Avalanche forecast

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Sawtooth Avalanche Center
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Avalanche danger

Thu
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Expect natural loose snow avalanches (sluffs) in steep, rocky, sunny terrain. You could trigger soft drifts or wind slabs near ridgelines in alpine and upper elevation terrain. While the sluffs and wind slab avalanches will not be huge, they could trigger larger, destructive slides as they move downhill. Avoid stopping or standing below very steep, sunny slopes.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2–3

    The 10-16" of snowfall in the past few days is just enough of a load to maintain the threat of triggering large, destructive avalanches on weak layers buried 3-6 feet deep. Triggering one of these dragons is unlikely, but we're still saying there's a chance. 

    Unfortunately, we don’t have good tools to accurately predict stability on an individual slope. We can see that the snowpack structure isn’t great (picture a large, sturdy house built on a foundation of sand), but we can’t see exactly where you or your sled might affect the weak snow underneath.

    Some thoughts to reduce the chances of getting in trouble:

    • Wind-loading: With a foot plus of recent snowfall, the light to moderate winds have probably loaded some exposed slopes. In alpine terrain, you may find some fresh drifts and wind slabs that demand respect on their own. Slopes with stiffer snow near the surface—whether from wind-loading or "spin drifting" below large cliff faces—are more likely to produce big avalanches than areas where the surface is loose powder.
    • Likely trigger points: You are most likely to impact the deeply buried weak layers on parts of a slope where the snow cover is thin, like near rocky outcrops. Heavily wind-affected areas where the wind created a complex pattern of scoured and loaded slopes are also more likely to be problematic.
    • Slope angle: Avalanches are most likely in "prime time" avalanche terrain, slopes around 35-45 degrees in steepness. However, remember that persistent slab avalanches can be triggered remotely: from gentler terrain below or to the sides of steeper slopes.
    • Solar-effects: Read the Forecast Discussion below for details on how the sun could affect the persistent slab problem today. In addition to the deeply buried weak layers, the sun could cause issues in the upper 1-2 feet of the snowpack.

  • Loose Wet

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

    This area received a foot or more snowfall in the past few days. Today's sunshine will likely cause steep, rocky slopes that directly face the sun to "shed" the recent snow. Expect small to medium-sized, natural loose snow avalanche activity to follow the sun around the compass: E-SE facing slopes initially become unstable this morning, then S-SW and eventually W aspects this afternoon. 

    Pay attention to what's above you during your travels. Sluffs releasing from thousands of feet above could pack quite a punch and may be large enough to bury people in terrain traps. Additionally, the loose snow avalanches could trigger the persistent slabs described above as they travel downhill. Today is not the day to dilly dally on or below big, complex, sunny avalanche paths. 

Avalanche Discussion

Snowpacks tend to get “stressed out” and react poorly to any rapid changes; intense snowfall, wind-loading, and steady rain are obvious examples.  You’ve probably heard about “rapid warming” as another red flag that can indicate unstable avalanche conditions. While “rapid warming” is not the ideal term to describe—or easiest way to understand—how today’s weather may affect snowpack stability, it is worth diving into how today’s sunny weather could create some spooky avalanche conditions.

Research and empirical evidence indicate the stiffness of the snow surface—the top of the slab—appears to be a key ingredient to slab avalanche release, whether the weak layer in question is several inches or feet below the surface. Some avalanche starting zones remain in a “conditionally stable” state; there’s loose powder at the surface, the slopes are not going to slide on their own, and skiers or snowmobilers would have to find just the right place to trigger them. Today’s intense sunshine could change the stability equation on some slopes. Unobstructed sunlight (solar radiation) will have several effects:

  • Sun-induced loose snow avalanches (sluffs) and/or wet loose avalanches are likely in very steep, sheltered terrain where at least 4-6” of snow fell yesterday. This is pretty straightforward. These small slides could trigger larger slab avalanches as they cascade down slopes.
  • On less steep slopes that received at least 4-6” of snow and directly face the sun, the fresh powder may rapidly consolidate, creating a slab near the surface where no slab existed before. You could see thin soft slabs form and become sensitive to triggering or even release naturally during the day.
  • The sun-induced changes in the surface snow could be “felt” by weak layers just beneath the past few days’ snowfall or by weak layers buried several feet deep. This is hard to envision, but it appears to happen in rare instances.

Why does this matter? These changes could cause natural avalanche activity. Most natural slides should only involve the past few days’ snowfall and be relatively small, but we could see some natural avalanches large enough to bury people. Picture the “conditionally stable” slopes as a loaded gun with the trigger partially cocked. Today’s solar radiation isn’t enough to dramatically change the stability equation, but any slopes that are already teetering on the edge don’t need much to be pushed to their breaking point. On these conditionally stable slopes, there may be more places today that skiers or riders could trigger large slab avalanches. The first day of intense sunshine following the recent snowfall could be that secondary factor that ultimately pulls the gun trigger and causes a big avalanche. A snowpack that would remain “happy” with cloudy skies and another 12” of low-density powder might get “angry” with the sun creating a very soft slab out of the 6-12” that has already fallen in the past few days.

Where does this matter? Slopes that face SE-S-SE-W, have buried persistent weak layers or crusts, and have received at least 6” of snow in the past few days are most prone to these phenomena.

What should you do? If you’re playing on slopes facing the southern half of the compass, pay close attention to how the sun is changing the surface snow. Rollerballs and pinwheels in steep terrain are obvious indicators the sun is having an impact, but the sun is also affecting the snow surface on slopes that are less steep or face the sun less directly. Consider building in some extra safety margin if you notice you’re able to make a snowball where the snow was blower powder at the beginning of the day.    

 

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