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.
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 may 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.
The storm system came in right side up with a cooling trend, so storm slabs will be a lower concern and possible in non-wind affected terrain.
Non-avalanche hazard: Despite our seasonally healthy snowpack, many creeks are open and difficult to cross due to the periodic warm temperatures and rain events.
Snowpack Discussion
Weather and Snowpack
A rain crust was buried on 2/17 when a storm cycle from February 17th-20th dropped 1-3 feet of snow along the west slopes.
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 thick surface crusts, especially on solar slopes in most areas by Friday and helped stabilize wind slab formed early last week.
A strong Pacific frontal system blew through the Cascades mid-day Sunday. A few inches of wet snow accumulated through the early afternoon above 4000 feet, but the strong winds were the main story. The most significant transport winds were from the W-SW and seen in the early afternoon post-frontal passage with gusts in the 40s or 50s below or near treeline with much stronger winds above treeline.
The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
Storm snow received Sunday and forecast for Sunday night will be the focus for avalanche problems moving forward along the west slopes including the Cascade Passes. NWAC observer Dallas Glass was at Kendall Peak Sunday morning but new wind and storm slab had not yet become an issue in his area.
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
