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Archived

Avalanche Forecast

Dec 17th, 2016–Dec 18th, 2016
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low

Regions: Olympics.

Caution, areas of new wind deposited snow are possible Saturday on unusual aspects, mainly near or above treeline. Watch for wind stiffened surface snow on a variety of aspects and avoid steep terrain with evidence of wind loading. 

Detailed Forecast

A weak disturbance will move across the region Saturday, causing increasing NW winds and very light snow showers. The main impact of the weather Saturday is that any light snowfall may bury intact a variety of weak surface snow types formed this week under fair cold weather.

Moderate to strong NW ridge level winds may build new fresh wind slabs on lee slopes, near and especially above treeline. 

Local wind slabs from the recent easterly winds is also possible Saturday. This should be mainly on westerly aspects near ridges.

Watch for firmer wind transported snow on all aspects, mainly near and above treeline.

In the Cascades, the December 8th layer continues to round and gain strength and should be less sensitive to trigger. We don't have any direct evidence of this persistent weak layer in the Olympics and it won't be listed in this zone as an avalanche problem. Do head for lower angle slopes and ratchet back your plans if you experience collapsing or whumpfing or have evidence of this layer such as from snowpits.

Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.

Snowpack Discussion

Weather and Snowpack

The last storm cycle to affect the Olympics was from late Thursday, December 8th through Monday morning, December 12th. About 2.6 inches of water equivalent and about 3 feet of snow fell at Hurricane Ridge ending Monday morning Dec. 12th. 

Fair and cold weather this week has been interspersed with periods of moderate E-NE winds both Wednesday and again Friday. These winds were strong enough to build fresh wind slabs on more atypical SW-NW facing terrain, mainly near and above tree line.

There has been about 12 inches of snowpack settlement since the storm cycle ended, allowing underlying or storm snow related weak layers to stabilize. 

Several clear nights with light winds this week have allowed for weak surface snow development. On wind and sun sheltered terrain, extensive near surface faceted snow or surface hoar have formed this week and this may be an important weak layer if buried intact by the expected return of light snowfall Saturday.

A thin sun crust is likely to have formed on steeper southerly facing slopes, which also may be a future interface for avalanches when loaded with sufficient wind transported or future storm snow.

Recent Observations

A storm related avalanche cycle was seen December 8th-11th. Some avalanches released on a December 8th persistent weak layer in the Washington Cascades, though the presence of that layer has not been verified in the Olympics as yet. 

The NPS ranger at Hurricane Ridge indicated widespread natural slides had reached the road in many places by Sunday morning, likely releasing overnight during moderate to heavy snowfall.

There is a report via the NWAC Observations tab from Sunday of a triggered slab avalanche on a steep east slope apparently above Lake Angeles.

No recent direct observations have been received since Sunday.

Avalanche Problems

Wind Slabs

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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1