Avalanche Forecast

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.

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.