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Avalanche Forecast

Archived

Dec 19th, 2015–Dec 20th, 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 and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Mt Hood.

Dangerous avalanche conditions are expected on Sunday with large storm or wind slab avalanches expected in some areas. Careful snow pack evaluation and cautious route finding will be necessary on Sunday.

Detailed Forecast

The next strong cool cold front will cross the Northwest Saturday night and Sunday morning. This will renew moderate to heavy snow in the Olympics and Cascades Saturday night and Sunday morning with a minor warming trend but the snow level will stay below the passes. It looks like the heaviest snow will shift to Oregon Sunday.

Rapid loading will cause new storm and wind slab will build Saturday night and Sunday. It looks like the most snowfall will be on the volcanoes including Mt Hood where 1-2 feet should be seen by later Sunday so the avalanche danger should be greatest on the volcanoes. This means that some large natural or triggered storm and wind slab should be seen in some areas during the main part of the storm Sunday morning at Mt Hood. But these conditions could easily last all day Sunday.

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 has been followed by about 1.5 feet of snow the past 2-3 days at Mt Hood with low snow levels.

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 at Mt Hood Meadows with reports from the pro patrol of widespread sensitive and sympathetic storm slab of 8-12 inches.

New storm slab layers have stabilized somewhat by Saturday. The Mt Hood Meadows pro patrol reported no more results from avalanche control on Saturday with much improved stability.

The general break in the weather should bring some further stabilizing on Saturday.

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.