Lawinenlagebericht

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Kernpunkte

The threat of wet snow avalanches will increase as the day warms up. These slides are most likely on very steep slopes that directly face the sun but are possible on shaded slopes at lower elevations as well. Be alert to changing snow conditions and seek out colder snow or low angle slopes as the crusts on the surface break down and the snow becomes wet. Large, dry snow avalanches are unlikely, but there is still a lingering chance of triggering a large slide on very steep, rocky, wind-affected slopes.

Lawinenprobleme

  • Loose Wet

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

    Last night was the second night of barely below freezing temperatures. The snow is melting during the day but warm overnight temperatures are preventing the snowpack from refreezing completely at night. Warm temperatures and intense sunshine today will quickly melt the crusts that formed at the snow surface last night. If you find yourself traveling on steep slopes with wet, slushy snow on top, triggering one of these slides will be possible. These slides will start at a point and fan out as they travel downhill and entrain additional wet snow. They may gouge deeply into the wet snow below, creating large, destructive avalanches.

    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 mid-boot it's a good idea to avoid steep slopes or go looking for colder snow to play in. Triggering one of these slides is most likely on very steep, rocky slopes that directly face the sun. These slides are also possible on shaded slopes at lower elevations.

    Large cornices exist along many ridgelines in this zone. Warm temperatures are causing these beasts to sag under their weight until they eventually tumble off the ridges they are perched on. This can be dangerous in its own right, but can also trigger loose snow and slab avalanches on the slopes below.

    ADDITIONAL DISCUSSION:Weak overnight freezes, warm temperatures, and buried weak layers 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 weakens the snow, particularly in areas with crusts or pre-existing weak layers (like the one described below). You can see two photos of wet slabs that occurred in our area in previous years here (crown, debris). Read more about wet slabs in the Forecast Discussion below. 

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Unlikely
    Größe
    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. In this zone, 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. 

    (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

Avalanche Discussion

WEAK LAYERS AND WET SLABS

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. But not all weak layers are created equally. They vary in distribution, character, sensitivity, and longevity. Some create just an avalanche or two before healing, while other stick around for months, giving us at the Avalanche Center plenty of fodder for headaches and sleepless nights. It's safe to say that the weak snow buried at the base of our snowpack this season is not a one-trick pony. This snow has behaved unpredictably since the day it was buried and it seems unlikely that it has run out of surprises for us.

The overlap of the seasonal transition and buried weak layers produces one of the most unpredictable types of avalanches: wet slabs. I'm writing about wet slabs, not necessarily 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 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. You can see two photos of wet slabs that occurred in our area in previous years here: crown, debris.

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 push meltwater 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 near freezing overnight this re-freeze is much weaker. Unfortunately, last night was the second night without a solid freeze and it is looking like we may have a few more nights with weak freezes on the horizon. 

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 and weakening the bonds between snow grains. 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. You can watch a video I made yesterday showing what this looks like here.

Based on remote weather stations and yesterday's field observations, the water in the snowpack did not completely refreeze last night. 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 deeply gouging wet loose avalanches and wet slab avalanches. 

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