in the snowpack have gone into a state of dormancy—they are still present, but the likelihood of triggering an avalanche involving these layers has decreased significantly. Of course, unlikely is different from impossible. The most problematic of these layers is the weak snow (depth hoar) that is buried near the ground. This layer has produced countless avalanches this season and has behaved in ways that surprise snow professionals and experienced backcountry users alike. You are most likely to trigger a slide involving this snow in very steep, rocky, wind-affected terrain. 

    ADDITIONAL DISCUSSION:Buried weak layers, warm temperatures, and weak overnight freezes are the recipe for generating wet slab avalanches. These types of slides are both notoriously unpredictable and highly destructive. As melt-water works its way through the snowpack it can weaken the snow, particularly in areas with crusts or pre-existing weak layers (like the one described above). You can take a look at a large, destructive wet slab that likely failed during the warm, sunny weather at the beginning of March here. Read more about wet slabs in the Forecast Discussion below. 

    (3/11/2021) Weak layers may be dormant, but they still exist. This snow profile shows soft snow near the surface that steadily hardens until the weak facets carved out near the ground. Photo: J. Preuss

  • Loose Wet

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

    A weak overnight freeze and another day of warm temperatures will keep wet snow problems on the table today. Increasing cloud cover should mute the impacts of this problem today, but a few hours of sunshine this morning may be all it takes to remelt the snow surface. If you find yourself traveling on steep slopes with wet, slushy snow on top, triggering one of these slides will be possible. Monitor the snow surface as you travel—hop off your sled or out of your skis and dig down with your hands. If you are finding wet snow (easy to make a snowball) deeper than your boots it's a good idea to avoid steep, rocky slopes or to go looking for colder snow to play in.

Avalanche Discussion

TRANSITION

We find ourselves in the middle of the seasonal transition from a dry winter snowpack to a consolidated spring snowpack. On upper and middle elevation slopes that don't directly face the sun, the snowpack is still cold and dry. At lower elevations and on all slopes that face the sun, the warm temperatures and direct sunshine of the past two weeks have begun to gradually push melt-water down into the snowpack. When the snowpack is freezing solidly at night it tends to behave in a predictable, diurnal pattern: the surface snow melts during the day and then refreezes at night. When this is happening, avalanches are usually restricted to surficial wet loose avalanches. However, when ambient air temperatures stay above freezing overnight and thick cloud cover prevents the snow from radiating out the energy it absorbed during the day, this re-freezing is much weaker. Last night was somewhere in the grey area in between those two ends of the spectrum. 

Periods of transition are awkward and unpredictable times for the snowpack. This is especially true when dealing with a snowpack that has buried persistent weak layers in it, as ours does. The overlap of the seasonal transition and our buried weak layers produces one of the most unpredictable types of avalanches: wet slabs. I'm writing about wet slabs, not because I think they are going to be happening everywhere today, but because they are likely to be a problem at some point in the near future and it is good for us to be thinking about them. We've received reports of a few small wet slabs and one large, destructive wet slab (information here) thus far this season. 

In many ways, wet slab avalanches are similar to other types of slab avalanches (wind slabs and persistent slabs). A cohesive mass of snow (a slab) detaches from the snow below it, producing an avalanche. However, there is a key difference with wet slabs. Instead of failing due to overloading, as is generally the case with dry slabs, they fail as a result of a decrease in strength at the failure plane of the avalanche. This decrease in strength is caused by liquid water moving through the snowpack. When overnight temperatures are cold and the snowpack is able to freeze solidly, there isn't much liquid water in the snowpack to worry about. But when warm temperatures at night prevent a solid freeze over the span of several days, significant amounts of melt-water can accumulate in the snowpack. Based on remote weather stations, the water in the snowpack probably did not completely refreeze last night. If this pattern continues over the next few days there is a good chance that we will see Avalanche Danger increasing due to wet slab concerns. If you are out in the mountains today and you are finding a snowpack that is deeply saturated with water you will be well served the think about the possibility of wet slab avalanches and give yourself a wide berth around the type of terrain where they will be most likely. What sort of terrain is that? Steep, rocky slopes that directly face the sun where the snowpack is thin.

You can catch up on what happened last week in Ethan's Weekend Update.

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