Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Fri
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

To reduce the chance of triggering avalanches, avoid slopes where the wind recently formed drifts or slabs. In areas more protected from the wind, triggering a 1-2 foot thick avalanche is less likely but not out of the question. Pay attention to the consequences of being caught in even small slides in terrain with rocks, cliffs, trees, and terrain traps.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2

    What is it?  Multiple weak layers exist in the upper part of the snowpack. The most prominent of these is buried surface hoar down 1.5-2 feet on many sheltered, shady (W-N-E) slopes. This layer produced a number of triggered slides during the second half of January. It's showing signs of improvement, but surface hoar is notoriously unpredictable. Newer weak layers consisting of facets or crust+facet combinations formed during recent dry spells and were recently buried. Recent observations from the Sawtooths report some early signs of instability on these layers.

    Where is it? You're most likely to find trouble on slopes steeper than about 35 degrees where the wind has formed drifts or slabs or stiffened the snow surface. The surface hoar is most widespread and problematic in somewhat protected, middle-elevation terrain that faces W-N-E. Surface hoar is a tricky weak layer. It can be "spotty" in its distribution—it may exist on some slopes, but not others. The newer, more shallowly buried weak layers exist on most aspects. 

    How can you recognize it?  Wind-affected slopes will have stiffer snow at the surface, keeping you from sinking deep into the snowpack. You may or may not experience snowpack collapsing and/or cracking while traveling. These red flags are obvious signs of instability. The layer of surface hoar is subtle and may not "jump out at you" when searching for it in a snowpit (see photo here).     

    How can you manage it?  You're most likely to trigger persistent slab avalanches involving either the surface hoar or the shallowly buried facets/crust+facet combos on slopes steeper than about 35 degrees with stiff wind-blown snow or slabs.You can reduce the chances of tangling with sizeable surface hoar-driven avalanches by avoiding very steep (greater than about 35 degrees), shady (facing W-N-E) middle elevation slopes. Persistent slab avalanches may break wider than expected or higher on the slope above you. Both of the weak layers could produce an avalanche large enough to bury you, but even small slides can be dangerous if they wash you into a gully, through trees, or over cliffs. 

    (1/23/2023) Avalanche apparently triggered by a skier on Banner Summit. Slab depth and terrain characteristics are consistent with early January surface hoar layer. 7250', NNW. The date of this avalanche is uncertain, but likely occurred since Jan 18th. Photo taken Jan 23rd.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    What is it? Expect to find some stiff or hard wind slabs in exposed terrain. Sunday's new snow and wind produced some natural avalanche activity, and today's snowfall and expected strong winds could continue building slabs.Though not huge, these wind slabs could be sensitive to your weight and are stressing the buried weak layers described in Problem 1 above. 

    Where is it?  You may find wind slabs near ridgelines, on the sides of gullies and chutes, near prominent mid-slope terrain features, and on aprons below chutes and couloirs. 

    How can you recognize it?  Cracks shooting out from your skis or sled are an obvious sign of instability. Watch for drifts, dunes, and ripples on the snow surface. Wind slabs can have a smooth, chalky appearance and feel and sound hollow, like a drum. Wind-deposited snow will be stiffer and keep your skis or snowmobile on or closer to the surface.       

    How can you manage it?  The easy move is to avoid obviously wind-loaded slopes steeper than about 35 degrees. Hard slab avalanches can behave unpredictably, breaking above skiers and riders. Small triggered wind slabs have the potential to "step-down" to buried persistent weak layers (Problem 1) lower on slopes, causing a larger destructive avalanche.  

    (2/6/2023) Cornices and evidence of recent wind loading in the Banner Summit Zone, south of Copper Mountain.

Avalanche Discussion

Wind-affected snow remains the most dangerous piece of the puzzle to triggering an avalanche. Slopes that house several weak layers can be stressed by this added weight on top of the existing slab. Recent storms are slowly building a slab over weak layers in the upper snowpack. We have yet to see a lot of avalanche activity on this layer, but snowpit test results continue to indicate poor results. The most recent avalanche occurred on buried surface hoar released near Titus Ridge on January 29th (report). This layer is showing signs of improvement. However, surface hoar is an unpredictable weak layer; most of us aren't ready to write it off just yet. In the Galena Summit & Eastern Mtns, and Soldiers Mtns, older weak snow exists where the snowpack is less than 3 feet thick. You are more likely to find this issue in the alpine, where slopes steeper than about 35 degrees have rocky, wind-affected thin trigger spots can be found. 

Moderate to strong northerly winds have transported new snow creating hard and soft slabs of the Forecast area. Most avalanches have been small, but where they overlay persistent slabs is the most concerning.  In the mountains surrounding the Wood River Valley, wind slabs are more dangerous than persistent slabs. 

A stockpile of cold days created an upper snowpack mainly composed of faceted snow (most prominent in the Wood River Valley). This weak snow doesn't like to bond well with other snow. Playing in terrain steeper than about 35 degrees can produce "facet-lanches" in this type of snowpack. These avalanches start at a point and entrain more weak snow as they descend the slope (see field report). They aren't usually big enough to bury you. However, they become dangerous when riders get knocked over and carried through trees, cliffs, gulleys, or other terrain traps. This loose snow problem can also occur when the intense, strong sun comes out today. Sunny, rocky slopes steeper than about 35 degrees can produce wet loose slides as the sun rotates around the compass and melts the snow. 

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