Avalanche Forecast
Issued: Nov 24th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
New storm and wind slabs may be significant Friday! Don't let the early season anticipation keep you from a heightened awareness of the conditions. Even small slides can be dangerous, especially in the early season with the many exposed terrain and vegetative hazards. Use these conditions to ease into your winter by practicing transceiver searches and testing your gear, especially if out for the first time this season.
Summary
Detailed Forecast
Diminishing wind and snow early Friday should give way to a few snow showers through the day. Freezing levels should remain relatively low Friday. Only light new snow accumulations are expected during the day Friday between weather systems.
This weather should allow for recent significant storm and wind slabs to begin slowly settling. However, don't let your early season enthusiasm catch you off guard. There have already been active avalanches toward the beginning of the current storm cycle and by Friday, storm layers may be significant and need ample time to settle.
The greatest danger should be in areas where the most significant storm snow has fallen, mainly volcanic peaks, particularly the Mt Baker area and Mt Rainier area. Watch for recent wind transported slab formations on lee terrain, ranging mainly from NW-ENE near ridges.
Caution at lower elevations with shallower snowcover for traps such as creeks, rocks and early season terrain hazards.
Snowpack Discussion
Happy Thanksgiving Snow! From bare cupboards to a filled banquet table in about two days time. Most of the new snow has fallen in the past 24 hours with the volcanic sites receiving 1-2 feet of new snow and significantly less along the east slopes and Stevens Pass.
Two day total snowfall has been between 1-3.5 feet, with the Mt Baker area receiving the greatest amounts. Hurricane Ridge in the Olympics has basically gone from very little snow to 30 inches over the past two days as well.
Below treeline, the current storm cycle is beginning to cover terrain anchors and fill in creeks, through in many areas there will be a significant amount needed to transform the landscape to full winter conditions.
Near and above treeline however, snow depths are climbing quickly, now ranging from 2-5 feet or more in many areas along the west slopes, Olympics and NE Cascades.
The early season snowpack is well settled and prior to the current storm, a significant surface crust existed in most areas.
Limited field observations over the past two days indicated new storm slabs were becoming increasingly easy to trigger in the Mt Baker backcountry and also reported at Mt Hood Meadows early Thursday as ski patrol prepared for opening.
In general, increasing storm or wind slabs overlie a melt-freeze crust above the settled early season snowpack.
This forecast is based on limited field observations. It will be imperative to assess local snowpack conditions as you travel this weekend and give safe margins when in doubt.
Problems
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
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, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Nov 25th, 2016 10:00AM
