Soldier & Wood River Valley Mtns

Triggering an avalanche is most likely where the recent snowfall and wind have formed drifts. A human-triggered avalanche large enough to bury a person occurred in the Soldiers yesterday on a slope that had hard slabs of windblown snow. Weak layers buried exist in the top several feet of the snowpack—while less likely, avalanches are also possible in areas sheltered from the wind.

If you're traveling in the upper reaches of the Warm Springs Ck drainage near Dollarhide Summit where more snow fell this week, refer to the Galena Summit & Eastern Mtns zone forecast. 

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What's old is new again. After dealing with widespread persistent weak layers for the first part of the winter, an extremely dry February left us with a stable snowpack and LOW danger by March. But now persistent slabs are back on the problem list—déjà vu anyone?

Many folks think of weak, faceted snow as an early-season phenomenon, but the clear nights and temperature swings of spring can be perfect for forming facet layers at the surface of the snowpack. Often times, these facet layers are mixed in with crusts formed by the sun and warming. During the first half of March, an alternating pattern of small spring storms and clear, cold weather created several of these weak layers. More consistent snowfall in the second half of March has now buried these persistent weak layers beneath 1.5-3+ feet thick slab.  

These persistent weak layers exist throughout the forecast area, but seem to be most common and reactive in the Galena Summit area and Smoky Mountains (including the Western Smokys and Salmon Headwaters). In these areas, there have been three, possibly four, human-triggered persistent slab avalanches since late-March. A number of natural and earthquake-triggered avalanches in recent days appeared to break down to these layers as well. Snowpack bservations from our northern mountains have been sparse, but these weak layers may be less common in the Sawtooths and the Banner Summit area—but I wouldn't rule them out.  In the Soldiers & Wood River Valley, there has been less recent loading on these weak layers.

Spring persistent slab problems make me nervous, especially when they follow an extended period of relative stability like we experienced in late-February into early-March. We all seem to understand that these problems are "normal" during the early-winter, and we get in the habit of choosing mellow terrain. But by spring, we expect a more stable snowpack and smaller, more manageable avalanche problems. Quite often that's the case—but not always. In fact, our area has had two avalanche fatalities in April, one major injury in late-March, and a number of late season close calls.

This isn't a typical spring, not on a global scale and not on the scale of our snowpack. There are a number of reasons to ratchet it back out there—and triggering a dangerous persistent slab avalanche is one of them.