Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Mon
Alpin
1 · Faible
Limite forestière
1 · Faible
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 7 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Avalanches are unlikely and most slopes are safe, but increasing wind speeds today may build isolated but touchy slabs in upper elevation terrain. Watch for these isolated wind drifts and cornices at and just below ridgelines. While small, encountering one of these slabs in very steep terrain could have high consequences. 

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1

    Last week, this zone picked up 5-10" of low-density snow. I spent a few days skiing in the Sawtooths (just south of this zone) this week and found a few natural slab avalanches, like the one pictured here. Chris was skiing near Banner Summit and found similar conditions (photo at right). These avalanches were failing in areas where winds had stiffened the snow surface and built slabs. In many areas, however, this new snow fell without wind leaving a nice blanket of powder. With time to rest, this generation of slabs has become much less sensitive to triggering.

    In the days since the last snowfall, cold temperatures, light winds, and plenty of sunshine dominated the weather. These conditions generated a combination of crusts and facets on slopes that directly face the sun and facets on more shaded aspects. Today, for the first time since last weekend's storm, wind speeds are forecast to increase. This wind will pick up the loose snow left over from last week's storm and build slabs in exposed, upper elevation terrain. As they increase in size today, these slabs are likely to be touchy, thanks to the weak snow surfaces that they will be deposited on. Snowfall starting late this afternoon will give the wind additional building material for wind slabs.

    While these wind slabs are likely to be small, encountering one in very steep, rocky terrain could have high consequences.

Avalanche Discussion

The weather pattern for our neck of the woods has been fairly stable for the month of February. A stubborn ridge of high pressure docked along the west coast has forced moisture to our north and kept things high and dry here in central Idaho. But even though the weather has stagnated, the snowpack never sleeps. Solid materials (snow) near their melting point are in a constant state of change. If you've been out this week for a few days in a row you've probably noticed some of these changes. Maybe the skiing actually improved in your favorite powder stash as the snow surface weakened and faceted. These facets can make for some good riding conditions ("recycled powder"), but they also make for reactive weak layers.

Today, a short-lived flattening of the ridge will allow for a little moisture to squeak into central Idaho. Ahead of this moisture, we should see an uptick in wind speeds for the first time since last weekend's storm. In our northern forecast zones, a good bit of unconsolidated snow (the remnants from that storm) can be found, particularly on shaded slopes. Our southern areas received very little snowfall during this storm, but the snow has faceted significantly, leaving a layer of weak snow resting on the surface. Across our forecast zones, today's increasing wind speeds will be picking up whatever loose snow it can find and transporting it. Near features that slow the wind down and force it to drop the load that it is carrying, such as ridgelines and mid-slope features like ribs and gullies, fresh slabs will build. These slabs will be much thicker and touchier in our northern areas because there is more snow available for transport, but even down south we may see some isolated wind slabs building. Across the region, these slabs will be deposited on the weak snow surface that formed this week.  

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