Bulletin d’avalanche

East Slopes Central

Northwest Avalanche Center
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Danger d’avalanche

Tue
Alpin
5 · Très fort
Limite forestière
5 · Très fort
Sous la limite
4 · Fort
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Points saillants

Heavy snow, strong winds, and warming temperatures will create extremely dangerous avalanche conditions Sunday night through Monday. Avalanches will run from upper elevations, could be very deep and destructive, and could break trees. 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

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    3–4

    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 terrain in the mountains through Monday.  

    (2/20/2021) This deep avalanche likely failed on the mid-Jan crust Saturday near Stevens Pass. S, 4900' Photo: Wash DOT NCR.

  • Storm Slabs

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

    Two to three feet of recent, heavy dense snow is rapidly accumulating over lighter, lower density cold snow from the past month. More new snow will fall, and temperatures will continue to slowly rise through Monday afternoon. Storm slabs will run naturally, and could be very large. 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
    Likely
    Taille
    1–2

    Rain on snow, or wet snow on dry snow will continue to cause wet loose avalanches. These could grow to become large on steep slopes. Wet slabs and glide avalanches aren't out of the question either, and may 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 Monday, even at lower 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

Avalanche Discussion

The brunt of an atmospheric river will arrive Sunday night into Monday, will rapidly alter our snowpack, and we expect a significant avalanche cycle in the mountains near the crest. We haven't seen rain above 3,500ft or so since mid-January, and with 5 feet of cold, dry snow on a crust near the crest, one of the major concerns is that persistent slab avalanches 3'-8' thick may begin to come crashing down. The avalanche cycle(s) may last through the day Monday. The recent avalanches on the mid-January crust may be a precursor to more of these 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 continues to be overloaded by wet, heavy rain and snow. During Extreme danger, we may see avalanches occur in areas where they don't often happen. Slides may run to valley bottoms, and fill up runout zones. Stay off of, and out from underneath avalanche terrain. Be careful of roof avalanches off buildings, as snow will begin sliding off with the warmup.

WSDOT reported triggering a D3 with control early in the morning on Sunday, in an avalanche path that has only run three times in the past ten years. A snowmobile triggered a D2 persistent slab near Colockum on Saturday (see photo below). A handful of other excellent observations have come in from the zone (Thank you!), be sure to check them out. 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). 

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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