Bollettino valanghe

Stevens Pass

Northwest Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

Heads up Monday: Expect a stormy, dynamic, and ultimately dangerous day. New snow, rain, and warming temperatures are expected to increase avalanche danger at all elevations. You could easily trigger storm and loose wet avalanches. Avalanches could be large and surprising above 5,500ft, where buried wind slabs and persistent weak layers may fail. 

Problemi valanghivi

  • Loose Wet

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

    Rain on snow and warming temperatures is expected to set off a round of wet loose avalanches on Monday. Expect these to grow larger as the day warms up, and as the rain line creeps up the mountain sides. Signs of wet loose avalanche activity are rollerballs, and fresh fan-shaped avalanches with chunky looking debris.

    Stay off of, and out from underneath very steep slopes once rain begins to fall on snow. Be especially careful of going underneath larger steep slopes where slides could pack more of a punch and hit you from above. If you do go out, plan to avoid this avalanche problem because the day will be dynamic with changing conditions.

    Example large wet loose avalanche

    Photo: ONP Rangers

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2.5

    New snow falling with warming temperatures should continue to pile up into the afternoon on Monday. The snow should stack up on top of colder, drier snow from Sunday. Warming temperatures are expected to create unstable and upside-down snow, and storm slabs could slide easily off steeper features at all elevations. 

    Plan to stay out of most avalanche terrain on Monday. Look for signs of instability like shooting cracks and small recent slides. Avalanches could be large, or even very large at higher elevations where the new snow will be covering up older wind slabs, and a deeper layer of faceted grains over a crust. Above 5,500ft, NW-NE aspects harbor weaker, faceted grains over a crust from early March. This layer could be down close to 3ft deep in some areas by Monday, and any avalanche that runs on it would be very large.

    Example storm slab avalanche.

    Photo: Stephen Semanski

Avalanche Discussion

Expect a significant increase in avalanche danger from Sunday to Monday, as a warm front sends snow levels to 5,000ft during the daytime and creates dangerous conditions. Conditions should be dynamic, and what you find first thing in the morning may be much different a few hours later. With the weather forecast models uncertain on how this storm will play out, the avalanche forecast is uncertain. High avalanche danger may occur if the storm shifts a little further to the east, so the forecast may be updated by 0700 on Monday morning. In either case, expect dangerous conditions with good chances for natural avalanches to run.

On Sunday observers triggered small windslab avalanches in the new snow, and small loose avalanches on steep features facing the sun. We've also been tracking a layer of faceted grains just above a crust from early March. Observers in the Chiwaukum range consistently found the March 8th facets above 5,000ft and reported significant wind loading on Saturday. On Friday, forecasters on Jim Hill and in the White Pine drainage confirmed the presence of the 3/8 facets and test results that indicate they could produce slab avalanches. See images and videos below. The layer could produce surprising avalanches where it exists under drifted slabs and may get overloaded with the storm on Monday. You can find the weak layer 2-3 feet below the surface on NW-NE aspects above 5,500ft.  Elsewhere in the Cascades, this layer has produced very large slab avalanches, and it may just be a matter of time in the Stevens Pass zone.

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