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Avalanche Forecast

Archived

Mar 28th, 2017–Mar 29th, 2017

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

Regions

Olympics.

Very dangerous avalanche conditions are expected on Wednesday and travel in avalanche terrain is not recommended in this area.

Detailed Forecast

A warm front is moving over the Northwest on Tuesday. A second frontal system will move over the Northwest Tuesday night and Wednesday. The second system will cause periods of moderate to heavy rain or snow with further gradually rising snow levels Tuesday night and Wednesday.

An avalanche cycle is expected by Tuesday night and should last into Wednesday. Snow that has accumulated since the mid-March avalanche cycle may be involved or entrained in avalanches.

SW alpine winds are likely to begin to build new wind slab on mainly N-SE slopes on Tuesday night and Wednesday. Alpine winds have been predominately from the SW-WSW the pasts few days with some SE winds seen at the top of Crystal. Hence NW to SE slopes will be indicated.

Loose wet avalanches will be likely in the near treeline and below treeline where there is significant rain and warming.

Recent cornices are very large. Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.

Although they won't be listed as avalanche problems, new storm slab is very likely at higher elevations and wet slab avalanches are possible at lower elevations.

Very dangerous avalanche conditions are expected on Wednesday and travel in avalanche terrain is not recommended in this area.

Snowpack Discussion

Weather and Snowpack 

The first week of March was very cool and snowy. This was followed by periods of heavy rain in the second week of March. This caused significant avalanche cycles in most areas March 9-10. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

Another strong low pressure system brought several inches of rain to the west slopes of the Olympics and Cascades on Friday, 3/17 through early Saturday morning 3/18. Rapid cooling later Saturday morning was followed by generally light snow showers with little in the way of new snow accumulation. The rain event 3/18-19 has formed a very strong crust layer, now buried by this past weeks storm snow.

During the past week, daily weather systems crossed the Northwest. In the Hurricane Ridge area, about 2 feet of snow has fallen over the past five days as of Monday morning.

Similar to most of the recent wind patterns, the latest front passing the area Sunday had moderate sustained S-SE winds. This has transported available snow to build fresh wind slabs in the Hurricane Ridge area.

Daily early spring warming temperatures has allowed surface snow melt and consolidation, at nearly the same rate as accumulations. With about 2 feet of snowfall received in the past five days, the total snowdepth has increased only about 10 inches. 

Recent Observations

NWAC pro-observer, Matt Schonwald was in the Hurricane Ridge area Thursday afternoon. Moderate S-SE winds were quickly building fresh 10-12" wind slab on lee aspects and scouring windward aspects to the most recent rain crust. Wind slabs near treeline were becoming increasingly sensitive by the end of the day. The new cornice formation was occurring along ridgelines. 

Matt was back in the Hurricane Ridge area on Friday. Hurricane Ridge continues to live up to its name because the wind was again the main story with fresh wind slab becoming deeper and more sensitive on lee slopes which included some W slopes near treeline. Fresh and large cornices were also building and deemed likely to fail. All wind loaded avalanche terrain was avoided. Generally shallow, loose wet avalanches occurred below treeline on solar aspects until the cloud cover increased late morning.  

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.