Avalanche Forecast
Issued: Nov 28th, 2016 9:58AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Fresh wind slab found in the near and above treeline bands will be the primary avalanche problem on Tuesday. Look for signs of recent wind loading and don't assume a lack of natural avalanche activity means a slope can't be human triggered!
Summary
Detailed Forecast
A weak front will approach the area Tuesday spreading high clouds over the area in the afternoon. After a cool start Tuesday morning freezing levels should moderate a bit into the afternoon.
Fresh wind slabs in the near and above treeline bands found mainly on lee easterly aspects will be the main avalanche problem Tuesday. Watch for signs of firmer wind transported snow that indicate wind slab. Storm slabs should be more difficult to trigger and isolated on Tuesday since storm snow instabilities have had time to settle.
Since tree-line extends vertically higher along the east slopes than the west slopes, the upper part of this elevation band will be included in the Moderate forecast but apply specifically to areas closer to the Cascade crest the have a deeper snowpack.
There currently aren't any indications of persistent weak layers in the snowpack along the Cascade east slopes.
Snowpack Discussion
Weather Discussion for the Cascade East Slopes
A frontal system passed through the Cascades Sunday night and was followed by NW flow aloft on Monday. This pattern favored the west slopes for significant snow accumulations. Near the Cascade crest along the east slopes, snow amounts were in the 3-5 inch range with lesser amounts further east. Moderate WNW winds continued through the day Monday as recorded at the Mission Ridge and Dirty Face weather stations.
Recent Reports for the Cascade East Slopes
There is little in the way of recent reports for the Cascade East Slopes. However on Friday a skier triggered a deep wind slab on a 40 degree northwest slope at about 7500 feet on Slate Peak near Harts Pass. The crown varied from about 2 to 7 feet and ran about 2-300 vertical feet with car sized chunks of debris. Fortunately the skier was not caught. He noted that windward slopes across the valley were scoured indicating recent wind transport.
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
Expected Size
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: All elevations.
Likelihood
Expected Size
Valid until: Nov 29th, 2016 9:58AM
