Bollettino valanghe

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Northwest Avalanche Center
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Pericolo valanghe

Wed
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

While the new snow is welcome, winds will continue to crank through Tuesday afternoon and transport new snow onto lee slopes. This is a good day to avoid all wind-loaded terrain where you might trigger a large avalanche. Travel with a large margin of safety in parts of the zone where deep persistent slabs have sporadically released over the last 2 weeks. 

Problemi valanghivi

  • Storm Slabs

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

    Think about wind and storm slab avalanches Tuesday.  Avoid steep open slopes near and above treeline, especially closer to the Cascade crest where more snow is forecast.  Assume you can trigger a larger avalanche on lee slopes where you see winds actively transporting snow.  Smaller storm slabs are possible at lower elevations or in more wind-sheltered terrain.  Avalanches may break within new snow instabilities or further down to the old snow surface formed prior to this storm cycle (sun crusts on previously sunny aspects or weak faceted snow on shady aspects).  Look for signs of instability like cracking in the new snow as you travel. Use small test slopes as a gauge before committing to more consequential terrain.   

    While conditions will cool during the day on Tuesday, you may find areas of wet punchy snow lingering at lower elevations during the day.  Use caution on very steep slopes if you find yourself sinking in deeper than your ankle. 

    Avoid terrain where avalanches within the new snow may overlap with the deep persistent slab problem. 

    Slab avalanches are most likely on steep rollovers like the one in this photo.

    Example of a natural storm slab from earlier this winter in the Hairpin valley. 

    Photo: Adam Butterfield

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    While it's unlikely you'll trigger a deep slab avalanche in much of the zone, the area near Harts Pass continues to be the area we're most concerned about based on recent information.  Here the persistent weak layer is about 3' below the surface. 

    What can you do to manage this low-likelihood/high consequence problem?   You can limit your exposure to large start zones and very steep slopes, especially those with big overhanging cornices (see photo).  Also, avoid complex and unsupported terrain, shallow, rocky areas, and unsupported slopes (see photo in Drew's ob).  

    On March 3rd and then again on the 9th, backcountry travelers in the Harts Pass area triggered large to very large avalanches on northerly aspects near treeline.  A cornice-triggered avalanche may have also occurred around the 8th or 9th on McGregor Mtn. You can find the late January weak layer (facets sandwiched between two crusts) 3-5' below the surface. Snowpack tests can help you identify the layer of concern. Observers continue to report sudden or propagating test results even as avalanches have become difficult to trigger. East of Washington Pass, the snowpack becomes shallower, and this interface may be nearer to the snow surface.

    (3/10/2022)

    In the center of the photo is a recent D2.5 deep slab, likely triggered by a cornice fall. 

    To the right of the photo, you can see a similar-sized avalanche that likely occurred during the natural cycle of 2/28-3/1.

    McGregor Mtn, NNE, 7,200ft.

    Photo: Matt Primomo

Avalanche Discussion

Following the late February/early March atmospheric river,  it's been a dry first half of March.  We haven't seen significant avalanches since March 9th when observers near Harts Pass remotely triggered a large, R3-D2 avalanche from 20-30 feet away. The avalanche occurred roughly around 6,600ft on a north aspect and failed on facets above a crust.  However,  a crown profile revealed a uniquely shallow snowpack suggesting previous avalanches may have already run on this interface.  On March 3rd, a size D2.5 avalanche was triggered nearby on Tatie Peak on a northeast aspect at 6,800ft. Fortunately, no one was caught in either of these avalanches.

Another avalanche likely occurred on the late January weak layer with cornice fall around March 8th or 9th on McGregor Mtn, NNE, 7000ft. Observers reported other deep crowns in the Hairpin Valley and near Azurite Pass after a widespread avalanche cycle on 2/28-3/1. 

In the upper snowpack, a rain crust can be found following the late February/early March atmospheric river up to around 6000' in most parts of the zone.  You may continue to find challenging travel conditions as lower elevations where you encounter shallowly buried avalanche debris or thin snow cover. 

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