Avalanche Forecast
Issued: Feb 28th, 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
Play it conservative on Monday and give new wind slabs time to settle by avoiding freshly loaded slopes. Wind slab may extend into the below treeline zone as strong winds Sunday and Sunday night transported snow further downslope than usual. Look for potentially reactive and recently buried weak layers in the upper snowpack of the northeast zone.
Summary
Detailed Forecast
Showers and at times blustery west winds Sunday night should taper down Monday morning. High clouds from an approaching warm front will spread over the area in the afternoon. Snow levels will remain relatively cool.
New wind slab should be sensitive on lee aspects Monday, with significant amounts of new snow transported Sunday and Sunday night. Play it conservative on Monday and give new wind slabs time to settle by avoiding freshly loaded slopes. Wind slab should extend into the below treeline zone as strong winds Sunday and Sunday night transported snow further downslope than usual. Watch for firmer wind transported snow and surface snow cracking as you travel throughout the terrain.
Shallow storm slabs will be a lower concern and possible in non-wind affected terrain mainly near the Cascade crest.
New wind or storm slab layers could be very sensitive and easy to trigger where they build on the latest surface hoar layers such as seen in the northeast zone.
Snowpack Discussion
Weather and Snowpack
Springlike weather under high pressure Wednesday and Thursday of last week led to abundant sunshine with daytime highs climbing into the 40's and 50's. The fair and mild weather caused surface crusts on solar slopes in most areas by Friday and helped stabilize shallow wind slab formed early last week. However, good powder snow was found on non-solar aspects. Also during this period wide-spread surface hoar formed in the NE Cascades, mainly surviving outside of steeper solar aspects.
A weak front brought light amounts of rain and snow Friday night to Saturday morning, burying the surface hoar layer in the Washington Pass area on 2/26.
A strong Pacific frontal system blew through the Cascades mid-day Sunday. A few inches of snow accumulated through the early afternoon above 3-4000 feet near the Cascade crest with rapid drying and very little accumulations further east. Alpine winds were strong with significant W-SW transport winds.
We aren't tracking any layers of concern formed earlier this winter in the mid or lower snowpack.
Recent Observations
Tom Curtis was out at Blewett Pass on Thursday and found cool wind transported snow on lee slopes near tree line that was not cohesive or reactive. Solar and windward slopes are bare or have a shallow snowpack.
NWAC pro-observer Jeff Ward was out near Washington Pass on Thursday and found powder and good ski conditions on north to east slopes. Several loose wet avalanches up to size 1-2 were seen on on steep solar terrain. Surface hoar up to 15 mm was also seen on non-solar slopes. A large cornice release was seen in the Cutthroat Creek drainage.
Jeff was out again on Friday and reported numerous loose wet avalanches to size 1.5 on the south slopes of Delancy Ridge. He noted widespread 6-8 mm surface hoar on non-solar slopes.Good cool snow was also still found on non-solar slopes. On Saturday, Jeff noted the surface hoar buried intact in the Washington Pass area, specifically the Cedar Creek drainage.
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.
Elevations: All elevations.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Feb 29th, 2016 10:00AM
