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Avalanche Forecast

Archived

Dec 23rd, 2016–Dec 24th, 2016

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

Regions

Olympics.

Don't be caught surprised by the unusual PWL at Hurricane on Saturday. Limit your travel to low angle slopes until we are sure this layer is stabilized.

Detailed Forecast

An upper trough will exit the Northwest on Saturday and high pressure will begin to build offshore. Light snow showers mainly along the west slopes Saturday morning should give way to partial clearing Saturday afternoon with light west to northwest winds and cool temperatures.

Don't be caught surprised by the unusual PWL at Hurricane on Saturday. Limit your travel to low angle slopes until we are sure this layer is stabilized.

Wind slab should also be possible on Saturday due to recent south winds. Watch for firmer wind transported snow mainly on northwest to southeast slopes.

Although cornices won't be listed as an avalanche problem avoid ridges that may have a cornice and slopes below cornices.

The same danger levels will be indicated in all the terrain bands in the outlook for Sunday due to the cool weather and until there is known favorable change in the PWL layer.

Snowpack Discussion

Weather and Snowpack

A front crossed the Northwest on Thursday followed by an upper trough on Friday. This has caused about 8-10 inches of snow at Hurricane. South winds Thursday became light on Friday a cooling trend Friday.

Recent Observations

NWAC pro-observer was at Hurricane Ridge on Friday and gave an important report so pay attention. He was triggering collapsing and whoomping in every open area that he visited on Friday. In 2 snowpits on slopes less than 30 degrees he found the December 17th PWL and preserved surface hoar and preserved faceted snow at about 46-50 cm below the surface with moderate Extended Column tests indicating propagation. While the ski conditions were excellent he was unable to access higher open terrain safely.

Matt also reported that cornices were growing on the lee northeast sides of ridges on Friday.

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.

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.