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Avalanche Forecast

Archived

Dec 18th, 2015–Dec 19th, 2015

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

Mt Hood.

Dangerous avalanche conditions should persist Saturday above treeline, though be improving. Best to be particularly cautious, especially near and above treeline, avoiding steep or wind loaded slopes, watching for sensitive storm layers and wind slab deposits.

Detailed Forecast

A transitory ridge of high pressure should move across the area Saturday to allow for cool weather, light winds and some possible sun breaks. This weather should allow for a slow improving trend as recent storm and wind slab slowly settle and strengthen. 

Avoid terrain where wind may have built isolated wind slab layers, mainly higher elevations near ridges, most likely on a wide variety of aspects..    

Avalanche problems Saturday should continue to be associated with recent storm and wind slab layers. Avalanches should be confined to the new storm snow since Thursday, but a few may step down to older storm layers or crusts in isolated locations.  

Terrain anchors are still causing significant anchoring at the lowest elevations. Use caution near creeks which are still open in some areas.

Snowpack Discussion

Another rain event Thursday on Mt Hood caused rain to about 7000 feet. This produced widespread storm snow conditions and released widespread avalanches Thursday. With reports from pro patrol at Mt Hood Meadows indicating sensitive natural and triggered slab releases were occurring as the storm progressed through the day Thursday and temperatures warmed. Many sympathetic and very sensitive slabs were reported, involving the 8-12 inches of storm snow that had been received by mid-morning Thursday.  

Cooling Thursday night through Friday, and a further 8-12 inches of snowfall is now building new storm layers above a newly forming crust.

Any expected avalanches should now be confined to new wind slab layers, and possibly some storm slab layers, though the cooling trend should limit the extent of storm slab.

 

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.