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 unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Snoqualmie Pass.

The snowpack has grown substantially this week at Snoqualmie Pass, but with that come a risk for avalanches. Avoid steep open slopes above 4500’ where underlying old weak snow may produce dangerous avalanches.

Discussion

Avalanche Summary

We have had numerous avalanches reported from the Snoqualmie Pass area this week. At least some of these avalanches failed on layers of old weak snow. On Wednesday, an NWAC Forecaster in the Snoqualmie Pass area experienced several large whumphs and felt collapses. These avalanche observations and non-avalanche snowpack failures are easy signs to interpret. This snowpack has the ability to produce large avalanches.

Snowpack Discussion

Regional Synopsis 20181213

Winter is here.

A productive winter storm pattern has elevated the avalanche danger and provided an interesting start to the season.

A quick breakdown:

Tuesday and Wednesday saw a significant localized avalanche cycles.
We have a lot of new snow...2’ to 5’ above 4500ft.
We’ve gotten a lot of wind.
We have weak layers near the ground.

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:

Snowfall totals so far this week (Monday morning to Thursday evening) illustrate more recent snowfall in the north than the south:
- Mt. Baker: 55”
- Washington Pass: 29”
- Stevens Pass: 37”
- Snoqualmie Pass: 28”
- Paradise: 38”
- Mt. Hood Meadows: 13”
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 hoar 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.

The uncertainty of how reactive our buried weak layers are will carry into the weekend. Be cautious and get home safe.

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

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.