Boletín de aludes

East Slopes Central

Northwest Avalanche Center
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Peligro de aludes

Thu
Alta montaña
3 · Notable
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 5 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

An incoming windy storm will build fresh slabs and dangerous conditions in exposed upper elevation terrain. Visual cues such as blowing snow or a window to the past such as wind-textured surfaces can help you avoid the problem. Avoid wind-loaded slopes steeper than 35 degrees above treeline or anywhere you suspect 6" of snow has accumulated.

Problemas de aludes

  • Wind Slabs

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

    Shallow wind slabs will quickly build at upper elevations as WSW winds buffet the East Central zone. You'll need to constantly re-evaluate your risk as uncertainty in snowfall amounts brings uncertainty to your danger. As you move into more exposed terrain, seek out small wind-loaded test locations where you can evaluate the thickness of slabs. Most slabs will form above a thin crust that you can feel with your pole or probe. Once these slabs approach 6" deep, you'll want to proceed more cautiously, avoiding all wind-loaded slopes steeper than 35 degrees. Frequent hand shears can help you identify whether the slabs have bonded to the crust or old snow surface. 

    In eastern parts of the zone expecting significantly less snowfall with significantly shallower snowpacks, you can still trigger a small slab in steep, unsupported or rocky terrain features at upper elevations.

     

    Example: Steep, wind-loaded and unsupported terrain below ridgelines will become increasingly susceptible to human-triggered avalanches on Wednesday.

    Photo: Lee Lazzara

Avalanche Discussion

There's a fair amount of uncertainty regarding incoming weather. We expect 5-10" of new snow to be accompanied by moderate winds. If we get amounts in the upper end of that range, a storm slab could develop at all elevations. If these conditions materialize, you can use frequent small test slopes to evaluate the developing slab, sticking to supported terrain features less than 35 degrees at all elevations to stay safe.

NWAC forecasters were on Dirtyface Mountain on Tuesday. They found wetted and denser snow above 4500 ft with drier low-density snow on the surface at lower elevations. We're keeping an eye on a layer of preserved stellars that remains intact down 14" at Dirtyface Mountain, and was the culprit in a crown profile from an avalanche attributed to the 12/18 widespread cycle. We don't expect but can't rule out the possibility that wind slabs will fail down to this layer on Wednesday.

You'll find significant differences in the upper snowpack as you travel around the zone. A ridge of high pressure on Tuesday accompanied an inversion that lifted temperatures above freezing temperatures at elevations greater than 4000 ft. This warming likely formed a crust in these upper elevation locations throughout much of the zone with colder, drier lower-density snow remaining below this line. The most recent 6-8" of snow fell Sunday night into Monday in the southwest part of the zone, but little if any snow fell north of highway 2. A significant storm and natural avalanche cycle occurred on Saturday. Old wind slabs from this storm event will be stubborn or unreactive to human trigger by Wednesday.

We have limited data from elevations above treeline and in the northern portion of the zone. In some areas, weak snow may exist above crusts, creating the potential for larger avalanches, so approach complex or large terrain cautiously. Use snowpack tests to determine how well bonded the overlying snow is to these crusts. Given the variability in incoming weather and snowpack structures, expect the avalanche danger to differ throughout the zone, and stability on one slope may not represent adjacent slopes. You may find avalanche danger to be significantly lower than forecast in the Blewett Pass or Wenatchee Mountain regions.

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