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Avalanche Forecast

Archived

Dec 21st, 2015–Dec 22nd, 2015

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

Regions

Mt Hood.

New or existing storm slab and wind slab are likely Tuesday, with new wind slab mainly forming on lee slopes ranging from N-SE facing, especially by afternoon as the next storm increases.

Detailed Forecast

Continued cool weather with snow showers and moderate winds overnight should build further storm slabs. A brief decrease is expected early Tuesday ahead of another approaching Pacific frontal system. The next system moves into the Mt Hood area early Tuesday afternoon with renewed moderate snow and strong westerly winds. This should load existing extensive areas as well as build new areas of wind slab on lee slopes below ridges and create more extensive storm slabs.

Dangerous avalanche conditions are expected in the above and near treeline bands Tuesday, especially later in the day.

Remember to give cornices a wide margin. Also above tree line strong winds recently have scoured snow from exposed slopes, leaving exposed rock and vegetation. Terrain anchors are still causing anchoring at the lowest elevations, but less and less with every storm passage!

The recent unconsolidated storm snow is getting very deep! There was a snow immersion fatality in a tree well in bounds at Snoqualmie in the central WA Cascades on Saturday, so always ski with a partner. 

Snowpack Discussion

Heavy rain fell to about 7000 feet on Mt Hood on December 17th. This has been followed by about 2-3 feet of snow the past 5 days.

NWAC pro observer Laura Green visited the Heather and Newton Canyon areas on Thursday and reported rapidly changing conditions with plentiful cracking and natural or easy to trigger heavy storm slab of 20-25 cm.

Widespread storm slab avalanches were also seen Thursday by the Mt Hood Meadows pro patrol with widespread sensitive and sympathetic storm slab of 8-12 inches.

Storm slab layers stabilized somewhat on Saturday. However the Mt Hood Meadows pro patrol reports increasing new 6-8 inch storm slab on the east facing slopes on Sunday releasing on the crust from December 17th.

Another strong storm Monday deposited very heavy snowfall through the day, with 10-15 inches of new snow accumulating through the day! 

Problems

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.

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.