Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Snowfall, wind-loading, and buried weak layers are creating dangerous avalanche conditions. Sensitive wind slabs exist in exposed terrain. These slabs are hazardous in their own right and they are also adding stress to layers of weak snow buried in the snowpack. You can dramatically reduce your risk of getting caught in an avalanche by sticking to wind-sheltered, low- and mid-elevation terrain.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Steady snowfall and light to moderate winds have been the theme in this zone for most of the month of February (view storm totals here). The pattern continued yesterday. This combination of wind and snow has built many generations of wind slabs in exposed terrain. Wind slabs will be most widespread at upper elevations and may be found both near ridgelines and mid-slope in exposed terrain. The evidence of past wind loading may be masked by the ongoing snowfall. It is safe to assume that most upper elevation slopes have experienced at least some amount of wind loading over the past week. Owing to the near-vertical nature of the Sawtooths, the wind tends to blow snow down cliff faces where it accumulates in spindrift slabs on aprons below.

    This zone has received enough snow over the past week that wind slab avalanches could be plenty dangerous on their own, but you're also more likely to trigger a persistent slab avalanche (Problem #2 below) on wind-loaded slopes. You can greatly reduce your risk of triggering an avalanche of any variety by sticking to wind-sheltered, mid-elevation slopes.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    There are two main persistent weak layers in our snowpack:

    1. Depth hoar exists near the ground on all aspects. This is the snow that we've been talking about all season long and seem to be destined to talk about for the remainder of the season. This layer is buried 4-6' deep in this zone, making it much more difficult (but not impossible) to trigger.
    2. In the upper 2-4' of the snowpack are several interfaces that have produced both avalanches and unstable test results in recent days. In this zone, these layers appear to be worst where they are accompanied by a crust.

    On Friday, Chris and I visited the scene of a very large avalanche in Smiley Creek that was triggered by a snowmobiler. Unfortunately, the man was caught and killed in the avalanche (photo at right/above, more details here). This avalanche failed on a crust/facet layer that was buried in late January. The next day, we went to Beaver Creek to look for this layer. We found it. It took some force, but it failed cleanly during snowpack tests. We also saw a natural avalanche that likely broke on this layer. Yesterday, a skier in the Alturas Creek drainage experienced a very loud collapse that likely involved this layer. Continuing snowfall and wind-loading are keeping this weak layer reactive.

    Given the uncertainty surrounding these weak layers, skiing or riding in "prime-time" avalanche terrain is a risky business. You are more likely to trigger an avalanche on upper elevation, wind-affected slopes steeper than about 35 degrees, but these weak layers exist in sheltered terrain as well. Most upper elevation slopes have received additional loading due to wind over the past few weeks, making them more dangerous. While triggering a wind slab (Problem #1 above) is more likely, a wind slab avalanche could step down into these persistent weak layers resulting in a much larger avalanche.

    (2/19/2021) Photo of a very large triggered avalanche that resulted in a fatality on Friday, February 19th, near the head of Smiley Creek. The snowmobile track of the victim is visible in the center of the photo. The avalanche released on a south aspect at 9100'.

Avalanche Discussion

BIRTH OF A WEAK LAYER

If you've made it this far down the page you are well aware that we have layers of weak snow in our snowpack. In fact, we have several of them. You may find yourself wondering, where do weak layers come from? Well, they don't just spontaneously appear in the snowpack. Most often, they develop at the surface and are then buried by the next round of snowfall. 

When we are out in the mountains we are generally looking for the best surface conditions that we find. After all, skiing and riding powder is more fun than skiing and riding breakable crust. If you've been out in the past few days, looking for powder, you have probably noticed that there is a crust on top of the snow on slopes that face the sun. Depending on the conditions that they form in and how they are buried, crusts can make for long-lived weak layers. In fact, the combination of a sun crust with some small, weak facets on top was the weak layer responsible for the avalanche that resulted in the death of a rider in Smiley Creek on Friday. 

As we look forward to more snowfall overnight and another shot later in the week we can think about the surfaces that will be buried. If you want some homework, spend a bit of time mentally mapping where you are finding crusts today. If the crusts that we are burying this week end up being persistent weak layers you will have some sense where they are located and how to avoid them. Of course, weak layers can form without a crust too... It's never simple out there.

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