Avalanche Forecast

Archived

Nov 30th, 2017–Dec 1st, 2017

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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Wind slab is the most likely avalanche problem to encounter on Friday, with deeper and potentially more sensitive slabs possible in the Washington Pass area. Approach cautiously and watch for firmer wind-transported snow on lee slopes near ridges. Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Detailed Forecast

On Friday, a frontal system should stall over the Olympics and north Cascades before finally sagging south in the late afternoon and evening. Alpine winds generally out of the SW will increase late Thursday night and stay elevated through Friday. New and generally shallow storm snow instabilities may develop in the Washington Pass zone Friday afternoon. However, fresh wind slab may build at higher elevations in all zones during the day.

Wind slab is the most likely avalanche problem to encounter on Friday, with deeper and potentially more sensitive slabs possible in the Washington Pass area. Watch for firmer wind-transported snow on lee slopes near ridges, generally northwest through southeast aspects, but expect wind deposited snow on a variety of aspects in areas of more complex terrain.

Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Snowpack Discussion

Avalanche and Weather Summary

Warm, wet weather before Thanksgiving caused first storm then wet snow avalanches in the Washington Pass area. A weather system last weekend that brought rain to the rest of the Cascades brought several inches of snow to Washington Pass. Post-Thanksgiving and outside of the Washington Pass area, little to no new snow has fallen over the central-east and southeast Cascades recently due to warmer temperatures and less precipitation. Snow depths are limited in these areas.

On Tuesday, a frontal passage brought a slight bump in snow levels into the 3000 ft range and deposited about 5-10 inches or more of new snow as of Wednesday morning. The greatest recent storm totals were received in the northeast Cascades. There is now about 12 -20 inches (30-50 cm) above the Thanksgiving Day crust in the Washington Pass area at 5400 feet.

Observations

North

On Tuesday in the Slate Creek drainage between about 5400-7250 feet, NWAC observer Jeff Ward found about 1-2 feet of low density snow bonding well to a thin crust from last week. The new storm snow was not slab like in the areas traveled and no avalanches were triggered or observed. There was potential for wind slabs in specific terrain above treeline, but again no avalanches were directly observed.

Central and South

None

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.