Soldier & Wood River Valley Mtns

Triggering a large avalanche remains possible in the eastern portion of this zone (the Little Wood drainage, southern Wood River Valley, Pioneer Mtns, and near Bald Mtn). There, the weak snowpack is still adjusting to the 16-20 inches of snow that fell last week. Avalanches can be triggered remotely—from flatter terrain below, above, or to the side of steeper slopes. Avalanches are most likely where fresh or recent wind drifts exist, but sheltered slopes are not guaranteed to be safe. Periods of direct sunshine could cause wet loose snow avalanches on very steep slopes. 

“Spring is nature’s way of saying, ‘Let’s Party!’” Robin Williams

Spring mountain weather—and its effects on snow stability—is a lot like the late Robin Williams' performances: unpredictable, challenging to keep up with, schizophrenic at times, and generally wild. We are in the middle of a classic spring weather cycle in the northern Sawtooths and Banner Summit (and maybe the White Clouds?) areas. These mountains received 12-18" of snow in the past couple of days, and the temperature, snow density, and winds varied dramatically throughout the storm. Let's dive into how the variable spring weather affected the northern mountains' snowpack and snow stability as well as what we see in the crystal ball going forward. 

• The Past:
  • The snow fell on a variety of typical spring surfaces, including crusts, old powdery snow, weak faceted snow, and combinations of these.  The variety of old surfaces means there is variability in how well the storm snow sticks to the old surface. Our limited observations indicate that storm snow is bonding fairly well to the old surfaces. We also need to consider a layer of faceted snow from early March that could be a problem on isolated slopes. It presents as a fairly obvious dirty stripe when you dig down to it, and avalanches could fail on this layer where it's heavily loaded. 
  • The storm started warm (dense snow on the bottom) and cooled off (lighter powder near the top), but there were temperature fluctuations during the "squalls" of more intense precipitation. This created several distinct layers within the new snow. Why does this matter? Layers can be weak or strong relative to the layers below them. You'll notice this is roughly the definition of a slab—a layer of snow that is stronger or more cohesive than the layer below it. The complex layering within the storm snow created several potential weak layers and slabs. This phenomenon can create very touchy conditions (easy to trigger) for a day or two.
  • Variable winds: The recent hourly wind speed averages indicate light to moderate winds. However, you may have noticed a big spread between the average speeds and the maximum gusts. This spread can result from periods of sustained moderate to strong winds during and following squalls (heavy wind transport) intermixed with periods of light winds with little or no snow transport. Both Chris and I observed significant winds and wind-transported snow in the northern Sawtooths (video) and Banner Summit area (Observation) the past two days, indicating the hourly average wind speeds are not telling the whole story. Expect to find more wind slabs than you would expect given the recent 10-15mph (light) average wind speeds.  The erratic winds exacerbated the complex layering issue described above. 
• The Present and Future: 
  • Wind slabs: Chris and I both observed touchy (easy to trigger) wind slabs that failed within the new snow. Some of these instabilities may have already "healed", and they typically last up to a couple of days. In exposed terrain, you're likely to find some sensitive wind slabs that will fail on layers within the new snow. These slabs will generally be less than 2 feet thick and won't extend far below ridgelines. In the Sawtooths, you may find mid-storm instabilities that mimic wind slabs where "spin drift" collects in cirques below large cliff bands.  
  • Loose snow avalanches: If the sun peeks out as expected today, it will warm the lower density snow at the surface and increase the cohesion of the storm snow. This process can/will produce loose snow slides, or sluffs. With up to 18" of new snow, we could see sizeable, widespread natural avalanche activity in very steep terrain. Call them wet loose, dry loose, or whatever type of avalanche you'd like; the important things to remember are they are likely, they could be large enough to ruin your day, and they occur naturally. You don't need to trigger loose snow avalanches. To compound problems, the loose snow slides could trigger larger slab avalanches as they run downslope (slabs formed by wind or spin drifting described above or the "weird" slab avalanches in the next bullet point). 
  • Esoteric or "warming" caused avalanches: The surface properties of a snowpack are very important in slab avalanche mechanics. As the sun warms the low density surface snow, it increases the cohesion or stiffness at the surface. In addition to promoting loose snow avalanche activity, this process can have surprisingly profound effects when a snowpack is already teetering on the edge of stability/instability. Increasing the surface stiffness can produce natural or easily triggered slab avalanches where the snow was stable before the solar input. The slabs could fail within the new snow or on faceted layers beneath the storm snow. This phenomenon is rarely seen and nearly impossible to accurately forecast. I've experienced only a handful of these scenarios in my almost 30 years of playing the avalanche game. That being said, today is the first day this season I have it front and center in my mind. 

How do we manage Robin Williams' springtime "Lets Party!" avalanche problems and scenarios? 

1. The answer is always terrain: If you're skiing or riding in 1) terrain sheltered from the recent winds, 2) that doesn't directly face the sun, and 3) is free of overhead hazard (big, steep avalanche paths above you), you're unlikely to experience any avalanche activity or signs of instability. As you check one or more boxes (wind-affected, directly facing the sun, overhead hazard), you'll be exposed to more potential avalanche hazard. Do not spend more time than necessary under large avalanche paths, especially very steep rocky paths. 
2. Keep your head on a swivel: Conditions will change rapidly over time and space. The layers beneath and within the new snow change dramatically as you move up or down in elevation and to slopes that face different directions. The pit you dug on one part of a slope may not be representative of what you'll find where the aspect slightly changes or what's a couple hundred vertical feet above or below you. Also, the late March sun is like a turbocharger for changing snow stability. Think minutes rather than hours when reevaluating stability and terrain choices.