Bollettino valanghe

West Slopes South

Northwest Avalanche Center
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Pericolo valanghe

Sat
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Both wet loose and also slab avalanches in areas of recently wind drifted snow are on the menu Friday. Look for signs of instability in terrain overhead with warming conditions. Identify and steer around wind loaded terrain features like steep slopes just below ridgeline and convex rollovers.

Problemi valanghivi

  • Loose Wet

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

    Loose wet avalanches are most likely earlier in the day before clouds roll in and start blocking the sun. Some areas may have already seen significant sun, warming, and wet avalanche activity earlier in the week while others did not. Either way, when the sun is out in mid-April it's best to keep track of what’s going on above you no matter what the aspect or elevation.

    If you experience warm temperatures and strong sun, start looking for the usual signs of instability like wet, unsupportable snow surfaces, rollerballs, or small avalanches. Avoid travel on and directly under steep slopes where you see the above. If a firm crust tops the snow surface then wet loose avalanches are unlikely.

    (4/12/2023)

    Human triggered avalanche in wet snow near Crystal.

    Photo: NWAC Public Obs

  • Wind Slabs

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

    Fresh and reactive wind slabs were found Thursday. All observed slabs were small but easy to trigger with ski cuts or by dropping small cornice chunks on the slope below. They will gain strength overnight but should still be on your radar.

    Northerly winds built fresh slabs on south and west aspects where they are less commonly found. Tune into the pattern of wind loading by using scoured snow surfaces, cornice-like features, and changes in snow texture to figure out where wind slabs might lie in wait. Most slabs will be found just below ridgeline but you might find them farther downslope in some areas.

    Test slopes and hand pits can help you monitor differences in snow stiffness and layering as you change aspects and gain elevation. Evaluate the bond of wind slabs before easing into steeper terrain - or just find a lower-angled line around areas like convex rolls where you are more likely to trigger one.

    (4/13/2023)

    These wind slab clues helped us determine the local wind patterns and potentially wind-loaded slopes.

    Photo: Dallas Glass

Avalanche Discussion

Clouds and cool temperatures were the themes for Thursday. Most mid-mountain and ridgeline weather stations recorded high temperatures below freezing. Wind speeds for most locations stayed light after slightly stronger winds overnight and into the early morning hours. Paradise picked up a few inches of new snow.

An NWAC Forecaster in the Crystal Backcountry found northerly winds had redistributed the recent 6” of new snow onto southerly aspects. Reactive wind slabs were not widespread but still easy enough to find and trigger. Signs of instability like shooting cracks were common. The sun made an appearance in the afternoon after a cool and cloudy morning - expect crust early Friday. Though southerly slopes warmed enough to soften no new wet loose activity was observed. The crust below the recent snow is about 12” thick and mostly boot supportable. The layers of small rounded grains below the lower crust were observed as strong and well bonded.

The atmospheric river last weekend dropped 2"-3" of precipitation (SWE) and spurred a round of large to very large slab and wet loose avalanches across West South. Most notable were very large (D2.5-3) wet slab avalanches triggered by Crystal Patrol with explosives and 5 large to very large slides picked up by seismic equipment used by the USGS on Mount St. Helens. Look out for frozen debris in the runout of avalanche paths.

 

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