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Avalanche Forecast

Archived

Dec 13th, 2018–Dec 14th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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Avoid open slopes greater than 35 degrees where you will be able to trigger avalanches. You are most likely to encounter dangerous avalanche conditions at higher elevations, in locations affected by wind,  and in areas that receive more than 8 inches of new snow.

Discussion

Avalanche Summary

We have received reports of a few large avalanches in the Crystal area. An NWAC professional observer in the Crystal area experienced whumphing, shooting cracks, and found weak old snow (facets and/or buried surface hoar) in the snowpack. These are all easy to interpret signs of unstable snow. While a high degree of uncertainty exist around the distribution this weak layer, more snow Wednesday night and Thursday combined with a weak snowpack means you will be able to trigger avalanches.

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.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.