Bollettino valanghe

West Slopes South

Northwest 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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Non aggiornato — questo bollettino è scaduto 3 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Take the time to assess new and wind-drifted snow layers before considering steep terrain. Avoid slopes over 35 degrees to reduce uncertainty and the likelihood of triggering a slab avalanche. Limit travel on or directly under steep slopes seeing direct sun or where the snow is mushy and wet. Anticipate locally higher danger on the volcanoes and other locations that received a foot or more of new snow.

Problemi valanghivi

  • Wind Slabs

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

    New snow and wind on Tuesday and Wednesday likely built fresh, mostly shallow slabs, especially in areas with the heaviest snowfall. These are generally easy to identify and avoid by looking for cornices and wind-scoured snow surfaces, tracking snow depth and stiffness, and other clues. 

    The bigger issue is where wind transport over the past week built thick slabs that have loaded up weak layers above the crust buried in early February. While not widespread in distribution, avalanches were likely triggered at these layers based on recent observations. It’s enough of a problem that it should be near the top of your list of concerns for Thursday.

    How to avoid triggering one? For one, this layering setup is generally found on northeast through west aspects but could, theoretically, be found anywhere. You can also track the snow stiffness and depth over the thickest crust. Where there is more and stiffer snow, you are more likely to trigger an avalanche - pretty simple. Snowpits and testing in representative terrain might also help you evaluate stability. Finally, you can just avoid steep wind-loaded slopes by using lower-angled terrain to navigate around potential start zones.

    (2/19/2024)

    Crown of a thick human-triggered wind slab avalanche.  North aspect at 6300' near Crystal.

  • Loose Wet

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

    Thursday isn’t forecast to be completely bluebird, but there is enough clearing and sun that wet avalanches are possible. The extent of wet avalanche activity will hinge on cloud cover, temperature, and the snow surface in each area of the forecast zone.

    Fresh, cold snow is nearly always more reactive to warming and sun than snow that has already seen some combination of sun, warmer temperatures, or rain. Thick crust takes longer to break down and become wet and uncohesive. Slopes scoured down to crust by strong winds will not likely produce much when it comes to wet avalanches On the other hand, deep new snow at upper elevations on St. Helens and other southern locations could produce some large ones on slopes facing the sun. Keep track of what is overhead as the day warms and the sun comes out.

    (1/23/2021) 

    An example of the kind of wet loose avalanches you could see on Thursday.

    Photo: Susie Glass

Avalanche Discussion

Observers around Crystal reported winter-like conditions with snowfall and light to moderate wind transport in exposed areas.  6” of new snow was observed in wind-sheltered terrain.  No observations were submitted from other parts of West South.

The Crystal Moutain patrol triggered a fresh wind slab avalanche with a cornice drop on a north aspect at 6600’. It released within recently wind-drifted layers as opposed to faceted snow above the thick rain crust buried in early February. No new avalanches were reported from backcountry terrain. 

Precipitation amounts varied widely across West South. Crystal and White Pass weather stations recorded about 0.25”of precipitation, while Paradise got a half-inch since Tuesday afternoon. Many weather stations in the southern reaches of the forecast zone received about 1” of precipitation, with the Swift Creek Snotel near St. Helens picking up over 2”. Precipitation generally fell as rain or very wet snow at low elevations. 

The upper snowpack consists of layers of new and settled snow, crusts, and facets over the Early February Crust (EFC), often 12-30 inches (30-75cm) below the surface. The exact depths and layering depend on aspect, elevation, and exposure to sun and recent strong winds. A layer of facets has been consistently identified above the EFC at Crystal, Paradise, and White Pass.

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