Avalanche Forecast
Issued: Feb 20th, 2016 10:34AM
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
Watch for new building wind or storm slab layers by the end of the daylight hours if you venture out on Sunday.
Summary
Detailed Forecast
A moderate front should cause increasing winds and spread increasing snow especially to the near and above treeline of the west slopes Sunday afternoon with a slight warming trend.
This weather should build new wind slabs on lee slopes mainly near and above treeline by the end of the day. Watch for new or previous firmer wind transported snow mainly on the lee N to SE slopes below ridges.
New storm slab is likely where more than several inches of snow rapidly accumulates Sunday by the end of the day.
A slight warming trend should help to enhance new wind and storm slab layers on Sunday.
Snowpack Discussion
Weather and Snowpack
Rain and mild temperatures dominated last Sunday and Monday forming the latest crust along the west slopes. NWAC sites in the near and below treeline recorded 2-6 inches of mostly rain over the 2 days ending Tuesday morning. The Paradise station recorded over 5 inches of precipitation (mostly rain) in the 24 hours ending Monday 4am!
An active and at times stormy, cooler pattern began to bury the crust on February 17th and has brought about 1-2 feet of storm snow mainly near and above treeline to the west slopes Wednesday to today.
Storm layers may linger in the recent storm snow but will have strengthened today. Bonding of storm snow to the February 17th crust is good in some areas and not in others along the west slopes.
The mid and lower snow pack along the west slopes should be a stable mix of crusts and layers of wet rounded snow crystals.
Recent Observations
Widespread skier triggered avalanches adjacent to the Mt Baker ski area were reported by the Mt Baker pro-patrol today. These were 12-18" deep x 100-200 yards wide storm slabs running fast on the February 17th crust on the northeast side of Shuksan Arm in the near treeline 4500-5000 feet.
NWAC pro-observers Dallas Glass and Ian Nicholson were on Mt Snoqualmie today and reported 2 skier triggered wind slabs 20 cm deep x 30-50 feet wide with ridges limiting the widths of the slabs. This was at about 5500 feet on a cross loaded southwest slope. No other avalanches were seen and they reported a good bond of storm snow to the February 17th crust.
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: 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: Feb 21st, 2016 10:34AM
