Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Mon
Alpine
3 · Considerable
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Out of date — this forecast expired 6 years ago.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

The avalanche danger will rise to CONSIDERABLE today as new and windblown snow begins to overload a very weak snowpack. Triggering an avalanche 12-16 inches thick will become likely on wind-exposed, upper elevation slopes. Smaller avalanches will be possible on very steep slopes in sheltered terrain. Choose lower-angled terrain to reduce your risk.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Recent storms have favored our northern forecast zones, and a 10-12" thick slab sits atop extremely weak, faceted snow that formed during the 3+ week dry spell since mid-November. This relatively small load was enough to produce avalanches large enough to bury people in the Western Smokys, Sawtooths, and Banner Summit area during the last storm. 

    While there's still not enough load on the weak layer to cause widespread instability, we're getting precariously close to the brink. Yesterday's wind was visibly transporting snow along higher ridgelines and in alpine terrain, adding additional weight to the weak layer. Forecasted snowfall and wind today will get us even closer to the edge. There's a good chance that by sometime tomorrow, we'll be there and we'll have fairly widespread dangerous conditions.

    Today will be a good day to start ratcheting back your terrain choices. I'd avoid any steep slope with evidence of wind effect. As the day progresses, triggering avalanches on steep, wind-sheltered slopes may become a real possibility.

    (12/18/2020) Avalanche in the Langer Creek area in the Banner Summit zone. It failed on very weak faceted snow that formed during the prolonged dry spell. 8000', NE

Avalanche Discussion

Drip. Drip. Drip. A trickle of loading from snowfall and, more recently, wind is ever so slowly loading the very weak facet layer that formed during the prolonged dry spell. Before snowfall buried the weak layer on December 11th, we had a faceted snow surface that was fairly uniform across the forecast area. This uniformity is beginning to break down as different portions of our forecast area get loaded at varying rates. While we're only talking differences in inches of snowfall here, with a facet layer as weak as ours, that can make all the difference.

Although the variation across our forecast area is more of a spectrum, it's usually helpful to simplify matters. So far, none of our area has received a sufficient load to produce widespread instability on the facet layer. But our northern zones (Sawtooths & Western Smokys, Banner Summit) received more recent snowfall, and as a result, have been closer to the brink. The last storm produced more widespread, and larger, avalanches in these zones.

The trend of a diverging pattern between our northern and southern zones will continue with today's weather system strongly favoring the north. It seems likely the Sawtooths & Western Smokys and Banner Summit zones will finally reach the tipping point at some point tonight or tomorrow. Watching what happens in these zones will likely foreshadow conditions in our southern zones once they catch up.

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