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Archived

Avalanche Forecast

Dec 2nd, 2017–Dec 3rd, 2017
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Cascades - East.

Isolated and stubborn pockets of wind slabs may be encountered on Saturday near ridges.  Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Detailed Forecast

Generally light and isolated showers Saturday morning should transition to more widespread shower activity in the afternoon. Showers are expected to be heaviest in the late afternoon and evening with some upslope enhancement for the east slopes of the Cascades. See the NWAC mountain weather forecast tab for more details. 

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, 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

Warm, wet weather before Thanksgiving caused first storm then wet snow avalanches in the Washington Pass area.  Post-Thanksgiving about 2 feet of snow has accumulated in the Washington Pass area. There is now about 1-2 feet (30-60 cm) above the Thanksgiving Day crust in the Washington Pass area.

Outside of the Washington Pass area, less recent snow has accumulated for the central-east and southeast Cascades during the week. Snow depths are limited in these areas.

Observations

North

None

Central and South

None

Avalanche Problems

Wind Slabs

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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1