Avalanche forecast

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Avalanche danger

Wed
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Intense spring sunshine will break down the crust on the snow surface, bringing the possibility for wet snow avalanches today. Monitor the snow surface you are traveling on and be ready to find colder snow or head for home when the crust is no longer supporting you.

Avalanche problems

  • Loose Wet

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

     Yesterday afternoon, I went up to Galena Peak to see how the snowpack was behaving. Even during a cooler day without sunshine, I found that the crust on the surface wasn't strong enough to support my weight (with skis on). As I skinned along I regularly fell through this crust, dropping to near the ground through a pile of warm, wet snow. Yuck. Professional observers in this zone found similar conditions and avoided avalanche terrain that had not frozen well. Fortunately, last night brought the coolest temperatures we've seen in a week which was a welcome sight. This has allowed the snow surface to freeze a bit more deeply than in the past few days. 

    As you leave your car this morning, I expect you'll find that the crust on the snow surface is stronger than what I found yesterday afternoon. However, underneath this crust you will still find a warm, wet snowpack that hasn't frozen for days. Today's strong spring sunshine will quickly go to work on this crust. As the crust melts the likelihood of encountering wet snow avalanches will increase. Your best defense against this problem is to get an early start and to monitor the snow surface throughout your day. Be ready to look for colder snow or head for home when the crust is becoming wet and unsupportable. And remember, if you are skiing and riding up high, the snowpack that you have to travel through to get back to your vehicle is probably warmer and wetter than where you are playing. 

    You can read more about how the snowpack melts and freezes in the Forecast Discussion below.

    ADDITIONAL DISCUSSION: WET SLABSWet slab avalanches are unpredictable beasts. These slides occur when melt-water in the snowpack pools on a layer in the snowpack and weakens the bonds there. The conditions that produce these types of slides are much more predictable than the avalanches themselves. We are most likely to see wet slab activity during extended periods of above-freezing temperatures. On Sunday, a sizable wet slab was triggered by a wet loose avalanche from above near Galena Summit (photos and more information here). These slides are a bit less likely today than they were during the heat of last week, but they will still be on my radar as I travel in the mountains today.

    (4/5/2021) Debris from skier-triggered avalanche on Senate Peak on 4/4/21.

Avalanche Discussion

RADIATION BALANCE and SNOWPACK COOLING

When we think about avalanche hazards related to a spring snowpack we are often thinking about melting and freezing. During the day, warm temperatures and sunshine melt the surface of the snow. At night, colder temperatures allow for the snowpack to freeze. Simple enough, right? Yes and no. There are a few things that complicate this picture:

  • During extended periods without a good freeze, liquid water penetrates much deeper than just the surface of the snowpack. This happened during the warm-up last week. In many areas, the entire snowpack deteriorated to an unconsolidated pile of mush, resulting in a significant increase in the likelihood of encountering wet snow avalanches. It is possible to eventually refreeze a snowpack that looks like this, but this won't happen overnight.
  • Cold air temperatures help the snow surface to freeze by a process known as cold air advection. However, this isn't the only process that cools the snowpack. An important, but less obvious factor is radiative cooling. Snow is a very effective emitter of longwave radiation and it can dump much of the energy it absorbs during the day when skies are clear at night. We observed this process last week, when the snow surface was freezing solidly even on nights when the air temperature remained at or above freezing. However, when skies are cloudy the radiation gets trapped by the clouds and it bounces back and forth between the snow and the clouds. On nights like this, the snowpack can barely refreeze even when temperatures are below freezing. Last night, skies were cloudy in our northern mountains and generally clear in the mountains above the Wood River Valley.
  • New snow acts as a thermal blanket, slowing the freezing process and complicating the radiative cooling. Snowfall amounts were light during yesterday's storm so the impact of this thermal blanket will be muted today. However, in areas that received snow, this new snow will quickly melt, sending water down into the snowpack and weakening the bonds in the crust below. 

All of these factors play a role in the type of snowpack you will find if you go out in the backcountry today. As you are traveling, take some time to observe how deeply the snowpack you are on has frozen. See if you can break through the crust with your ski pole or if your track drops through the crust when you blip the throttle. Hop out of your skis or off your sled and see if the crust supports your weight. Keep doing this throughout the day, observing how the results of these simple tests are changing. Have a plan for where to go to find colder snow when the surface crusts are breaking down. Heading home and enjoying a cold beverage of your choice in the sun is a great way to spend a spring afternoon. 

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