Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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In sintesi

Several fresh, large, destructive slab avalanches were observed in the Smoky, Boulder, and southern White Cloud Mtns yesterday, as well as countless wet loose avalanches in all forecast zones. Today will bring more of the same. The large slab avalanches can occur naturally or can be triggered remotely—from a distance above, below, or to the side of steep slopes. The slab avalanche problem appears to be more widespread in the western Smoky Mtns than in the Sawtooths closer to Stanley. Wet avalanches will release naturally on many aspects. Avoid areas with wet, soggy snow. Do not linger below large avalanche paths.

Problemi valanghivi

  • Persistent Slabs

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

    We observed a 1500' wide, destructive slab avalanche on Boulder Peak yesterday (sunny slope), a wide slab avalanche on Bear Peak (shady slope), and a sizable slab in the Newman Ck drainage (partial sun/shade). Observers reported natural slab avalanche activity near Horton Peak (sunny slopes) and in the Owl Creek drainage (shady slopes). This is in addition to the  4 large avalanches skiers and riders triggered since last week's storm, many of them remotely (from a distance above, below, or to the side of a steeper slope). Do you see a trend? It's sketchy out there right now.

    Yesterday's activity involved sunnier aspects (SW-S-SE) as well as the problem faceted layers on E-NE-N-NW aspects we've been talking about for days.  You're more likely to trigger persistent slab avalanches in areas where the wind has stiffened the snow surface or formed drifts, but they are possible in sheltered terrain as well. As we are in the midst of the first major warm-up of the season, the wise move is to stand back and watch the show instead of trying to outsmart exactly where the persistent layers are most likely to fail. 

    Dealing with persistent weak layers is tricky. Snowpack test results become less reliable, and obvious signs of instability may be absent. The first clear sign of an unstable snowpack may occur when you trigger a large avalanche. These types of slides can be triggered remotely—from a distance above, below, or to the side of a steeper slope—or by smaller loose snow avalanches or cornice fall coming from above

    ADDITIONAL DISCUSSION: Thin, soft, fresh wind slabs may exist in exposed, upper elevation terrain. Be alert to the consequences of triggering a small to medium-sized slide in steeper, rocky terrain. The heat and sun are increasing the chances of natural cornice fall. Small slides or cornice fall could trigger a thicker slab lower on slopes.

  • Loose Wet

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

    I counted over 60 natural wet loose avalanches yesterday on a short trip in the Smoky and Boulder Mtns. Chris saw several wet loose slides in the Sawtooths. Today's intense sunshine and warm temperatures will heat and melt the surface snow, producing wet loose avalanches. These slides can occur naturally, without a human trigger. Natural cornice fall can also trigger them. Where the snowpack is thin (less than about 2 feet thick), some of the wet loose avalanches could gouge down to the ground. Where is this problem most likely?

    1. On upper elevation slopes that directly face the sun: NE-E-SE aspects in the morning, S-SW aspects near noon, and SW-W-NW facing slopes later in the afternoon. Slopes sheltered from the wind are especially dangerous.
    2. On middle and lower elevation slopes that still have a foot or more of snow, regardless of the direction they face.

    Keep an eye on the condition of the snow surface while you're traveling. When it gets wet or when you see wet snow rolling downhill, it is time to move to gentle terrain or slopes where the snow surface is colder and drier. Wet loose instabilities can develop rapidly; think minutes rather than hours. Minimize the time you spend below all large, steep slopes that directly face the sun. 

Avalanche Discussion

Snowpacks do not like rapid change. The hot, sunny weather this week is something this snowpack has never seen, and it's predictably misbehaving. The upper 3 feet of the snowpack is officially a mess in much of the forecast area. Choosing which weak layer or avalanche problem to prioritize varies dramatically over space (both weak layer distribution and recent snowfall amounts change as you move up or down in elevation and to slopes that face different directions) and time (how is the intense, April sun changing the stability equation). It's like picking a winner in a crowded, wide-open Kentucky Derby field. With that in mind, here's today's updated "Daily Avalanche Form" to handicap the avalanche field:

  • 1:9  Surficial wet loose avalanches: The hands-down favorite again today. I saw at least 60 of these yesterday in the Smoky and Boulder Mtns, and Ben, Chris and other observers counted dozens of additional wet loose avalanches. While these avalanches will not be huge, they have the finishing kick to knock you down and carry you places you don't want to go. In the wrong terrain, they could be large enough to bury you.  
  • 1:1 Natural cornice fall: A new entry in today's race. Cornices will sag under the hot April sun. Some of them will break. They could trigger wet loose or persistent slab avalanches when they strike the slopes below them. 
  • 3:1 Persistent slab avalanches: We saw several fresh, large slab avalanches yesterday, including a monster on Boulder Peak. People triggered at least 4 large avalanches since the earthquake and the storm ended. These are big enough to bury you and can be triggered remotely or fail naturally.
  • 3:1 Full-depth wet loose avalanches: In areas with a thin snowpack and in rocky terrain, today's intense April sunshine will make larger, deeper wet loose avalanches a contender. Give this choice serious consideration if you're sinking into a thin, wet snowpack or are traveling in or below steep, rocky terrain. 
  • 3:1 Wet slab avalanches: These will have a better chance of finishing first each day week as the temperatures heat up. These beasts are most likely in the Soldiers and Wood River Valley Mountains where the snowpack is thin (less than 3 feet thick) and weak (lots of sugary facets beneath the snow from March storms). 
  • 7:1 Wind slab avalanches: Although the southern and western part of the forecast area picked up 5-12" of snow this week, winds were light. In alpine terrain, you could find isolated, soft wind slabs.

Daily Double options: Watch out where these problems overlap. Wind-drifting and persistent weak layers/slab avalanches form a potent one-two punch. Also, small wet loose slides and /or cornice fall could trigger slab avalanches lower on slopes as they move downhill. 

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