Bollettino valanghe

West Slopes North

Northwest Avalanche Center
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Pericolo valanghe

Fri
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Thursday will bring the warmest temperatures and strongest solar radiation since the storm ended this past weekend. Stay alert to changing conditions as concern for wet loose avalanches will increase throughout the day. At upper elevations, continue to steer around areas of wind drifted snow on slopes greater than 35 degrees. 

Problemi valanghivi

  • Loose Wet

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

    Loose wet activity occurred each day since Monday, but cooler temperatures and afternoon clouds limited a widespread cycle. Thursday we expect the most wet avalanche activity yet. The sun will first need to break down a melt-freeze crust on solar aspects. By the time this happens, snow surfaces should be wet, weak, and primed for loose wet avalanches. Monitor changing conditions throughout the day, and keep your return routes in mind. The loose wet hazard will be limited early morning and where you find firm and frozen snow surfaces. The best chance for loose wet avalanches will be during the afternoon, especially on south and west aspects. Watch for snow shedding from trees and rocks, and rollerballs and pinwheels following soon after. Although most loose wet avalanches are expected to be small, some could grow large and run far downslope. Steer around steep terrain with evidence of wet snow and move to shaded or lower angled slopes to avoid the problem. 

    Example photo of loose wet avalanches crossing common travel routes on the south side of Table Mountain

  • Wind Slabs

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

    You are most likely to trigger a lingering wind slab above 4500ft on northerly convex slopes steeper than 35 degrees and in areas below cornices or rock bands. Several human-triggered wind slabs and a few close calls occurred over the past several days. Fortunately, there were no full burials or injuries. Much of the recent avalanche activity occurred above treeline, but observers on Tuesday reported shooting cracks and reactive wind drifts near treeline on Mt. Herman. Avalanches in wind drifted snow could break 6in-3ft deep on a variety of mid-storm interfaces or on a crust that formed up to 6000ft on March 4. Continue to approach upper elevation terrain cautiously, and give cornices a wide berth. These unpredictable features are large and overbuilt and can break further back than you may expect or fall onto slopes below acting as huge triggers. Seek out terrain less than 35 degrees where you find recent avalanches, cracking, collapsing, or evidence of wind drifted snow. 

    (3/8/2021) A snowboarder-triggered avalanche on Shuksan Arm that broke 1-3ft deep, 200ft wide, and ran 300 vertical feet. The snowboarder dropped the cliff above the slope and triggered the avalanche upon landing. They were caught and carried but remained on the surface with no injuries. Photo: Pete Durr

Avalanche Discussion

From March 4-7, over 2ft of new snow accumulated above 4000ft throughout the West North Zone. Deposits of wind-blown snow are now a few days old, but still reactive to human traffic. Several human-triggered wind slabs were reported March 6-8, along with natural wet loose activity during periods of sunshine March 8-10. Triggered wind slabs occurred in upper elevation terrain on Shuksan Arm, Table Mountain, and on the south side of Mt. Baker. An impressive icefall avalanche also occurred on Mt. Shuksan's Hanging Glacier on Tuesday (photo below). Be heads up with these transitional spring conditions as ice fall and cornice fall may continue Thursday. Minimize exposure to these and other overhead hazards especially if you venture into the alpine. 

There are dormant layers (mid-January PWL) in our snowpack buried over 10ft deep that produced a widespread avalanche cycle in the Hwy 542 and Hwy 20 corridors in early February, and isolated but very large natural avalanches as recently as February 25. In other parts of the region (West Central, Stevens, Snoqualmie) very large and widespread natural cycles on the mid-January layer occurred in late February, and as recently as March 1. With quiet weather, no new deep slab avalanches for 1-2+ weeks, and consistent and favorable results in snow profiles and tests, there is enough evidence and confidence to remove it as an official problem for now. Regardless, we will continue to track the Mid-January PWL throughout the region for the remainder of the season. We will not soon forget the very large deep slab avalanches from February, and recognize these types of avalanches are challenging to accurately assess and forecast. It is not out of the question for large triggers like cornice fall to still cause a deep slab avalanche, and a significant weather event may even be enough to reactivate the layer and bring the problem back to life at some point again this season.

During the ongoing Covid-19 pandemic, continue to follow the latest guidance from local, state, and federal health officials. Avoid unnecessary risks that could impact first responders and stress medical resources.  Wear a mask when appropriate, practice social distancing, and consider recreating closer to home.

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