Bollettino valanghe

Stevens Pass

Dec 12 – 13, 2018Northwest Avalanche Center
Vedi sulla mappa
Archiviato · scaduto 8 years ago

Pericolo valanghe

Fri
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
Apri la mappa completa

In sintesi

Significant winter storms are continuing to create very dangerous avalanche conditions. Stay out of areas where avalanches can start, run, and stop. Avalanche danger will rise and peak during times of heavy snow and warming temperatures.

Problemi valanghivi

  • Storm Slabs

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

    Natural activity within the new snow is likely to spike with the heaviest precipitation rates on Thursday morning. This will coincide with a rapid warmup. In addition, slabs from three to four feet thick are sitting on old weak snow. It is uncertain whether or not this storm will overload this layer to a breaking point, but it may. Any failure on this deep, weak layer would be surprising and scary. It will be prudent to avoid avalanche terrain and avoid entering terrain with overhead hazard.

  • Loose Wet

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

    At lower elevations, the danger from loose wet avalanches will rise in the morning as a result of rising freezing levels. Stay out from underneath steep slopes if you are noticing roller balls, very wet surface snow, and of course fan shaped avalanches!

Discussion

Avalanche Summary

During the day on December 11th, professional observers reported natural, and remotely triggered avalanches within the storm snow and on an old snow layer. These were on a variety of aspects and as low as 4700ft. On December 12, professional observers reported whumphs and collapses. Some of these collapses propagated long distances through a weak layer in the snowpack. With another round of heavy snowfall, a warming trend and strong winds, we may tip the scales for very large avalanches on this layer.

Since December 10, the total snowpack depth has more than doubled on Stevens Pass. The first 9 days in December were clear and cold. Surface hoar grew large during this time, and facets developed near the surface of the cold, low density snow. On December 9th this layer was covered up and preserved on many aspects and elevations. Currently, this layer is two to three feet down in the snowpack and tests are showing that if triggered, it will likely propagate.

Snowpack Discussion

Regional Synopsis 20181211

Welcome winter!!!

The NW snowpack is evolving quickly and a near constant pattern of winter storms with fluctuating freezing levels is making things interesting for our 1st week of forecasting.

Here’s what we know:

  • Tuesday saw a significant avalanche cycle.

  • We have a lot of new snow...2-3’ above 4500ft.

  • We’ve gotten a lot of wind.

  • We have weak layers near the ground.

  • We are going to get more snow before the weekend.

All in all that’s enough to indicate that dangerous avalanche conditions will persist over the next couple of days. That said, it is early season and the flavor of avalanche you may run into is likely a function of elevation, timing, and dumb luck.

Here are some basic emerging patterns:

  • Storm total (Monday morning to Wednesday evening):

    • Mt. Baker: 24”

    • Washington Pass: 22”

    • Snoqualmie Pass: 10”

    • Paradise: 17”

    • Mt. Hood Meadows: 10”

  • Upper versus Lower Elevations: The change in the snowpack is still pretty dramatic with elevation. Height of snow decreases rapidly below 4500’ at Baker and Washington Pass, 5500’ at Crystal/Rainier. The Passes have better low elevation coverage, but it's still pretty thin below 4000’. With additional warm storms in the forecast, this pattern is expected to continue for awhile.

  • East versus West: Loading along the East slope has been more incremental, and a  variety of buried facet and surface hoard layers may be found. This is most pronounced near WA Pass. Although significant snowfalls and precip totals have resulted in thicker, more homogeneous snowpack in the western zones, lingering weak layers near the ground will persist through the week.

Regioni coperte