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Avalanche Forecast

Archived

Dec 20th, 2015–Dec 21st, 2015

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
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.

A storm with strong shifting winds, heavy snow and rapid loading is expected at Mt Hood on Monday. Back country travel is not recommended above and near treeline at Mt Hood on Monday.

Detailed Forecast

A strong low pressure system will move west to east across Washington on Monday. There should be a big difference in the weather and avalanche conditions between the north and south Cascades with the strongest winds and heaviest snowfall in the south Cascades. But a different track of the low could bring much different than expected weather and avalanche conditions on Monday so the forecast is somewhat uncertain.

This storm should bring strong shifting winds, heavy snow and rapid loading to Mt Hood on Monday with potentially deep new storm slab and wind slab. Back country travel is not recommended above and near treeline at Mt Hood on Monday.

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

Snowpack Discussion

Rain was seen to about 7000 feet at Mt Hood on December 17th. This has been followed by about 1.5 feet of snow the past 2-4 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 east slopes on Sunday releasing on the crust from December 17th.

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.