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Avalanche Forecast

Archived

Mar 27th, 2017–Mar 28th, 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.

Fresh wind slabs and cornices will be the main problem Monday. Loose-wet avalanches will become likely on steeper sun exposed slopes during extended sun breaks Monday. Keep terrain selection simple and conservative. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches. 

Detailed Forecast

Showers and moderate ridgetop winds overnight Sunday and early Monday should maintain unstable wind slabs on lee slopes below ridges, near and especially above treeline. 

Recent winds have been mostly S-SE, so firmer wind slab should be found mainly on W-N-SE slopes near and above treeline. 

Recent cornices are very large. Make sure to avoid areas on ridges where there may be an overhanging cornice as well as travel on steep slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.

Snowpack Discussion

Weather and Snowpack 

The first week of March saw a very active winter weather pattern with deep snow accumulations, 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, generally 20-25 inches in the Hurricane Ridge area.

Since Tuesday, 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 Sunday afternoon.

Similar to most of the recent wind patterns, the latest front passing the area Sunday, has 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 westerly 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.

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.

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.