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Avalanche Forecast

Archived

Apr 14th, 2017–Apr 15th, 2017

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

Mt Hood.

Recent wind transport, continued cold temperatures and the potential for larger wind slab will raise the avalanche danger to Considerable above treeline. Afternoon sunshine will increase the loose wet potential on steeper solar aspects Saturday. Initially shallow loose wet avalanches could entrain deeper layers and become large in specific locations. Give cornices a wide safety margin.

Detailed Forecast

The cool upper level trough that's been plaguing the Pacific Northwest for several days should finally begin to loosen it's grip on the area Saturday. Clearing is expected in the Mt. Hood area Saturday afternoon. Recent wind transport, continued cold temperatures and the potential for larger wind slab will raise the avalanche danger to Considerable above treeline. Fresh wind slab should be sensitive and found on lee slopes in steeper terrain, primarily on NW-SE slopes and above treeline on Saturday.  

Afternoon sunshine will increase the loose wet potential on steeper solar aspects Saturday. Initially shallow loose wet avalanches could entrain deeper layers and become large in specific locations. Be especially wary of the loose wet avalanche potential near terrain traps. 

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

Heavy 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. 

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

A weather system passing through the area Wednesday night produced roughly half an inch of water at Mt. Hood stations through Thursday morning. This translated to a few inches of new snow at the Timberline and Mt. Hood Meadow base stations. Showers increased during the day on Thursday with light additional snow accumulations mixing with graupel during more intense showers. Periods of moderate S-W winds occurred Wednesday night and Thursday transporting recent snow above treeline. By Friday afternoon an additional 4-8 inches of snow had accumulated above 5000 feet. Friday was also cool, cloudy and showery with continued moderate westerly transport winds. 

Recent Observations

Updates from the Meadows patrol Tuesday and again Wednesday indicated daily fresh wind slab above treeline on isolated features, mainly N-NE facing terrain. These were hard slabs ranging from 4-12 inches and stubborn Tuesday with some 1-2 ft wind slabs by Wednesday. These slabs were mostly released with explosives with good propagation and some were sensitive to ski trigger. Cornices were again noted to be very large!

Laura Green was touring in Newton Canyon Thursday. She found firmer wind slab above treeline that was less reactive in snowpack tests than on Wednesday. No natural avalanches were noted and loose wet activity was limited by the cooler temperatures and increased cloud cover. Laura noted a graupel layer well distributed throughout the terrain she covered on Thursday; a layer to watch where fresh wind slab built Thursday night and Friday.   

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.