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Avalanche Forecast

Archived

Dec 10th, 2015–Dec 11th, 2015

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

Regions

Snoqualmie Pass.

Quick update for increased storm and wind slab avalanche hazards along the west slopes of the Cascades. Only the danger rating has been changed for Day 1.

Detailed Forecast

Quick update for increased storm and wind slab avalanche hazards along the west slopes of the Cascades. Only the danger rating has been changed for Day 1. 

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Cascades West Slopes Forecast

An occluded front should move south to north over the Northwest Wednesday night and Thursday. South to southeast alpine winds Thursday morning should generally shift to southwest Thursday by afternoon. Look for mostly moderate amounts of snow by later Thursday.

Along the west slopes watch for possible new wind slab on lee slopes and new storm slab in calmer less exposed areas. The mostly moderate amounts of snow and the cooling should somewhat limit the extent of new wind and storm slab. Remember that the avalanche danger will be greater if you encounter stronger winds and heavier snowfall. Evaluate snow and terrain carefully Thursday. Change your plans if you find yourself in unexpectedly prolonged stormy weather.

Snowpack Discussion

Cascades West Slopes Weather:

Snowfall over the west slopes the first 7 days of December was about 1-4 feet with an overall warming trend. Then an atmospheric river arrived the past couple days (Main Fork of the Pineapple Express) and heavy rain was seen over the west slopes ranging from about 5-8.5 inches of water in just 2 days ending this morning. Wow!

But snowfall has resumed on Wednesday with a cooling trend. Good news!

Cascade West Slopes Snowpack and Reports:

The snow and warming trend the first week of December followed by the heavy rain, building on crust layers and on faceted snow and surface hoar near Stevens and Snoqualmie, caused extensive instability and extensive avalanches mainly Saturday to Tuesday, as verified by reports from Baker, Stevens and Snoqualmie. There were a couple close calls with skiers caught and totally or partly buried at Stevens and Alpental.

Thankfully the weather the past few days has led to a greatly consolidated and stabilized snowpack. Persistent weak layers especially at Stevens and Snoqualmie, should have been eliminated. Reports from the Mt Baker and Stevens Pro Patrols on Wednesday indicate stable, consolidating, wet, rounding grains well down into the snowpack.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.

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.