Bollettino valanghe

East Slopes Central

Northwest Avalanche Center
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Pericolo valanghe

Thu
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Continued snow showers and winds will continue into Wednesday. You can trigger avalanches in the new snow, and in areas where the wind has drifted snow, these avalanches may be large enough to bury you. Avoid steep terrain near ridges and open terrain where you find stiff, hollow-sounding snow. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    On Monday, observers throughout the East Central zone noticed wind-affected snow and small soft wind slabs building near ridges. There is still plenty of low-density snow is available for transport, and forecasted winds will build new slabs and increase the size of existing wind slabs. Slabs may be larger and more touchy at upper elevations. On steep solar aspects, these slabs could also be resting on top of weak snow on a crust and break surprisingly wide. At lower elevations, wind slabs may linger on open slopes where the wind deposits snow on the lee sides of gullies as it blows across the slope. 

    Use quick hand pits or your probe to look for strong over weak layering. Small test slopes that produce shooting cracks or collapsing are red flag signs that you should avoid steep terrain. Wednesday will be a good day to travel conservatively and steer around wind-affected areas with large pillow-like drifts or hollow-sounding snow. 

    (1/4/2022)

    Recent avalanches in open wind-affected terrain such as these on Blewett Pass are a red flag sign of instability. 

    Photo: Matt Primomo

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    While some areas of the Cascades experienced a brief period of dry weather, lingering showers continued in areas near the Cascade Crest in the East Central zone. Recent avalanches from Blewett Pass and stability test results near Lake Clara indicate lingering storm slab instabilities. We expect any avalanches to be within the storm snow from Jan 2nd- 4th, but there is a slight possibility that smaller avalanches could step down to deeper instabilities in the snowpack. Therefore, it will be worth digging into the snow and identifying crusts and interfaces where the overlying snow slides off easily. 

    Some possible convergence may be in store for the I-90 corridor on Tuesday night, which may bring 6-8 inches of new snow to the Salmon La Sac area. If this convergence sets up, storm slabs may be slightly larger and more sensitive in this particular area of the East Central zone.

    Storm slabs most likely linger in steep sheltered terrain, and unsupported slopes or convex rolls are probable locations to trigger these avalanches. Approach new terrain slowly and use small test slopes to observe how the recent snow bonds to the old snow interface. 

    (12/25/2021)

    Example of shooting cracks in new snow.

    Photo: Ron Martin

Avalanche Discussion

Wednesday will be the proverbial calm before the storm, with the main event bringing warming temperatures and heavy snow/rain into the region Wednesday night and Thursday. This storm will initiate a rapid increase in avalanche danger. While we don’t expect heavy precipitation to impact the region until late on Wednesday, if you venture out in the evening hours, keep in mind that conditions will begin to change very quickly.

There have been abundant observations of weak snow, surface hoar, and facets on the existing surface. Underlaying the weak snow is a variety of surfaces, from sun crusts and wind crusts to soft snow. As a result, adjacent slopes and slight variations in aspect may have very different snowpack conditions. We are particularly concerned about steep solar aspects with a few inches of preserved stellar snowflakes, surface hoar, and faceted crystals on top of a sun crust. There is a lot of uncertainty but keep in mind that avalanches may break farther and become larger than a typical storm slab. 

Shuksan Arm, the Chair Peak basin, and Mt. Mastiff have all had avalanches on deeply buried weak layers in the snowpack. And Mission Ridge Patrol reported that an explosive triggered slab stepped down to the Solstice crust. These observations seem sporadic and inconsistent, but they indicate uncertainty with the spatial distribution of these deeper weak layers. This uncertainty, along with the potentially high consequences of these very large avalanches, warrants extra caution as smaller avalanches may step down to deeper interfaces. 

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