Bulletin d’avalanche

East Slopes Central

Northwest Avalanche Center
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Danger d’avalanche

Mon
Alpin
4 · Fort
Limite forestière
4 · Fort
Sous la limite
4 · Fort
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Points saillants

Heavy snow, strong winds, and warming temperatures will create very dangerous avalanche conditions on Sunday. Avalanches will run at all elevations and could be very deep and destructive. Avoid going into, near or underneath anywhere an avalanche could start, run and stop, including hiking trails and roads that cut across steep slopes.  

Problèmes avalancheux

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    1–2

    A foot or more of new snow will fall, and temperatures will rise significantly first thing in the morning on Sunday. Storm slabs may become easily triggered and could run naturally. These won't' be your light and fluffy kind, they will be thick and dense. Very strong winds will blow, creating thicker and more reactive slabs in any exposed, open slopes. You could trigger storm slabs from distance, and they could step down to become unsurvivable deep slab avalanches. Stay off of, and out from underneath avalanche terrain.

    Example storm slab in a wind loaded area from Hyak. [Photo: Erik Olson]

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    1–3

    Rain on snow, or wet snow on dry snow will cause wet loose avalanches. These could grow to become large on steep slopes. Wet slab avalanches aren't out of the question either, and become very large and destructive. Wet slides could run over common hiking trails and snowmobile trails in the zone. Stay off of, and out from underneath any steep slopes on Sunday, even at low elevations.

    Example wet loose avalanche at 3900'. It crossed the summer hiking trail at Eightmile creek and entrained a large old tree trunk. Photo Matt Primomo

  • Persistent Slabs

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

    Persistent slabs appear to be waking up. On Saturday afternoon a very large avalanche occurred in steep SW facing terrain at Stevens Pass, and a snowmobile triggered persistent slab occurred on Southeast facing terrain near Mission Ridge. Warming temperatures and heavy snowfall may overload this layer in a more widespread way on Sunday through Monday. As evidenced by this recent slide, and propagation tests at 4,000ft, persistent slabs can occur even well below treeline. Tests on the mid-January crust/facet combination that was responsible for many large to very large avalanches over a week ago continue to indicate propagation potential in tests. These scary avalanches could run into mature timber and become very destructive. Stay off of, and well out from underneath any steep open terrain at any elevation on Sunday through Monday.  

    (2/20/2021) This very large deep slab avalanche occurred in the afternoon on Saturday at Stevens pass. It appeared to be triggered by a skier but details are unknown. Southwest aspect at 5,000ft. Crown estimated at 4-5' deep.

Avalanche Discussion

Well, it has been a good stretch of cold, powder riding. Cities like Leavenworth and Plain have a fairly deep snowpack, and much of it is still dry snow while I write this. Unfortunately, that is about to change in a big way. An atmospheric river will rapidly alter our snowpack over the next two days, and we expect a significant avalanche cycle. We haven't seen rain above 3,500ft or so since mid-January, so one of the major concerns is that persistent slab avalanches 2'-8' thick may begin to come crashing down starting on Sunday. The avalanche cycle may last through the day Monday. Saturday's deep slab at Stevens Pass may be a precursor to more very large avalanches. In any case, large storm slabs and wet loose avalanches are expected to run from steep slopes as our once beautiful cold, dry snow becomes overloaded by wet, heavy rain and snow. This is not your average High avalanche danger, and we may reach Extreme danger by the end of Sunday. Stay off of, and out from underneath avalanche terrain. Be careful of roof avalanches on Sunday, snow will begin sliding off buildings with the warmup.

Over 5 feet of cold, dry snow has fallen near the Cascade crest since last weekend...! Storm totals tapered dramatically from west to east across the zone. Last week a handful of large natural avalanches were observed, and some avalanches appeared to have stepped down into older snow layers (see Miller Peak photo below). Large to very large avalanches were triggered Monday and Tuesday by Stevens Pass WSDOT in the new storm snow.  Smaller wind slab avalanches were intentionally triggered in the Mission Ridge area during control. 

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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