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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

Snoqualmie Pass.

Dangerous avalanche conditions are expected on Sunday with large 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 where 1-2 feet should be seen by later Sunday so the avalanche danger should be greatest there. 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 especially on the volcanoes. But these conditions could be seen elsewhere and 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

A major rain event was seen December 7-8th. A series of cool storms since then has deposited increasing storm snow at relatively low freezing levels. There has been about 1.5-2.5 feet of snowfall at sites along the west slopes the past 2-3 days.

Widespread sensitive storm slab was reported by the Baker, Stevens and Alpental pro-patrols on Friday. Slab were mostly in the 1 foot range with some to 2 feet at Alpental when the wind shifted to west and there was warming there.

A party of two were caught and injured in the Alpental Valley Thursday midday at the height of the increased storm slab instabilities. Fortunately, no major injuries or worse occurred.

NWAC pro observer Dallas Glass was at Paradise on Friday and found weaknesses at 15 and 45 cm and restricted skiing to slopes 30 degrees or less. Several small triggered storm slab avalanches were noted. A report via the NWAC Observations page for Paradise on Friday also indicated a reactive layer at 22 cm on rain crust.

New storm slab layers have stabilized somewhat by Saturday. The Mt Baker pro patrol reported a few more 8-18 inch storm slabs were triggered on control Saturday morning. The Alpental pro patrol reported one more ski triggered storm slab of 1 foot that stepped to 2 feet to the rain crust of December 7th and 8th and with storm slab still expected in the back country near the ski area.

The general break in the weather should bring some more 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.