Lawinenlagebericht

Banner Summit

Sawtooth Avalanche Center
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Lawinengefahr

Thu
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 4 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

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.

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    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

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    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

While there are still some concerns about deeper weak layers—especially in the windswept, alpine reaches of the Pioneers, Boulders, and White Clouds—the limited recent avalanche activity occurred in the upper 1-2 feet of the snowpack.

If you've been following the forecast, you've read about a layer of surface hoar that was buried in early January. This layer was most reactive in the Sawtooths & W Smokys and Banner Summit zones that received the most snowfall, but triggered slides also occurred in the Soldiers and near Galena Summit. The most recent avalanche on the 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, especially during a snowfall + wind-loading event.

Dry weather during the second half of January produced newer weak layers near the snow surface, ones that we're more familiar with:  facets and crust+facet combinations. Since January 27th, two smallish storms (both combined with wind) slowly loaded these newer weak layers, predominantly in our northern zones that received more snowfall. Early signs of instability such as cracking and poor snowpack test scores were observed last week in the Sawtooths, but there hasn't been quite enough snowfall yet to create a widespread problem. The current storm will continue to incrementally load these new weak layers, making isolated avalanches involving these layers possible.

While the danger is MODERATE and conditions aren't especially dangerous, there is enough complexity in the upper snowpack—primarily in the mountains north and west of the Wood River Valley—that it's worth using additional caution. Slopes over 35 degrees, especially those with recent wind loading, deserve respect. Before committing to riding or skiing in avalanche paths, spend a bit of time thinking about the consequences of triggering and being caught in a slide. Even small avalanches can cause big, bad outcomes on slopes with cliffs, trees, and terrain traps. 

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