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Avalanche Forecast

Archived

Feb 10th, 2017–Feb 11th, 2017

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, human triggered possible.

Regions

Olympics.

Strong winds Friday and Friday night will have built fresh wind slab on lee aspects mainly near and above treeline. Watch for new wind slab in the upper portion of the below treeline band. Easterly aspects will be highlighted in the elevation/aspect diagram but watch for firmer wind transported snow on a variety of aspects.

Detailed Forecast

Light to moderate showers Friday night should taper off after midnight with only isolated light snow showers expected on Saturday. The same trend is expected for the moderate to occasionally strong W-SW winds forecast for Friday evening with winds easing considerably heading into Saturday.

Strong winds Friday and Friday night will have built fresh wind slab on lee aspects mainly near and above treeline. Watch for new wind slab in the upper portion of the below treeline band. Easterly aspects will be highlighted in the elevation/aspect diagram but watch for firmer wind transported snow on a variety of aspects.   

Despite the cooling trend, new and shallow storm slab instabilities may be locally sensitive. 

Wet slabs will not be listed as they are very unlikely due to the cooling trend. However, for one more day avoid travel below unsupported slopes, especially below treeline, with the low likelihood/high consequence of large wet slab avalanches in mind. Wet slab releases are hard to predict and can happen a day or two after peak warming and rainfall.   

Snowpack Discussion

Weather and Snowpack

A strong storm cycle was seen Friday through Monday with about 3 feet of snow accumulating at Hurricane Ridge. 

A strong plume of moisture brought heavy rain to the west slopes of the Cascades and Olympics. About 2 inches of water fell mainly as rain at Hurricane Ridge from Wednesday afternoon through Thursday afternoon with significant snowpack settlement. A strong cold front swept through mid-day. Bands of light to moderate showers in SW flow followed the front with a slow cooling trend beginning Thursday afternoon.  

Moderate showers and strong W-SW winds along with a cooling trend was seen on Friday. About 10 inches of snow accumulated at Hurricane Ridge through 6 pm Friday. 

Recent Observations

NWAC observer Matt Schonwald was at Hurricane Ridge on Friday and found good skiing in non-wind affected areas well below ridges. Closer to ridgeline, new wind slab was quickly building on lee easterly slopes up to 1 ft deep and very sensitive to ski triggering with cracking readily observed on test slopes. One large loose wet avalanche was observed by the road crew Friday morning, with a few small trees in the 6-8 ft debris pile. Large cornices that had built on W-NW aspects earlier in the week were quickly being eroded by the strong W-SW winds.  

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.