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Avalanche Forecast

Archived

Apr 12th, 2017–Apr 13th, 2017

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

Regions

Olympics.

Watch for fresh wind slabs on lee slopes in steep terrain mainly above treeline. Sun breaks can quickly create loose wet avalanche conditions on steep slopes facing the sun. Give cornices a wide safety margin.

Detailed Forecast

An approaching front will move across the Olympics Wednesday night and early Thursday with strengthening southerly winds.

Light to briefly moderate snow and strong winds will likely build fresh wind slabs on lee slopes below ridges, near and especially above treeline by Thursday. 

Overnight fresh snow may fall on the previous day's melt-freeze crust, which will act as a good sliding surface, especially during sun breaks and daytime warming.

Watch for new wind slabs, mainly on NW-SE slopes above treeline. Keep an eye out for firmer wind transported snow that is a sign of wind slab.

Strong winds and new snow will continue to build and grow recent large cornices. There have been numerous recent cornice failures with some being very large in the WA Cascades. Similar conditions are expected in the Olympics.

Recent cornices are very large and resulting slab avalanches are dangerous and unpredictable. Five people were tragically killed by a cornice release in BC on Saturday. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

Snowpack Discussion

Weather and Snowpack 

Heavy rain in mid March has left behind a well consolidated old snowpack with one or more strong melt freeze crusts in the upper snowpack.

The most recent strong storm arrived Friday morning through Saturday morning, 4/8. This storm deposited about a foot of snow at Hurricane Ridge with very strong S winds Friday afternoon, averaging over 40 mph with gusts over 90 mph! This weather would have built large wind slabs on lee slopes below ridges as well as new fresh cornices along ridges.

A calmer, cooler pattern was seen Saturday and Sunday with little new snow. A period of fair weather and sunshine or filtered sun Sunday allowed for more consolidation and some surface snow melt on all but steep shaded slopes in higher terrain.  

Several weak disturbances have been moving from south to north across the region from Monday through Wednesday. Each depositing about 1-4 inches of snow and accompanied by periods of moderate to strong crest level winds, generally from the SE-SW. The recent shower bands or frontal passages have been building fresh wind slabs, and large cornices daily through the week. Older wind slabs should settle quickly, but be replaced with fresh from the latest passing front.  

Recent Observations

No recent observations from Hurricane Ridge.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.