Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Sat
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Triggering a large avalanche is most likely in very steep, rocky areas where the snowpack is less than about 4 feet thick. Intense sunshine and warm temperatures may cause loose snow avalanches on slopes that directly face the sun. These slides could trigger larger avalanches as they move downhill. 

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    Dry, warm weather over the past 5 days is giving the weak layers in the upper 4' of the snowpack a well-deserved vacation. Although obvious signs of instability concerning these layers are decreasing, their mere presence and track-record are enough to encourage conservative terrain choices. You can view photos of recent persistent slab avalanches in the Media Gallery below. Here are some areas where the odds of triggering one of these slides is greater: 

    • On steep (>35 degrees) middle and upper elevation slopes that received the most snowfall and wind-transported snow during last weekend's storm.
    • In shallow areas where the snowpack is thinner than about 4' thick. These spots can be found near windswept ridges and on the edges of wind-loaded slopes. They are also common in craggy terrain where shallowly buried rocks bring weak layers closer to the snow surface. 

    Don't let the spring-like weather lull you into complacency. These weak layers are improving in many areas but It's a gradual process, and we're not out of the woods yet. 

    ADDITIONAL DISCUSSION:Numerous wet loose avalanches have occurred since Monday and similar slides are possible again today. So far the majority of the natural wet loose activity has been relatively small. However, small avalanches can spell bad news if they sweep you off your feet or push you into a terrain trap like a stand of trees or bury you in a gully. Fortunately, this avalanche problem is fairly predictable. 

    You're most likely to find this problem on very steep, rocky slopes that directly face the sun and are sheltered from the wind. This type of instability follows the sun around the compass, beginning on E-SE aspects in the morning, moving to SE-S facing terrain near noon, and finishing up on SW-W aspects later in the afternoon.

    Direct sun and warm temperatures will also weaken the large cornices that have formed throughout the season, which may sag and break during the heat of the day. Cornice breaks, as well as wet loose avalanches, can trigger larger slab avalanches as they travel downhill. We've seen evidence of this in the Boulder Mountains (photo, photo). 

    (2/26/2021) Small natural avalanche observed on Banner Summit on Friday. This slide likely ran on a weak layer at the old snow surface.

Avalanche Discussion

Warm weather and climbing sun angles have many of us thinking about spring. Daytime high temperatures have increased steadily this week and will creep into the low to mid-40s F again this afternoon. Couple that with direct sun and light wind and you could be easily fooled into thinking it was late March. The snow surface has reacted as expected, releasing wet loose avalanches on very steep, rocky, sun-drenched slopes. Yesterday observers found melt-water percolating deep into the snowpack. However, this process is confined to sunny slopes and the effects of warming are more surficial in shaded terrain. This stark contrast will produce two very different snowpack scenarios, something to keep in mind as you head into the mountains today. 

This warmup is just the first in a series of warming events that we'll see this spring. When people imagine a "spring snowpack", they're generally thinking of a snowpack that's seen several melt-freeze events that help to transform the dry, weak, faceted layers that plague much of our area into a denser, stiffer, stronger, and more uniform snowpack.

This transition happens in different mountain ranges, aspects, and elevations at different rates. Generally, we will see this transition sooner in the low elevation terrain of our southern mountains, and it will take a bit longer on shaded, cooler, middle, and upper elevations as you head north. Because this process takes time, and the transition varies by elevation and mountain range it can be hard to track. 

There's no doubt that the weak layers in our snowpack are on the mend, a steady decline in obvious signs of instability support that. Many of the weak layers near the surface have been more problematic on sunnier aspects, which this week, have taken a wallop from the sun—helping them heal. There are a lot of factors edging us in the right direction. The transition to spring has begun, but let's remember to be patient—it's important not to jump the gun. 

 

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