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Avalanche Forecast

Archived

Dec 24th, 2015–Dec 25th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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A consolidating and stabilizing trend should begin by Friday. But recent wind slab, storm slab and loose dry snow will all still be key features to avoid on Thursday.

Detailed Forecast

An upper trough and cool air mass will sink to the south of the Northwest on Friday. Alpine winds should decrease and become northwest with light snow showers along the west slopes ending Friday morning. Some cloud breaks may be seen along the west slopes Friday afternoon. Fairer weather will be seen along the east slopes.

This weather should not build new snow pack layers and a gradual consolidating and stabilizing trend should begin by Friday. Cool temperatures may somewhat slow the stabilizing but the avalanche danger should be less than the previous couple days.

Recent wind slab will be most likely to linger on north to east slopes in the above and near tree line. Wind and storm slab avalanches should remain within recent storm layers but could still move fast. Test for inverted strong over weak storm snow and give cornices a wide margin.

Beware of loose dry avalanches in steep non-wind affected terrain that could have unintended consequences such as knocking you off your feet and into a terrain trap.

Remember to discuss plans with your partners or steer clear of high consequence terrain until there is more information and we know more about what is going on out there.

Snowpack Discussion

We have had nearly a week of heavy snow and cool temperatures in the Olympics and Cascades. Sites east of the crest have had 2-4 FEET of snowfall during this time!

A report via the NWAC Observations page indicates 2 skiers were caught in a storm or wind slab avalanche and carried a short distance on Driveway Butte on Tuesday in the Methow River area. The crown at the trigger point was 6 inches but ranged up to 2 feet. This was on a north slope at about 5840 feet.

A snow pit report from 5700-6500 foot range in Stemilt Basin near Mission Ridge also on Tuesday indicated a buried surface hoar at 45 cm. This layer gave some cracking and whumping.

NWAC pro observer Tom Curtis was at Iron Mountain a bit west of Blewett Pass on Wednesday and found some wind slab on steep open slopes even to below tree line and often above terrain traps like creeks or trees. A storm layer was seen at 35 cm with wind and storm slabs reacting to tests to this interface. He noted wind slab releases of 20-35 cm on northeast to east slopes near tree line and storm slab releases near and below treeline on steep terrain features of varied aspect and on small steep slopes above Forest Service roads. He also found some difficult trail breaking in deep powder.

A report from the North Cascade Mountain guides near Washington Pass on Wednesday indicated a storm layer at 15-25 cm but ski cuts on test slopes did not give any results.

The southeast zone should have a shallower snow pack. However, we have no recent observations from the southeast zone.

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.

Loose Dry

Loose Dry avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.