Bollettino valanghe

Stevens Pass

Dec 23 – 24, 2018Northwest Avalanche Center
Vedi sulla mappa
Archiviato · scaduto 8 years ago

Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
Apri la mappa completa

In sintesi

Recent storm snow will need more time to bond to an underlying crust. Above treeline, the low density snow is easily transported into thickened wind slabs. Deep, large avalanches are still possible, especially where the snowpack is more shallow. Conservative terrain selection is recommended. 

Problemi valanghivi

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1

    The most dangerous slopes will be at upper elevations where thick slabs may have formed from deeper accumulations and wind. Stay off of steep slopes near and above treeline if finding these slabs. The storm snow is sitting on a thin crust buried on December 20th up to 5,500ft, and low density old snow above that. This weak interface was inhibiting the bonding between the storm snow and the old snow surface. Carefully evaluate the new snow depth, it’s cohesiveness, and how it is bonding before choosing to travel in avalanche terrain. Use small, inconsequential slopes and tests to check if the storm snow will slide.

    Small loose wet avalanches may become likely in the afternoon on steep, southerly aspects. Limit your time spent on steep slopes during the middle of the day, and if noticing roller balls or recent loose avalanches. 

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2

    You may still be able to trigger Deep Persistent Slabs in the eastern portion of the zone near and above treeline. While these avalanches may be difficult to trigger, they are also very difficult to predict. If you hit the wrong place in the snowpack, it could result in a very large and dangerous avalanche. Large triggers are more likely to initiate a deep slab. Steep, unsupported slopes are the most likely terrain where these can occur.

    The best way to reduce the risk of this low likelihood, but high consequence situation is to minimize your exposure to slopes capable of producing large avalanches.

    -Attempt to put a significant distance in between where you are traveling and where avalanches start, run, and stop.

    -Limit the amount of time you spend on or near slopes 35 degrees and steeper.

    -Don’t underestimate how far and wide these could run when identifying safer areas to stop and regroup.

Discussion

Snowpack and Avalanche Discussion:

Stevens Pass received about 8” of new snow with 0.62” of water equivalent as of 4pm on December 23rd. Temperatures were initially cool and winds were light, but strong enough to move snow near and above treeline. During the day on the 23rd, skiers were able to trigger avalanches about a foot deep and 100ft wide on the new/old interface at 6,200ft on a Northeast aspect of Jim Hill. Out of any wind effected zone, loose avalanches were possible on very steep slopes.

Between the Pacific Crest and the eastern slope there is a large difference between snowpack depths. This is nothing unusual, and more often than not the west side of the Cascades and the passes get more snow than areas further east. Over time, this adds up. Right now, we are starting to see that difference grow. For example, at 3950ft on Stevens Pass there is 55” of snow on the ground. At 4,100ft on Blewett Pass the snowpack is only 20” deep.

The persistent weak layer that we have been tracking since December 9th has been observed to be rounding out, gaining strength, and sintering near the pass. Further east in the zone we are still gathering information, but this layer is still in the forefront of our minds in areas with a more shallow snowpack. A very large natural avalanche cycle occurred between December 18th and 20th on these layers in the East Central and East North zones. It appeared that the north side of Jim Hill may have ran during this time as well. 

Snowpack Discussion

Coming December 24th, 2018

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