Soldier & Wood River Valley Mtns

Triggering a large, 1-3 foot thick slab avalanche is not likely, but skiers and riders could get unlucky and find the wrong spot on the wrong slope. Large avalanches are most likely where the wind has formed drifts or stiffened the snow surface. To reduce your risk, steer clear of obviously wind-loaded terrain and don't linger on or below very steep slopes that directly face the sun.  

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.