Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Sun
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 6 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

While not likely, you could trigger an avalanche large enough to bury a person. The snowpack is still stabilizing from the recent storm that brought 2-3 feet of new snow and strong winds. Large avalanches are most likely in wind-loaded terrain but are possible on sheltered slopes too. With warming temperatures today, wet sluffs will become possible—primarily at lower elevations.

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Avalanche problems

  • Persistent Slabs

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

    Late-March storms buried weak layers of faceted snow that formed earlier in the month. These layers have been responsible for four, possibly five, human-triggered avalanches in other parts of the forecast area over the past week. These layers seem less common in the Banner Summit zone—with the important caveat that observations from this zone have been limited.

    Yesterday, I was north of Highway 21 looking for these persistent weak layers. I did not find them in every snowpit, but I did find them. On an E aspect, I found a thin layer of facets on a crust beneath a 3-foot thick slab. It took some force, but I was able to get a block to slide cleanly during snowpack tests. Whether or not these weak layers are widespread, 2-3 feet of recent snowfall, strong winds, and—let's not forget—a large magnitude earthquake are justification enough to give this zone more time to stabilize. 

    ADDITIONAL DISCUSSIONYesterday I found that the lower elevation snow surface was getting moist and sticky. Warmer temperatures today could lead to wet snow slides, sluffs, and "dribbles". This will primarily be a lower elevation problem but could creep into the mid and upper elevations if the sun stays out longer than forecast.

Avalanche Discussion

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, the Smoky Mountains (including the Western Smokys and Salmon Headwaters), and the Soldiers. In these areas, there have been four, possibly five, 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 observations 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 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.

Regions covered