Bulletin d’avalanche

East Slopes Central

Northwest Avalanche Center
Voir sur la carte

Danger d’avalanche

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

Points saillants

Cooling temperatures and only light precipitation will reduce the overall hazard. In most locations, avalanches in wind drifted snow are the primary concern, so avoid steep terrain where you find deep drifts of snow. However, In the Wenatchee Mountains, there is a possibility of triggering very large avalanches on buried weak layers, lower angle terrain is the safest bet in this area. 

Problèmes avalancheux

  • Wind Slabs

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

    Elevations above 5500ft likely stayed as snow throughout this storm, burying various old snow surfaces and building deep wind slabs on lee aspects. We have significant uncertainty and limited information on snowpack conditions from the terrain at these elevations. However, continued light to moderate snow with 10-20mph winds is enough to build unstable wind slabs. These shallow fresh wind slabs may form over slick melt-freeze crusts, potentially making them sensitive to triggers. You may still be able to trigger older wind slabs where they formed over weak snow layers, and they also are likely to be much larger and more dangerous.

    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. Textured surfaces, cornice formations, and drifts behind trees and rocks can also point to likely wind slab locations. Watch for signs of instability, such as shooting cracks or collapsing. Avoid steep wind-loaded slopes by traveling on the scoured windward side of ridges or sticking to wind-sheltered terrain. 

    (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

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

    Before this storm, there were numerous reports of persistent slab avalanches, failing on buried surface hoar, in the Wenatchee Mountains near Mission Ridge and Blewett Pass. While rain has destroyed this layer at most elevations up to around 5500ft, an NWAC forecaster found buried surface hoar preserved and reactive in stability tests at 6700ft near Mission Ridge. This layer is below the recent storm snow, and a moderately supportable crust around 1 inch thick exists on the surface. This setup will probably make triggering an avalanche on this layer difficult and require a large trigger, such as a snowmobile or a cornice fall. However, the possibility exists 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. It will be best to approach all slopes as suspicious until we gain more information and confidence regarding this particular problem. 

    (1/7/2023)

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

    Photo:

  • Loose Wet

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

    Low-elevation trailheads below 3500ft may continue to see precipitation in the liquid form, making wet loose avalanches possible. 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 Central zone has been in a sustained and stormy period for what feels like weeks. 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, particularly in the eastern portion of the zone. With unconsolidated wet snow and slick crusts, you will likely find challenging travel conditions at low and mid elevations. Obstacles such as stumps, creeks, and rocks are now likely back on the surface at these lower elevations.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. An NWAC forecaster found a 1 inch thick crust up to 6700ft in the Mission Ridge Area, but we don't know much from other locations in the zone. A previously spatially variable snowpack and not much new information 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. 

Régions couvertes