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Avalanche Forecast

Archived

Dec 25th, 2014–Dec 26th, 2014

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

Regions

Mt Hood.

Exercise caution and expect potentially dangerous avalanche conditions on wind loaded slopes near and above the treeline Friday, especially on more southerly aspects.  

Detailed Forecast

Isolated snow showers, sun breaks, light to moderate NW winds and cool temperatures should do little to change the avalanche danger.

Wind slabs on lee slopes near and above treeline should continue to be sensitive to human triggering and should be avoided. Recent NW winds have continued to load lee slopes, especially more southerly aspects. Newly formed cornices should also be given a wide berth.  

In general, exercise caution and expect potentially dangerous avalanche conditions on wind loaded slopes near and above the treeline Friday.  

 

Snowpack Discussion

A warm plume of moisture moved over the Pacific Northwest last weekend. Especially strong winds and heavy precipitation were recorded at Mt Hood; 9-12 inches of water fell at NWAC stations over 72 hrs ending 4 am 12/22 along with frequent gusts over 100 mph in the alpine, wow! A natural avalanche cycle occurred on the upper mountain.   

The frontal system that impacted the area Tuesday evening through Wednesday started warm and ended cold for a right side up layering. Moderate to strong winds turned northwesterly with the cool-down... loading easterly aspects during the day. Winds were SW at the onset of this storm... and above treeline slopes were cross loaded. 

New NWAC pro-observer C.J. Svela reported substantial wind transport Wednesday in White River Canyon in the near and above tree-line elevation bands. This translated to cracking on wind loaded slopes and sensitive cornices in the canyon. 

News arrived from the Mt. Hood Meadow pro-patrol Thursday of a post control release. NW winds and additional snowfall continued to load non-traditional southerly lee slopes. A wind slab avalanche was skier triggered in bounds where 40-60 cm of stiffer snow overlaid a weaker layer at 6600 ft on a southerly aspect. Ski cuts produced few results but the fracture was able to propagate along this interface in extended column tests.  

 

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.