Avalanche Forecast

Archived

Jan 3rd, 2015–Jan 4th, 2015

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural and human triggered avalanches likely.

Regions

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The avalanche danger will increase sharply by late Sunday afternoon and evening with human triggered slides becoming likely. Choose a conservative travel plan and make sure your plan allows for a safe exit if conditions deteriorate quicker than expected.

Detailed Forecast

Light snowfall Saturday night and Sunday morning will increase in intensity Sunday afternoon along with increasing winds. A warming trend should lead to inverted storm snow by later Sunday afternoon. Snow should change to rain at lower elevations outside the cooler Cascade Passes Sunday afternoon. Loose wet avalanches will locally be an avalanche problem in these areas.

All of this will lead to increasing avalanche danger Sunday. Expect human triggered avalanches to be likely as the afternoon wears on. Be prepared to curtail your trip early if conditions become sensitive earlier than forecast.

Shallow storm slab avalanches releasing within the storm snow may step down to a variety of surfaces including newly buried surface hoar below treeline in certain areas. New wind slab should form on NW through SE aspects near and above treeline.

More significant warming and precipitation is expected Sunday night and a natural avalanche cycle is expected. An avalanche watch has been issued Sunday night through Monday. An avalanche watch means HIGH avalanche danger may develop for all three elevation bands Sunday night and Monday along the west slopes of the Cascades.

Snowpack Discussion

After last weekend's storm, north through east winds, strongest in the alpine, developed locally sensitive wind slab on unusual aspects for the central and south Cascades near and above treeline. High freezing levels and sunshine activated small loose wet slides on steeper south facing slopes midweek. Northerly aspects that were not wind affected had soft, right side up snowfall generally well bonded to the underlying crust. Very light snowfall accumulated along the west slopes Friday due to a weak weather system.

One of our biggest concerns along the west slopes of the Cascades heading into the upcoming weekend storm will be the new storm snow's ability to bond to a variety of snow surfaces; near surface facets, wind board, melt-freeze crusts, etc.

Also of concern will be a thin layer of surface hoar formed midweek in sheltered areas below tree-line. In the Stevens Pass zone, WSDOT avalanche forecaster Brandon Levy found 2 mm of surface hoar buried intact at Stevens Pass Saturday morning.

A generally strong mid and lower snowpack is expected west of the crest. One caveat is a persistent weak layer noted near a crust about 1 meter down in the Stevens Pass area last week, but no known avalanche activity has occurred on this layer.

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.

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.