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Archived

Avalanche Forecast

Dec 3rd, 2017–Dec 4th, 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
1: Low
The avalanche danger rating below treeline will be low

Regions: Cascades - East.

Watch for mostly small wind-loaded pockets on steeper terrain, particularly above treeline and especially in the northeast zone.  Approach ridges cautiously and watch for firmer wind-transported snow. 

Detailed Forecast

Sunny weather should be seen on Monday with light northwest winds and recent snow layers will continue to stabilize. 

Wind slab is replacing storm slab as our primary avalanche problem as storm snow will be stabilizing and increasingly less likely to trigger.

Small wind slabs should remain generally on NW through E aspects on Monday and should become more stubborn or difficult to trigger. Watch for areas of firmer, wind-transported snow mainly on lee terrain features. 

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 accumulated in the Washington Pass area and this amount has settled above the Thanksgiving Day crust.

A series of frontal systems produced snow over the past week at all stations in the Cascades since the Thanksgiving warm period: Locations on the east side of the Cascade crest have received 5-15".

Recent snow accumulation for the 48 hours ending on December 3 along the Cascade east slopes have been light, ranging from 1-8".

Since about Monday November 27th, temperatures have generally been steady in the upper teen's to mid twenties and winds at higher elevations have generally been WSW in the single digits to the low twenty mph range.

Observations

North

NWAC pro observer Jeff Ward was near Washington Pass on Tuesday and found 30-50 cm of low density snow on the Thanksgiving crust. Some small loose dry avalanches were triggered late in the day. There was evidence of buried wind slab, but the layers were not reactive to ski tests.

Central and South

On Saturday, backcountry professionals skiing at Ingalls Peak reported generally stable conditions with some small, unreactive wind slab.

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: All aspects.

Elevations: Alpine.

Likelihood: Possible

Expected Size: 1 - 1

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Unlikely

Expected Size: 1 - 1