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Avalanche Forecast

Archived

Dec 28th, 2016–Dec 29th, 2016

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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.

The 12/17 persistent slab problem should remain the main problem at Hurricane Ridge. Continue to avoid steeper slopes especially in less skied areas at Hurricane Ridge. New or recent wind slab and new storm slab are also very likely on Thursday.

Detailed Forecast

West southwest flow aloft will carry the next front across the Northwest on Thursday afternoon and night. Winds should increase in most areas on Thursday but significant new snow during the daylight hours is expected mainly in the Olympics and northwest Cascades. A warming trend should also be seen with temperature inversions possible Thursday in the central west and southwest Cascades.

The 12/17 PWL persistent slab problem should remain the main problem at Hurricane Ridge. Recent and new loading may make this layer more sensitive to triggering. Remember that persistent weak layers are generally involved in larger avalanches. Continue to avoid steeper slopes especially in less skied areas at Hurricane Ridge.

Recent and renewed west to southwest winds make wind slab most likely on northwest to southeast aspects on Thursday.

New storm slab is also very likely at Hurricane due to new snow and the warming trend on Thursday. 

 

Snowpack Discussion

Weather and Snowpack

A system Thursday and Friday 12/22-12/23 with low snow levels deposited about 8-10 inches of snow at Hurricane.

A strong front and strong west flow aloft was seen over the Olympics and Cascades on Monday and Tuesday. The NWAC station indicated strong south to southwest winds Monday and Tuesday with almost 2 feet of new snow for the 48 hours ending Wednesday morning with a cooling trend.

Recent Observations

NWAC pro-observer, Matt Schonwald was at Hurricane Ridge on Wednesday and reported the 12/17 PWL is still causing collapsing on all aspects especially in less skied areas below ridge lines. Recent natural wind slab releases of 10-12 inches where also seen on N-NE slopes off ridges at about 6000 feet. A 2-4 foot x 150 foot wide wind slab crown on the convex north slope below the visitor overlook was seen which may have released on a buried surface hoar layer from around Christmas Eve.

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.

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.