Bollettino valanghe

Galena Summit & Eastern Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

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

Recent strong winds transported snowfall from the past week, forming fresh, stiff wind drifts and stressing buried weak layers. Dangerous avalanche conditions exist in terrain exposed to the wind. You can dramatically reduce your risk of getting caught in an avalanche by sticking to slopes that are completely sheltered from the effects of the wind.

Problemi valanghivi

  • Persistent Slabs

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

    Two large, natural avalanches released Monday night or Tuesday in wind-loaded, south-facing terrain just north of Galena Lodge (photo to right/below, view observation). Recent avalanches were also observed on a SE-facing, alpine slope in the Smoky Mtns and on an E-facing, alpine slope near ridgeline in the Boulder Mtns (view observation). Strong winds in the past couple of days moved snow onto many slopes, stressing the  weak layers near the ground and in the upper half of the snowpack:

    1. In the upper 1-3 feet of the snowpack, multiple interfaces have produced avalanches and unstable test results recently. Yesterday near Galena Lodge, I found multiple weak layers in the upper half of the snowpack (photo, video). Ben found a thin snowpack with multiple buried weak layers near Baker Creek on Monday (observation, photo). These layers appear to be worst on slopes that face the sun, where they are accompanied by a crust. 
    2. Depth hoar exists near the ground on all aspects. This is the snow that we've been talking about all season long and seem destined to talk about for the remainder of the season. This layer is buried deeper and is more difficult (but not impossible) to trigger.

    Given the uncertainty surrounding these weak layers, skiing or riding in "prime-time" avalanche terrain remains risky. You are more likely to trigger an avalanche on wind-affected slopes steeper than about 35 degrees, but these weak layers exist in sheltered terrain as well.

    (2/23/2021) These 2 large slab avalanches released naturally during the strong NW wind event Monday night or Tuesday in the Westernhome Gulch drainage (9650', S-SE).

  • Wind Slabs

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

    In the past two days, professionals in the mountains near Smiley Creek reported extensive wind-loading in exposed upper and middle elevation terrain. Yesterday, I observed thin wind-deposits in exposed middle elevations. These slabs sat on temperature and rain crusts that formed Monday.

    Expect to find wind slabs near ridges, on the sides of cross-loaded gullies, and lower on slopes near prominent terrain features. Continued light snowfall may mask some of the obvious signs that the wind has been at work, but it's a safe bet to assume upper elevation slopes and many middle elevation slopes have been wind-loaded in the past two days. 

    Recognize that the recent wind event also added stress to the weak layers buried deeper in the snowpack (described in Problem 1 above). Any wind slab you trigger could step down to these deeper layers, producing a much larger, more dangerous avalanche.  You can greatly reduce your risk of triggering an avalanche of any variety by sticking to slopes that are sheltered from the wind.

    (2/23/2021) Active wind transport on McDonald Peak in the southern Sawtooth Mtns Tuesday afternoon. Note the shiny crusts on the SE aspect on the left of the photo.

Avalanche Discussion

TODAY'S SNOW SURFACE, TOMORROW'S WEAK LAYER

A storm will arrive today bringing snowfall to much of the western and northern reaches of the forecast area including Banner Summit, the Sawtooths, the Smokys, and the Soldiers. As the current snow surface is buried, it's a good idea to take stock of the interface that may prove to be our next problematic weak layer. 

We've had a grab-bag of weather this week including rain Monday afternoon, snow and strong wind Tuesday, sun, light snow, and graupel yesterday, and a clear, cold night last night. This mix of weather created a smorgasbord of snow surfaces that will sit beneath tomorrow's storm.

If I had to guess, the touchiest interfaces we'll see are those that involve facets above or below a crust (or two). Crusts exist on many slopes near or just below the snow surface. Warm ambient air temperatures Monday climbed into the low 40s F. Snow surfaces at low and middle elevations (both sunny and shaded) melted during the day and refroze overnight. The warm air also turned some snow showers to rain up to 9,000'. 

Crusts also formed on most steep slopes that face the sun, even at upper elevations. Check out this cool photo Scott took on Tuesday where you can see the sun reflecting off of a crust high on McDonald Peak in the southern Sawtooths.

Middle and upper elevation, shaded aspects are the areas most likely to be crust-free, but they don't necessarily get to duck out of the potential weak layer category. As skies cleared last night dry, very cold air likely promoted faceting in many areas. 

Throwing in recent wind effect and snowfall differences across the area I'd expect the patterns of weak layer distribution to be highly variable. Stability may change over short distances as you climb in elevation or wrap to different aspects. This storm will also add stress to known weak layers in the middle and at the base of the snowpack. As a complex picture becomes even murkier, plan to build a larger margin for error into your travel plans. 

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