Bollettino valanghe

West Slopes North

Northwest Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 6 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Avoid travel on or directly under steep slopes where you see fresh rollerballs, small wet avalanches, or where the snow is wet and sloppy. Wet avalanches and cornice fall can also trigger a much larger avalanche on the slopes below. Expect greater danger if rainfall is heavier than forecast or lasts longer into Saturday morning.

Problemi valanghivi

  • Loose Wet

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

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    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.

Backcountry observers reported that yesterday’s wet snow surfaces firmed up enough overnight to form a breakable crust below about 5000 feet. A few inches of soft snow topped the crust, but it didn’t do much to improve riding conditions. The crust became softer with elevation gain, eventually forming a stiff slab of moist snow over softer, drier layers below. This very shallow slab produced localized cracking and small reactive pockets on test slopes. Temperatures warmed slightly through the day, but only a few rollerballs were reported.

Regioni coperte