Boletín de aludes

East Slopes Central

Northwest Avalanche Center
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Peligro de aludes

Tue
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 4 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

At higher elevations, triggering avalanches in wind drifted snow should be your primary concern, so avoid steep terrain where you find deep drifts of stiff snow. In areas near Blewett Pass and Mission Ridge, there is a possibility of triggering very large avalanches on buried weak layers, so choose lower angle terrain in this area. 

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–2

    We have significant uncertainty and limited information on snowpack conditions from alpine terrain. However, 4-6 inches of snow have accumulated at the upper elevations in the past 24hrs along with moderate ridgeline winds. This pattern built shallow, sensitive wind slabs on lee aspects. These shallow slabs may rest over slick melt-freeze crusts, potentially making them easy to trigger. You may also still be able to trigger larger, more dangerous, older wind slabs where they formed over weak snow layers. Probable locations to trigger these avalanches are near ridges, the base of cliffs/rocks, or convex rolls in terrain. You can recognize wind slabs by their pillow-like form and potentially hollow sound. Light snow may obscure wind textured surfaces. So, instead, use cornice formations and drifts behind trees or rocks to identify likely wind slab locations. You can also use your probe or pole to feel for stiffer layers over softer layers. Steer around these wind slabs in steep terrain, and any shooting cracks or collapsing is a sign to dial back your slope angles. 

    (1/3/2023)

    These pillow-like drifts in the foreground and the cornices in the background are signs that wind slabs exist on the slope below. 

    Photo: Katie Warren

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1.5–2.5

    Before this storm, numerous persistent slab avalanches failed on buried surface hoar near Mission Ridge and Blewett Pass. While rain has destroyed this layer at most elevations up to around 5500ft, it is still preserved at upper elevations. This surface hoar layer is 1-2 feet deep, and a supported crust caps the snowpack. A melt freeze crust and a strong overlaying slab will make triggering an avalanche on this layer difficult and likely require a large trigger, such as a snowmobile or a cornice fall. But you may also trigger these persistent slabs in thin spots near rocks and scoured ridgelines, maintaining the possibility of triggering a very large avalanche that breaks widely in surprising ways. With limited information, we have uncertainty over the reactivity and distribution of this layer, so if you are traveling above 5500ft in the Wenatchee Mountains, make sure to dig down and look for feathery surface hoar buried deeply in the snowpack. The terrain we are most concerned with is relatively sheltered terrain, with sparse tree cover. Meadows and glades are prime locations for preserved surface hoar lurking in the snowpack. Avoiding steep terrain in any place where you find this layer is the only way to manage this problem.

    (1/7/2023)

    An example of what buried surface hoar looks like within the snowpack.

    Photo:

  • Loose Wet

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–2

    The East slopes continue to squeeze out more precipitation, with rain likely below 3500ft. This additional water infiltrating an already saturated snowpack sustains the possibility of wet loose avalanches. Where there are steep rocks and cliffs, these wet avalanches may break as wet slabs and be large enough to bury a person. Avoid steep terrain where you are sinking deeply into wet, heavy snow, and be cautious of traveling below steep rocks and cliffs. 

    (1/13/2023)

    Example of a wet slab that released from a steep rock slab in Tumwater Canyon on Friday.

    Photo: Harlen Sheppard

Avalanche Discussion

The east slopes of the Cascades keep performing with these storms, and it feels like weeks since I have seen the higher peaks poke out from the clouds. This storm pattern is still ongoing, and we will likely squeeze out a few more inches of additional snow with lower freezing levels and moderate winds. You will find challenging travel conditions at low and mid elevations with unconsolidated wet snow and slick crusts. Other hazards should also be on your mind as creeks have opened up and other obstacles are poking their way back out of the snowpack.

Before this storm, some suspect avalanches occurred at higher elevations in the Icicle Creek area, along with observations of weak faceted grains and unstable snowpack test results. At the same time, observers in other locations found a relatively stable snowpack. This is due to a multitude of previous snow surfaces that varied substantially throughout the terrain. We have minimal information from above 5000ft since this storm. There are observations of preserved buried surface hoar in the Mission Ridge area above 5500ft, but we don't have much information from other locations in the zone. A previously spatially variable snowpack and few recent observations give us significant uncertainty in slope stability in alpine terrain. Anytime there is increased uncertainty, larger margins of safety are necessary to account for unknown conditions. So if you are venturing out into higher elevations, approach new terrain slowly and assess the snowpack as you travel. As you travel, wind slabs, weak snow layers, and icy surfaces should all be on your radar. Look for signs of instability, such as recent avalanches, actively blowing snow, shooting cracks, and collapsing. This is bullseye information on unstable snow, meaning you should dial back your terrain to low-angle slopes. 

Thursday's storm resulted in a widespread natural cycle with wet avalanches up to D3 and many smaller storm/wind slabs. Friday morning, the WSDOT NCR avalanche crew reported multiple D2 wet slab avalanches, with a couple up to size D3. The snowpack is draining water, temperatures are cooling, and the main event seems to be behind us. We are just beginning to get a handle on the extent of this avalanche cycle and the current state of the snowpack. 

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