Bollettino valanghe

Olympics

Northwest Avalanche Center
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Pericolo valanghe

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2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Wet slabs or glide avalanches remain a low likelihood, but consequential threat. They could become large and release without warning as rainwater continues to drain through the snowpack, or you can trigger one. Reduce your time on or under steep, open slopes at middle and upper elevations.

Problemi valanghivi

  • Wet Slabs

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

    Wet slabs are a difficult to manage avalanche problem because neither the forecaster, nor the field-based traveler can assess the movement of water through the snowpack in both space and time with accuracy. Although ONP Rangers did not observe signs of instability on Thursday, visibility was limited. We're expecting more rainwater to enter the snowpack on Friday. That increases the risk of wet slabs, particularly if water pools on the 5 cm thick 2/13 crust. Stick to lower consequence terrain features and limit your time crossing beneath steep slopes. Wet slabs are more likely to release during or shortly after periods of heavy rainfall, but they aren't fully off the table until we get a hard surface refreeze.

    After 5 days of rainfall, water is fully draining the snowpack in some areas, increasing the risk of the wet slab's cousin: glide avalanches. Glides occur when the entire snowpack slides on a smooth or bedrock underlying slope. Limit your exposure to "brown frowns" where the snowpack is starting to peel off, but hasn't fully released.  Note that wet avalanches of any kind will be larger and more consequential in thicker snowpack areas. In shallow snowpack areas, terrain traps amplify your risk.

    Example: A wet slab releasing on a steep, open slope. 

    Photo: Dallas Glass

Avalanche Discussion

As we enter into the final 24 hours of a multi-day atmospheric river, the snowpack has absorbed just over 1.5" of rain water at Hurricane Ridge since temperatures rose above freezing the morning of 3/15. This warm storm cycle has not been kind to the Olympics. Snow depths at Hurricane Ridge peaked around 4 ft on 3/14 and have consolidated by 25% over the last 5 days to 3 ft. This has erased the snowpack gains from last week. Lower elevations are back to bare ground on all but shaded, open slopes where coverage is thin. Middle elevations have patchy coverage on sun-drenched aspects. Travel cautiously on Saturday due to low coverage and the challenging riding conditions anticipated in heavy, saturated snow.

ONP Rangers traveled in the Victoria Bowl area. On a NW aspect at 5200 ft, the snow depth is approximately 43". The upper 14" of the snowpack was moist and ski supportable, but variably supportable to boot. In isolated areas, boots sink in past the knee or thigh. The moist surface snow overlies a 5 cm 2/13 crust, which is gradually breaking down, but still contains ice lenses. Polycrystals extend to the ground below. 

Although the snowpack structure is simple, how the water flows through it is the major question. You might gain some insight by digging down to the 2/13 crust and looking for water pooling above the crust. Observing moist rather than pooled water at this interface reduces your likelihood of a wet slab, but it doesn't completely eliminate it on a nearby slope.

We're expecting around 0.5" of water equivalent to arrive at Hurricane Ridge through Friday. Thursday night and early Friday, snow levels remain well above the highest peaks, lowering into middle elevations late in the day. As the transition to snow occurs, persistent moderate WSW winds could form wind drifts near upper elevation ridgelines. Steer around any drifts 6" or deeper in the unlikely event you ascend high enough in the terrain to find them.

Regioni coperte