Bollettino valanghe

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Northwest Avalanche Center
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Pericolo valanghe

Mon
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Winds and fresh snowfall will create dangerous slabs you can trigger on wind-loaded slopes steeper than 35 degrees. Near and below treeline, you could trigger a storm slab on a steep or unsupported slope. Slabs atop weaker snow or poorly bonded crusts could lead to surprising avalanches.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Low-density snow will be easy to transport with this storm and you could easily trigger them. These slabs will grow with time, but will be most reactive during periods of heavy snow and wind loading. A cold storm arriving Saturday night and continuing on Sunday will load wind slabs onto potentially poorly bonded interfaces including facets, surface hoar, or crusts. Use your terrain selection rather than slope tests or pit results to avoid a challenging wind slab problem on Sunday. Shooting cracks, whumphs, or active wind loading are all warning signs. Avoid all wind-loaded slopes steeper than 35 degrees, particularly slopes below growing cornices or in areas where you find more snow to entrain above the 2/19 crust.

     

    Example: Wind slab on a steep slope.

    Photo: Sam Luthy

  • Storm Slabs

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

    Typically, it takes around 8" of snow with high precipitation rates for several hours to get storm slab instabilities and we're expecting about 8-10" by late Sunday. You might find it only takes 4-6" of new snow to form slabs on colder, shaded aspects that held weaker snow prior to the storm. On aspects with a surface crust prior to the storm, slabs or dry loose avalanches may run fast and far. Typical tools you use to evaluate slab reactivity may not give you a full picture and should not be relied upon. Steer around open slopes steeper than 35 degrees and recreate safely in the trees. 

    An example of a small storm slab. If the new snow is cohesive, you may be able to trigger similar avalanches. If not, it may run as dry loose.

    Photo: NWAC public obs

Avalanche Discussion

The new snow sits above settled cold snow with widespread weak surfaces existing on shaded aspects prior to the storm. These interfaces produced touchy slabs as winds arrived in the Mt. Baker backcountry on Saturday (see photo in gallery) and we expect similar to play out with any new snow or wind loading over these interfaces on Sunday. 

Reactive simple slope tests may confirm the problem, but an unreactive test shouldn't lure you onto steep, open slopes. Red flags including recent avalanche activity, whumphs, propagating cracks, or cleanly shearing interfaces all point to the problem. If these are present below treeline, the danger may be higher than forecast.

Observers reported fantastic riding conditions on Thursday and Friday, with those conditions limited to shaded aspects by Saturday. 2-3" of fresh snow had fallen since 2/22 with very low density "snow flurries" continuing to pile up.  8-10" of snow now sits over the 2/20 crust.  Snow had redistributed with a variable 2-16" of snow above the crust with the larger amounts on easterly aspects.

Snow pits at 5000 ft on an E aspect on Thursday show two crust sandwiches (see media gallery). The upper one was formed warmer daytime temperatures 2/20 and 2/19 with some snowfall in between. Snowpack tests show a slight weakness just below the 2/20 crust proper, but it does not appear concerning. The lower double crust formed around 2/13. The layer of snow between the two crusts has consolidated significantly since last week.

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