Avalanche forecast

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Avalanche danger

Sun
Alpine
4 · High
Treeline
4 · High
Below
4 · High
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Highlights

Very dangerous avalanche conditions exist. The likelihood of large natural and human-triggered avalanches will increase dramatically throughout the day as the storm rages on. Expect to encounter a range of storm-related avalanche problems including wind slabs, storm slabs, and loose snow avalanches. Weak layers buried underneath 3-6 foot slabs will produce very large, very destructive avalanches that can be triggered from long distances away. Rain and warm temperatures at lower elevations will bring on wet snow problems this afternoon. Travel in avalanche terrain is not recommended today. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2–3.5

    Despite the month on the calendar, we have a wintertime snowpack with two sets of persistent weak layers (video at right/above). 

    1. March Weak Layers: Layers of weak snow buried 1-2.5 feet below the snow surface will be facing a real test today. The slab over these weak layers that has been gradually building over the past 10 days will double in thickness with today's storm. The most recent avalanche involving this snow occurred yesterday on Johnstone Peak, east of Ketchum (photo here). These layers appear to be at their worst on slopes that face the sun, where crust+facet combinations exist. The slab over these weak layers is thick enough that avalanches involving them are possible even in sheltered terrain.
    2. February Weak Layers: A very dense, 3-6 foot thick slab of snow is uncomfortably resting on weak layers underneath. This combination was responsible for two historic natural avalanche cycles in mid-March. Very large natural avalanches failed on all aspects of the compass. The most recent avalanche involving this snow occurred yesterday in the Frenchman's drainage near Smiley Creek when riders remotely triggered a slide that broke 4-5 feet deep and 800+ feet wide (photo here). They were at least 500 feet away on the ridgeline above when they triggered the slide. Blocks of snow the size of small cars remained intact as they traveled downslope, breaking green trees along the way. These types of slides are unsurvivable.

    Today's storm will add significant stress to these layers, dramatically increasing the chances of large natural and human-triggered avalanches. Smaller storm snow avalanches will be effective triggers for these deeper weak layers. Persistent slabs are challenging beasts to deal with. You can trigger them from long distances away from where the avalanche releases. Slides may break much wider and deeper than you expect, wrapping around large terrain features. The only effective management strategy today is to stay out of the way. 

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Certain
    Size
    1.5–2.5

    Today's storm will bring 10-20" of new snow to this zone. Snowfall will be accompanied by strong winds blowing out of the W-SW. The combination of new snow and wind will produce a range of storm-related avalanche problems. These will become larger and easier to trigger as the day progresses. Expect to see wind slabs, storm slabs, loose snow avalanches, and failing cornices. Avalanches failing within the new snow will be effective triggers for the deeper weak layers described above.

    Read the Forecast Discussion below for more information about the range of storm-related problems you may encounter today. 

Avalanche Discussion

We have two distinct persistent slab problems that set the stage for today's storm (read the Persistent Slab problem discussion above). These problems were keeping many experienced backcountry users out of large avalanche terrain, even before this storm. The weight of the new snow will make them much more reactive, and the smaller natural avalanches that will frequently occur will be effective triggers for these deeper weak layers. Here is a look at the types of storm-related avalanche problems you can expect to encounter today:

DRY SNOW: In upper and middle elevation terrain we will see a range of dry snow avalanche hazards, ranging from small to very large. 

  1. Wind slabs: Strong to extreme winds will accompany today's snowfall. These slabs will be thickest and most widespread in upper elevation terrain, but reactive slabs will be forming at all elevations.
  2. Storm slabs: Today's snowfall will be intense. Expect to see some impressive precipitation rates. Rapidly falling snow will create storm slabs that will be easy to trigger, even in terrain sheltered from the wind. 
  3. Spindrift slabs: In steep terrain, snow will be cascading down cliff bands and couloirs to the aprons below. Large, sensitive slabs will be forming here.
  4. Loose snow: Rapid snowfall rates will produce frequent loose snow avalanches. These slides will be easy to trigger. They'll start out small, but may pick up a lot of speed as they travel downslope, particularly where they are running on crusts.
  5. Cornices: This year has been snowy, windy, and cold. Massive cornices exist along many ridgelines. Today's storm will add a hefty load on top of these huge chunks of snow. Small to very large cornice failures are expected today. You don't want to be nearby when these beasts become unmoored. 

WET SNOW: At lower elevations, the snowpack is beginning to go through its springtime transition. You'll find icy crusts on the surface this morning, but it is warm and moist underneath. As today's warming temperatures and low elevation rain melt the snow we will see some weird, wet snow activity at lower elevations. Loose, wet snow avalanches may gouge deeply into the snowpack, producing destructive avalanches. Large, dry snow avalanches that run down into wet snow below can be very destructive. Wet slab avalanches are unpredictable, but they are certainly well within the realm of possibility today.

The overlap of an intense storm with proven weak layers is producing very dangerous avalanche conditions. Today is the first day of April, but you'll be well served to travel like it is a dangerous day in January. The only effective management strategy today is to stay out of the way. 

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