Bulletin d’avalanche

West Slopes Central

Northwest Avalanche Center
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Danger d’avalanche

Sun
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 6 months ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Signs of wet instability—rollerballs, small wet avalanches, or sloppy snow—mean it’s time to back off steep terrain. Wet avalanches and cornice fall may also trigger larger slides on slopes below. Expect conditions to worsen if rainfall is heavier than forecast or persists into Saturday morning.

Problèmes avalancheux

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–1.5

    Warm temperatures, high snow levels, and light rain will keep wet avalanches on the radar Saturday. Most rainfall should occur overnight, but drizzle and mild temperatures may continue to add moisture to the upper snowpack through the day. The already-moist and dense top foot of snowpack may handle some rain. However, softer and drier layers can still be found above about 5000 feet and may become reactive with additional rain or warming. 

    Cornice fall is also a concern. Large ridgeline cornices could weaken with rain and warm temperatures, potentially triggering avalanches on the slopes below. Shallow wet slab avalanches are also possible at upper elevations. Keep track of terrain above you and be ready to limit your exposure if conditions become more active than expected.

    (4/5/2021)

    Rollerballs released beneath this small cornice as slopes warmed. This is oftentimes a precursor to wet loose avalanches. 

    Photo: Andy Harrington

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    2–3

    The likelihood of a person triggering a persistent slab avalanche is very low considering the amount of precipitation over the past four days, the time and warm temperatures that allow weak layers to settle, bond, and gain strength, and the isolated distribution of the problem.

    The concern is that cornice fall or a smaller natural wet avalanche could provide the larger trigger needed to release a persistent slab. Give large cornices a wide berth—definitely don’t have lunch in the rain under one. Also, avoid traveling below steep slopes where you observe fresh wet avalanches or other signs of instability.

    Example of a slab triggered by a cornice shedding off the ridgeline above.

     

Avalanche Discussion

If weather conditions play out as forecast, avalanche danger could effectively be Low, with most wet avalanche activity occurring overnight. However, the day could feel quite Considerable if a falling cornice triggers a slab on the slope above you. Fortunately, an event like this is very unlikely and generally easy to anticipate by evaluating the terrain ahead.

Observations from across the central Cascades show variable conditions by elevation and location. In northern areas, wet snow surfaces firmed overnight into a breakable crust below about 5000 feet with a few inches of soft snow on top. Higher up, the crust transitioned into a shallow slab of moist snow over softer layers below, producing localized cracking and small reactive pockets on test slopes. Around Stevens Pass, 4 to 6 inches of new snow fell Thursday with moderate to strong west winds, but little wind transport or slab formation was reported.

The likelihood of triggering a persistent slab appears to be decreasing, though isolated signs of instability—including whumphing reported near Lake Valhalla over the weekend—suggest the problem has not disappeared entirely. Snow coverage remains below average in many areas, leaving exposed logs, creeks, rocks, and other obstacles as ongoing travel hazards.

Régions couvertes