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Avalanche Forecast

Archived

Dec 24th, 2014–Dec 25th, 2014

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

Regions

Mt Hood.

Exercise caution and expect potentially dangerous avalanche conditions on wind loaded slopes above the treeline Thursday.  

Detailed Forecast

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

However wind slabs on lee slopes near and above treeline should continue to be sensitive to human triggering and should be avoided. NW winds likely continued to load lee slopes Wednesday night.  Newly formed corncies should also be given a wide berth.  In general, exercise caution and expect potentially dangerous avalanche conditions on wind loaded slopes above the treeline Thursday.  

 

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 likely 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. New snowfall amounted to only a few inches at NWAC stations by 4 pm Wednesday, but 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 likely 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.  He also reported a break-able crust below the most recent snow.  

 

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.