Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Limite del bosco
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In sintesi

Last night was the third night in a row with above-freezing temperatures in the mountains. Wet avalanches are possible at the start of the day, and will become more likely as the relentless April sun bakes the snow. Start your day early and be prepared to pull the plug and go home early. If your skis, boots, or machine are sinking deeply into wet snow, you've overstayed your welcome. Loose slides that start small may become large and destructive. Large, destructive wet slabs are an unnerving, unpredictable possibility. 

Problemi valanghivi

  • Loose Wet

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

    Last night was the third night in a row with above-freezing temperatures in the mountains (temperature graphs and stream gauges in media gallery below). Skies remained clear overnight, allowing the snowpack to dump some of its thermal energy to the clear sky, producing a crust at the snow surface. Underneath this crust, the snowpack is warm and wet. The wet snow avalanches you'll deal with today come in two forms:

    1. SURFICIAL: Loose snow avalanches involving the upper 4-12 inches of snow are the most likely type of slides. These will be easy to trigger on very steep slopes where the upper snowpack is wet and will continue to dribble down naturally under the influence of the sun and hot temperatures. These types of slides can occur on all aspects, as temperatures are warm enough to melt the snow surface even on shaded slopes. These slides are relatively predictable and manageable, as long as they stay near the surface. They'll start at a point and fan out and pick up additional snow as they travel downslope.
    2. DEEPER: On most slopes, the snowpack is moist or wet all of the way to the ground. Hot temperatures and intense sun will rapidly melt the crust at the snow surface, pushing more meltwater into the snowpack. As this water works its way down it weakens the entire snowpack. Surficial slides that start small may gouge deeply into wet snow below, entrain lots of debris, and become large and destructive. When meltwater is pooling on top of and lubricating prominent crusts in the snowpack, wet slab avalanches become possible. We have not seen any large wet slabs or gouging wet loose slides out of the current cycle of heat and sun, but the recipe for both of these types of slides exist in our snowpack. 

    Small wet loose avalanches and larger cornices (photo here) are likely to be failing naturally today. Pay attention to the terrain above you as well as the terrain you are moving through. Give cornices a wide berth, these mountain gargoyles are sagging in the heat and unmooring themselves from the ridgelines they are perched on. They can  break much further back than you might expect.

    ADDITIONAL DISCUSSION - DRY SNOW: If you grin and bear it through all the wet snow, you'll emerge into a slightly more wintery world above ~10,500'. Here, you may trigger small sluffs and thin wind slabs. Neither of these are likely to be large, but both can knock you off your feet. Asthese slides run down into wetter snow lower on the slope they will take on a more destructive, wet character. 

    (4/6/2024) Debris from a large wet slab that released in Basset Gulch near Baldy. This slide failed on a NNE aspect at 7,000', likely sometime on Thursday or Friday.

Avalanche Discussion

PLUMBING THE SNOWPACK - A Story in 3 Acts

I:  Fresh snow is a mix of air and frozen water. It falls, accumulates, and settles throughout the season. This produces a layered or stratified snowpack. The layering in the snowpack is fundamentally why we have slab avalanches, the type that are most dangerous for people. During the winter, these slab avalanches occur when the snowpack is entirely dry. As we move into the springtime we begin to introduce liquid water into the snowpack and things become more complex. Read on. 

II:  The season's snowpack is a record of the weather, written in ice. Storms build large, generally cohesive slabs of snow. While these slabs make up the vast majority of the snowpack by volume, they fall in relatively short periods of time. Between storms, sun and rain produce crusts at the surface of the snowpack. Periods of drought produce facets and surface hoar. Though thin, these layers between larger slabs of snow represent a majority of the season's time. The snowpack that develops is unique, every year. 

III: The seasonal transition from a dry, winter snowpack to a wet or frozen spring snowpack unfolds differently each year. The character of this transition is dictated both by the nature of that season's snowpack and by the weather that the snowpack experiences. As this transition occurs, water fundamentally changes the snowpack. It gets sponged into the thick, once-dry slabs and pools at crusts and weak layers that formed earlier in the season. As a network of plumbing is established, the snowpack becomes more efficient at flushing water out of the bottom of the snowpack and into the ground. We can tell that this process is happening right now by watching local waterways. Creek gauges are reporting a significant increase in discharge, a clear sign that the meltwater produced at the surface is being flushed through the snowpack. There is a time delay between when meltwater is produced at the surface and when it is flushed out the bottom of the snowpack. 

The stage is set and we are part way through the third act. Thus far, our snowpack has been reasonably well-behaved. All of the wet snow avalanches that we have observed this week have been relatively small, predictable, and manageable. As the snowpack continues to melt and weaken, the trend in avalanche activity is toward larger, less predictable, and less manageable avalanches. Heading into our third day without solid overnight freezes, you may encounter wet snow slides that gouge deeply into the snowpack and entrain large amounts of debris. Or even worse, large, very dense slabs of water-saturated snow that fail as wide, destructive wet slab avalanches. Predicting the onset of this type of destructive wet snow avalanche activity is a crapshoot at best. 

So what can you do about all this? We can't tell the snow how to behave, but we do have control over ourselves. Head into the mountains with the expectation that you might not get to do any of the things that you are planning on. And remember to have an exit in mind if/when things get nasty out there; the terrain you traveled through/underneath on your way in will be more dangerous on your way out.  Our snowpack will need at least a solid refreeze or two to come back into safer, more favorable, more predictable conditions. 

Regioni coperte