Bulletin d’avalanche

East Slopes Central

Northwest Avalanche Center
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Danger d’avalanche

Sat
Alpin
4 · Fort
Limite forestière
4 · Fort
Sous la limite
3 · Marqué
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Multiple very large avalanches occurred on Thursday, and dangerous avalanche conditions will persist into Friday. There are complex and overlapping avalanche problems with the potential for very large avalanches to occur naturally. Avoid all avalanche terrain, including the runout of overhead avalanche paths.

Problèmes avalancheux

  • Storm Slabs

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

    Lingering precipitation with temperatures continuing to creep higher will maintain the possibility of easily triggering storm slabs, at least where precipitation falls as snow. Winds are also forecasted to increase, drifting any soft snow into deeper, more dangerous slabs. Near and Above Treeline, this heavier snow is falling on cold, low-density snow from earlier in the week. This upside-down storm snow is a perfect recipe for sensitive storm slab instabilities. While storm slabs may break in the new snow or on the old snow surface, we're really concerned about storm slabs stepping down to deeper weaker layers in the snowpack outlined below in the persistent slab problem. On Friday, avoid all slopes steep enough to avalanche and avoid traveling below large paths where avalanches may run into lower angle terrain from above. 

    (12/20/2022)

    If you're seeing natural storm slabs like this below treeline, you already know conditions are touchy.

    Photo: Alexa Cathcart

  • Wet Slabs

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

    Wet slabs are notoriously hard to forecast. They sometimes release after peak precipitation, but temperatures are still warm. A layered snowpack with persistent weak layers and a thick crust from Christmas is a perfect setup for these destructive avalanches. They will be more likely Near Treeline and at upper elevations in Below Treeline terrain, where rain saturated 2-3 feet of dry snow. Avoid traveling on steep slopes where you may trigger one of these large avalanches. With this avalanche problem, you should also avoid exposure to steep open terrain above you, as these avalanches may release naturally and run long distances. Confined creeks and gullies are notorious for funneling large amounts of avalanche debris into lower elevations, so be especially cautious of traveling past creeks and gullies in flat terrain at valley bottoms. You will likely see smaller wet loose avalanches at lower elevations where less new snow rests on the Christmas Crust. 

    (12/23/2022)

    Example of a deep wet slab avalanche. Similar avalanches are still possible on Friday with continued warm temperatures. 

    Photo: NPS Rainier Rangers

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1.5–2.5

    Heading into this storm, we had already had several consecutive days of people triggering persistent slab avalanches on weak layers buried 1-2' down in the snowpack. These layers were most reactive in sheltered locations from 4000-6000ft. However, the multitude of snow surfaces before the January 4th storm lends significant variability and uncertainty to this avalanche problem. This variability, coupled with some late observations of surprising avalanches and unstable snowpack test results from higher elevations, indicates it will be best to treat all higher elevations and aspects with suspicion. In addition, precipitation and warming temperatures are adding stress to these weak layers. These persistent slab avalanches can break widely, wrap around terrain features, and you can trigger them from lower-angle terrain adjacent to steep slopes. Friday is not the day to mess with this avalanche problem. Instead, stick to simple lower angle slopes that are not connected to large avalanche paths. 

    (1/8/2023)

    Very wide shooting crack on Tronsen Head at Blewett Pass. This is typical of a persistent slab problem.

    Photo: PKS

Avalanche Discussion

The last week has been very active with avalanche activity, and this warm, potent storm has added weight to this situation, figuratively and literally. Looking at the long list of overlapping avalanche problems above should highlight the complicated snowpack situation in East Central. Before this storm, persistent slab avalanches 1-2 feet deep in the Wenatchee Mountains were widespread on N-E aspects, failing on a layer of buried surface hoar. Skier triggered wind slabs on unknown layers occurred at higher elevations on northerly aspects, including one incident where the party was caught and carried a significant distance. Also, observations of weak faceted grains with unstable snowpack test results and sensitive storm snow instabilities came in from the Icicle Creek Area. At the same time, some other observations and locations found a relatively stable snowpack. This spatially variable snowpack setup leads to significant uncertainty in slope stability throughout the zone. Anytime there is increased uncertainty, larger margins of safety are necessary to account for unknown conditions. Thursday's storm has buried these persistent weak layers deeper in the snowpack at higher elevations, and rain has saturated the lower elevations. As a result, a widespread natural cycle with wet avalanches up to D3 and smaller storm/wind slabs occurred on Thursday. We expect Friday to be similar, with warm temperatures and lingering precipitation. So continue to let the storm pass through while enjoying the wet stormy weather from low-angle terrain, far from the runouts of large avalanche paths. 

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