Bulletin d’avalanche

East Slopes Central

Northwest Avalanche Center
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Danger d’avalanche

Sat
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Dangerous conditions exist due to recent snowfall and strong winds. The snowpack is complicated and variable. Many slopes may be nearing a tipping point, and need only the weight of a person to initiate a wide and surprising avalanche. Friday is a good day to avoid all large open slopes steeper than 30 degrees at upper elevations.  

Problèmes avalancheux

  • Persistent Slabs

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

    Persistent slab avalanches are dangerous and can catch even the most experienced backcountry travelers by surprise. They can break widely around terrain features and be triggered from lower angle slopes. In the western half of the zone we may be reaching a tipping point where the triggering of these slides is fairly easy to do.  This instability dominates the way we think about, and plan to travel in the backcountry because all slopes steeper than 30 degrees are suspect. Look for a thin layer of weak snow above a crust roughly 12"-20" down from the surface. There may be a layer of buried surface hoar a few inches above this crust as well. Whumphing and collapsing of large sections of snow direct signs that the snowpack is unstable. 

    On Thursday experienced travelers remotely triggered a 500ft wide, 15"-20" deep slide that ran on the prominent 12/9 crust in neighboring Stevens Pass zone. On Wednesday, forecasters in the Lake Wenatchee area were finding weak sugary snow (facets 1.0-1.5mm) 8-14" above a firm crust in the near treeline elevation bands. On Monday, observers in the Highway 2 corridor near Lost Lake, as well as close to Stevens Pass, reported collapses up to 100ft in diameter and snowpack tests that indicated propagation.

    (12/17/2020) Widely propagating persistent slab avalanche that was triggered from a distance and below the slope. Northeast aspect at 5,800ft in Lanham Lakes drainage near Stevens Pass.

  • Wind Slabs

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

    As the front blew through on Thursday, strong winds creating blowing snow at upper elevations. Look for visual clues to recent wind drifting such as textured surfaces right next to smooth areas. Feel for wind stiffened snow as you approach open areas that may be more exposed to being breezy. These may be largest and thickets on north to east to south aspects. Any slab that is triggered above 5,000ft has the potential to step down to the prominent 12/9 crust, becoming a much larger avalanche than you may anticipate. You may find some evidence of wind slabs even at lower elevations, but these are likely to be smaller and more isolated in nature. 

    (12/17/2020) Long shooting cracks with textured, stiff and wind deposited snow like this one are indicators of wind slab instabilities.

Avalanche Discussion

A solid round of snow fell in the mountains of the East Central Zone on Wednesday night. Leavenworth picked up 6" with 0.7" of water equivalent, and Mission Ridge reported 5" with about 0.4" water equivalent, and lots of wind drifting. The active weather pattern will continue, and the snowpack will get deeper with each storm. Above 5,000ft each storm will be incrementally loading potential weak layers that formed during the dry spell of early December.  

The avalanche danger continues to vary greatly across the zone, with the higher hazard on the west side closer to the Cascade Crest. In the Wenatchee Mountains near Blewett Pass and the eastern foothills of the Cascades, you may find less dangerous conditions along with less snow.  Expect shallow snow cover at low elevations particularly on low south-facing slopes. Typical early season hazards can be expected with open creeks, stumps and rocks. 

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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