Avalanche Forecast
Issued: Jan 30th, 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
The greatest avalanche problem should be wind slab on lee slopes near and above treeline mainly NW-N-SE facing slopes below ridges. Watch for signs of firmer wind transported snow.
Summary
Detailed Forecast
Light winds and periods of light snow showers with continued cool temperatures should be seen on Sunday. A skiers delight! But continue to evaluate snow and terrain carefully on Sunday.
This weather should bring a gradually decreasing avalanche danger on Sunday. Storm layers that may exist above the new crust will slowly settle but could remain reactive to human triggers mainly in the above treeline Sunday.
The greatest avalanche problem should be wind slab on lee slopes near and above treeline mainly NW-N-SE facing slopes below ridges. Watch for signs of firmer wind transported snow.
Watch for storm slab in areas that had rapidly accumulating snowfall.
Watch for newly formed cornices along ridges as well. These may be sensitive to human triggers.
Below treeline, the old wet snowpack should continue to drain and slowly refreeze.
Snowpack Discussion
Weather and Snowpack
Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on January 3rd and 11th. Two heavy rain events, one about January 21st and one last Wednesday-Thursday have likely eliminated the January persistent layers and they will be removed from the avalanche problems in the west slopes zones.
A strong occluded front with strong winds crossed the Northwest on Friday. NWAC stations along the west slopes had about 1-1.5 feet of new snow by Saturday morning. Some areas are reporting a strong bond to the new crust and some a poor bond depending on when new snow arrived. However, even where good bonding exists there may be sensitive storm layers within the new snow. These storm layers may make for some sensitive triggered slabs and will take a bit of time to stabilize.
Recent Observations
NWAC pro-observer Simon Trautman ventured a ways into the white out conditions in the Mt Baker back country Friday afternoon to test the new storm snow. New sensitive wind slabs were quickly forming on a variety of exposed aspects near treeline during heavy snowfall and very strong westerly winds. Drifts were 8-18 inches and increasing. There was an intact layer of low density stellar crystals just above the crust forming the weak layer and reactive to skis on wind loaded terrain. See image below.
New sensitive wind slabs forming in the Mt Baker back country Friday afternoon 1/29. S. Trautman
Other observations received Friday afternoon from the pro-patrol at Alpental indicated the crust had already formed prior to significant snow accumulations so bonding was poor. Winds were very strong and limited consistent uniformed slab formation.
Less activity was reported on Saturday. The Crystal pro-patrol reports isolated 6-8 storm slab via ski cuts on varied aspects. The Alpental pro-patrol reported no significant avalanches and that new snow was not cohesive.
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: Jan 31st, 2016 10:00AM
