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Avalanche Forecast

Archived

Apr 13th, 2017–Apr 14th, 2017

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

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Watch for fresh wind slab on lee slopes in steeper terrain primarily above treeline. Sunbreaks can quickly create loose wet avalanche conditions on steeper solar aspects. Give cornices a wide safety margin.

Detailed Forecast

A cool upper trough will sit over the Pacific Northwest on Friday with shower activity increasing in the afternoon and evening. Sunbreaks are most likely during the morning through mid-day before convective clouds build in the afternoon.   

Watch for firmer wind transported snow, mainly on NW-SE slopes and above treeline on Friday. Fresh wind slab may have locally bonded poorly to graupel layers received Thursday. 

After a cool night Thursday, snow levels will remain low on Friday. This should generally restrict the loose wet potential to the shallow amounts of new snow found on steeper solar aspects near and below treeline. This time of year even brief sunbreaks combined with subtle daytime warming can quickly activate loose wet avalanches. 

Recent cornices are very large and resulting slab avalanches are dangerous and unpredictable. There have been numerous recent cornice failures with some being very large in the Washington Cascades. Five people were tragically killed by a cornice release in British Columbia 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

Warmer temperatures with periods of rain received in mid March has left behind a well consolidated snowpack with one or more strong melt freeze crusts in the upper portion of the snowpack in the near and below treeline elevation bands. 

A series of strong spring storms occurred during the first week of April.

A front crossed the Northwest on Sunday night and Monday, depositing up to a few inches of snow along the east slopes of the Cascades. Moderate winds built fresh wind slabs above treeline on steep lee terrain features.

Mostly sunny weather Tuesday helped storm snow to begin to settle, however, winds remained strong enough above treeline Tuesday to continue to transport surface snow. 

A weather system passing through the area Wednesday night produced roughly half an inch of water across east slopes of the Cascades through Thursday morning. This likely translated into a few inches of new snow around 4000 feet with slightly higher amounts above 6000 ft. Showers increased during the day on Thursday with light additional snow accumulations mixing with graupel during more intense showers. 

Recent Observations

North

No recent observations.

Central

Tom Curtis was on Irving Peak near Poe Mountain, NE of Lake Wenatchee on Wednesday, 4/12. There was about 6-8 inches of recent storm snow over the most recent melt-freeze crust on shaded slopes. This snow became moist to wet through the day and with little effort produced rather large loose-wet avalanches, even on steeper N-NE aspects. Several large to very large loose wet slides were observed and heard across the valley, releasing on N-NE facing terrain on Nason Ridge, including higher elevation band above treeline. Cornices were also very large. These conditions limited any travel on steep terrain of consequence.

South

No recent observations.

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.