Bollettino valanghe

East Slopes Central

Northwest Avalanche Center
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Pericolo valanghe

Sun
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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Strong winds have drifted recent snow into slabs on open slopes at all elevations. These may break in surprising ways, or could step down to weak older snow layers to become very large avalanches. Limit your exposure to steep slopes at upper elevations, and avoid areas of fresh or firm wind drifted snow on steep, open slopes. 

Problemi valanghivi

  • Wind Slabs

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

    Strong to even extreme westerly ridgeline winds have transported much of the recent snow into reactive slabs on lee slopes, and will continue on Saturday.  You'll find firmer wind-affected snow near and above treeline and even on open slopes below treeline.  Shooting cracks or hollow-sounding snow are signs you could trigger an avalanche on similar nearby slopes.  Avoid open slopes steeper than 35 degrees where you find freshly drifted snow, sticking to lower angle or wind-sheltered terrain.  

    In wind-sheltered terrain and for areas near the Cascade crest, you may trigger storm slab avalanches up to a couple feet thick.  Steer around convex rollovers and very steep unsupported slopes in areas you find deep, recent storm snow.

    Example of a skier triggered wind slab on a steep, wind exposed north aspect at 5,800ft (NTL). Windy Knob, near Blewett Pass.

  • Persistent Slabs

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

    We know there are several weak layers 2 - 5 feet down below the surface. In the central and eastern parts of the zone, you can dig down in the snow to look for a thin layer of surface hoar or weak facets from February 11th (see media gallery below). At and below around 3,500ft you may find very weak faceted grains over a recent stout crust to be wary of on steep slopes or windloaded areas.  Above that elevation the mid-January crust/facet combination that was responsible for many large to very large avalanches over a week ago is still on our radar, and recent tests continue to indicate propagation potential.  

    While it's been becoming less likely you'll trigger a persistent slab avalanche, they can be unpredictable and very large avalanches. Look for indications you could trigger one of these such as whumphing collapses and shooting cracks. Closer to the Cascade crest, persistent slab avalanches breaking beneath the recent storm snow could be very large and destructive.  For areas further east, it may be easier for you to affect these layers directly.  Either way,  continue to limit your exposure to large and consequential avalanche paths and avoid putting your entire group in a hazardous area.

    (2/9/2021) Persistent slab avalanches that may have run naturally on an east aspect at around 4900 ft in the Knox Creek drainage above Cle Elum Lake. These are suspected to have run on the mid-January crust later in the storm cycle of Feb 5-9th.

Avalanche Discussion

Most areas picked up 4"-7" of snow into Friday morning, with the exception of areas near Stehekin and Lyman lake, where over 13" fell.  Winds drifted snow at all elevations, and reactive windslabs were observed resting over buried surface hoar and/or suncrust in the 3-4,000ft range in both Icicle Canyon and Blewett Pass (see photo below) on North to Southeast aspects. Underneath the recent snow,  persistent weak layers lurk in the form of facet/crust combos and buried surface hoar.  As we enter a new storm cycle,  there haven't been reports of persistent slab avalanches since the weekend (remotely triggered in the Icicle drainage) and 10 days since human-triggered very large avalanches, which included an avalanche fatality, released down the mid-Jan facet crust combo.

Over 4.5 feet of snow has fallen near the Cascade crest since last weekend...! Storm totals tapered dramatically from west to east across the zone. Some 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. 

An atmospheric river looks to slowly enter the region on Sunday, along with rising temperatures, winds, and precipitation turning to rain at low elevations. With warming temperatures, heavy snowfall and strong winds, we can say conditions are expected to become quite a bit more dangerous on Sunday. If you get out on Saturday, help contribute to the forecast by submitting an observation!

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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