Avalanche Forecast

Archived

Dec 12th, 2018–Dec 13th, 2018

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

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Strong winter storms are continuing to create very dangerous avalanche conditions. This is the time to stay out of areas avalanches can start, run, and stop. Avalanche danger will rise and peak during times of heavy snow and warming temperatures.

Discussion

Avalanche Summary

We have received reports of natural avalanches in the Mt Baker area for the past three days. There is no simpler sign of unstable snow than recent avalanches. While observations from backcountry travelers on Wednesday showed less reactivity than earlier in the week, reports of whumphing, shooting cracks, and sudden snowpack test results continue to remind us that the snowpack is weak. More snow Wednesday night and Thursday combined with a weak snowpack means 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.