Bollettino valanghe

Northern Wallowas

Wallowa Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Poor snowpack structure and recent loading have created a persistent slab problem. Where a foot or more of cohesive snow (slab) exists above weak buried facets it is possible to trigger an avalanche large enough to bury a person. The only way to control your exposure to avalanches is to avoid terrain steeper than 30 degrees, including runout zones.  

This forecast is sponsored by Robert Brooks Painting in La Grande.  

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2.5

    The thickness of the overlying slab varies from 1.5-2.5' (or more with wind loading). The persistent weak layer below can be summarized:

    • On shady and wind-sheltered northerly aspects, February's snow has not bonded well to the 10-12" of weak, sugary facets and/or surface hoar.  
    • On solar aspects, a buried melt-freeze crust may provide a slick bed surface and harbors weak sugary snow below it.  We have had one confirmation from the S Wallowas of a persistent slab triggering below the 1/31 melt-freeze crust.  

    With each day of snow or wind loading, the weak layer is stressed more.  There is uncertainty about when it reaches its tipping point on a particular slope and is reactive to a human trigger.  

    (2/7/2025)

    The surface hoar layer propagated in numerous PST's showing its vulnerability.  

    Photo: Victor McNeil

  • Wind Slabs

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

    Winds increased in the afternoon, creating fresh wind slabs at mid and upper elevations.  An observation from the Flagstaff Butte area highlighted the rapid stiffening of the snow surface, as shooting cracks were present.  If you see signs of wind slab activity such as dune-like formations, textured snow surface and certainly shooting cracks steer clear of slopes >30 degrees.  Adding a slab along with a trigger to the poor structure we've observed could ruin your day.  

    (2/5/2025) Photo: Caleb Merrill

Avalanche Discussion

A persistent slab needs two ingredients: a weak or soft layer of persistent grains (the facets we've been finding for weeks) and a stronger or harder layer above (the new snow this week).   As the name suggests, the persisting layer will stick around and be slow to strengthen. 

What to do:   A persistent slab avalanche needs two more ingredients: a slope greater than 30 degrees to initiate, and a natural or human trigger.  With uncertainty about which slopes have the right combination of slab ingredients to be unstable enough, you can hedge your bets by choosing low-angle terrain or, at the least, inconsequential pieces of avalanche terrain. 

As always, be on the lookout for the freebie signs of unstable snow: recent natural avalanches, shooting cracks, and whumphing of the snowpack.   With a persistent slab problem, the absence of these clues does not mean that a slope is safe to ride, and careful evaluation should be used. 

Observations:  We had an observation from a group skiing East of Flagstaff Butte yesterday in the SW Wallowas of increasing winds in the PM, building fresh wind slabs.  Forecasters traveled to West Eagle Meadows on Friday and found the poor structure that is the culprit of our persistent slab problem.  Thanks to Motorized Pro Observers Danny O'Neal and Ryan Phillips for their observations you can read here and here.  

Elkhorns:   Forecaster Tom Whipple traveled in Angell Basin Thursday and found unreactive soft windslabs above 7900' and a persistent slab structure on northerlies Near Treeline, but not Above Treeline.  

Add value to your avalanche forecast.  Help us reduce uncertainty.   If you get out today, please submit an observation sharing where you traveled and if you saw avalanches or signs of instability.  How deep your boots sink into the snow is an easy gauge for how much snow the wind might transport.  

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