Avalanche Forecast
Issued: Nov 27th, 2016 10:00AM
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
Avalanche problems should be found in new snow layers on Monday.
Summary
Detailed Forecast
A snowy system is headed our way Sunday afternoon to Monday. This will cause some strong alpine winds and should be a good snow producer especially for the Cascade west slopes.
Slowly decreasing northwest flow should carry a cool, slightly unstable air mass and snow showers to the Olympics and Cascades on Monday. The west slopes will continue to be favored and convergence is likely in the central Cascades.
Any avalanche problems should be found in new snow layers. Watch for signs of firmer wind transported snow that indicate wind slab. Storm slab is most likely in areas that rapidly accumulate more than several inches of new snow.
While this forecast is for a moderate avalanche danger mainly above tree line the report above sure indicates you better be watching for potential wind slab. The potential for triggering does not seem great but such an avalanche can have bad consequences. Remember to watch for signs of firmer wind transported snow
A cooling trend should help give favorable right side up new snow profiles in sheltered areas where new snow does not accumulate too rapidly or too deeply.
There currently aren't any indications of persistent weak layers in the snowpack along the Cascade east slopes.
In most areas in the below treeline band there is not enough snow yet for avalanches. Any avalanche problems would be only in the upper part of this band.
Snowpack Discussion
Weather Discussion for the Cascade East Slopes
A weakening front with a lot moisture flowing south to north along the frontal boundary moved slowly into the Northwest with a warming trend late Friday and Saturday. There a brief break by Sunday morning with a cooling trend but the next front is beginning to arrive on Sunday afternoon. Generally light amounts of new snow were seen along the Cascade east slopes on Saturday morning and Sunday morning.
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Nov 28th, 2016 10:00AM
