Bulletin d’avalanche

East Slopes North

Northwest Avalanche Center
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Danger d’avalanche

Sat
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Small wet loose avalanches on steep sunny slopes and shallow wind slabs near and below ridgelines are the most likely avalanche problems you'll encounter in much of the zone.  In eastern areas of the zone with a shallower snowpack, the buried surface hoar layer in the upper snowpack may be easier to trigger.  If you encounter whumpfing or rolling collapses, step back to simpler lower angled terrain without overhead exposure. 

Problèmes avalancheux

  • Wind Slabs

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

    Observers in the Cutthroat drainage found blowing snow along ridgelines on Thursday.  Wind slabs were small (less than 6") but some slopes were being cross-loaded with the recent shift to W-NW winds.  Along with shallow wind slabs, a fresh cornice (D1.5) had failed within the last 24 hours.   

    While winds will ease on Friday, you'll still need to contend with recent and newly formed wind slabs on a variety of aspects.  Use caution on steep slopes near and below ridgelines, and generally avoid confined chutes, or the base of cliffs where wind-drifted snow has accumulated.

    Visual clues such as newly formed cornices or pillow-like piles can help you identify which slopes to steer around. If you see red flag signs like shooting cracks or new avalanches, choose lower angled and more sheltered terrain. 

    (1/17/2023)

    Natural wind slab avalanches near the base of cliffs. 

    Photo: Katie Warren

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1.5–2.5

    While there are several weak layers to talk about, the one closest to the surface that we're targeting is the Jan 5th buried surface hoar layer.  In the Washington Pass area, this layer is about ~2' below the snow surface and hasn't been involved in any human triggered avalanches since 1/12.  Further east, observers on 1/19 in the Tiffany Mt area saw recent avalanches on this layer, shooting cracks and felt collapses, indicating that triggering an avalanche is far more likely in this portion of the zone. 

    Deeper weak layers involved in the very large Liberty Bowl/Blue L. Peak avalanche are detailed below and worth tracking when and if we return to an active storm pattern or other significant new snowpack stressors. 

    Depending on where you traveling in this spacious zone we call East North, you'll need to adjust your travel practices. Digging snow pits and looking for propagating test results can help you identify these weak layers in the top 2-3 feet of the snowpack in western portions.  Further east, you need to have a more conservative mindset.  Put plenty of space between where you travel and large unsupported start zones and shallow areas near rocks and cliffs.  Watch for long shooting cracks and audible collapses as you travel.  Persistent slab avalanches often surprise travelers, breaking further across the slope and around terrain features and/or from lower angled terrain.

    (1/19/2023)

    Crown lines from a recent Avalanche near Clark Peak (Tiffany Mt area). Probably released on buried surface hoar. (N 6900', HS-N-D2-R2-O)

    Photo: Willie Webster

  • Loose Wet

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

    In wind-sheltered areas, there are a few inches of recent snow from Wednesday's storm.  Another day of sunshine combined with slightly warmer temperatures on Friday should help increase the likelihood of small human or natural wet loose avalanches where this loose cold snow remains. On very steep slopes or extreme terrain features these avalanches might surprise you if you're knocked over.  If you see roller balls or natural wet loose avalanches, avoid terrain traps where a small avalanche could increase the consequences of a small avalanche. 

    (1/19/2023)

    You may see wet loose avalanches on sunny slopes Friday. 

    Ex: Wet Loose avalanche occurred as a result of a tree bomb during a period of sunshine on 1/19.

    Photo: Zack Novak

Avalanche Discussion

Professional observer Willie Webster traveled to the Tiffany Mt area on Thursday and found what is likely the Jan 5th surface hoar layer about a foot down.  This layer produced some whumps and indicated propagation was likely in snowpack tests.  Small wind slabs could be easily triggered below ridgelines on test slopes. 

Wednesday, NWAC observers went to take a look at the very large slide that occurred in Liberty Bowl/Blue L Pk. While a known failure date is not confirmed, this slide is believed to have occurred between 1/13 and 1/14. Upon investigation, it was determined that the upper crown failed on the Christmas Crust, while the step down was to the mid-November facets (11/22). A crown depth of 5-8 feet was measured.  This avalanche highlights the spatial variability with deeper weaknesses in the snowpack that come out in different ways during large-scale loading events. 

On Monday, observers at 6,200ft, NW in the Hairpin valley reported sudden and propagating test results on the Jan 5th surface hoar, 2 feet (70cm) below the surface. The results were very hard (beyond the limits of standard tests) and the surface hoar was small and rounding (0.5mm). They also reported moist snow, but no crust in the upper layers. 

A significant cycle of wet avalanches occurred last week (1/13 and 1/14) in the lowland hills with some avalanches gouging to the ground. Check out the media reel below and our observation platform to see more media of the impressive slides. 

Régions couvertes