Avalanche Forecast
Issued: Nov 27th, 2017 11:30PM
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
Watch for the development of new storm and wind slab layers, especially if you venture to higher elevations. Make more conservative terrain choices as the day progresses.
Summary
Detailed Forecast
On Tuesday a front crosses the Cascades in the afternoon, causing increased winds, initial warming followed by cooling, with Baker and Paradise getting the most new snow.
This weather is likely to build new denser storm and wind slab on previous snow from Sunday night. Storm snow avalanches should primarily involve snow from Sunday night and new snow. These layers lie on a moist to wet older snowpack, which may provide a bed surface. Underlying older snow should continue to gain strength.
Watch for the numerous travel hazards such as open creeks, barely buried rocks and trees, and glide cracks, creating poor and challenging travel conditions, especially below treeline.
The avalanche danger should increase during the day Wednesday for all elevation bands.
Remember that closed ski areas without avalanche mitigation are equivalent to backcountry terrain!
Snowpack Discussion
The great start to the 2017/18 PNW Winter took a giant step backwards over the last week. During an extended period of warmth and wet weather in the days leading into Thanksgiving, an initial round of wet snow and glide avalanches occurred, especially on steep unsupported slopes and rock faces. Another round of rain Saturday night through mid-day Sunday likely did not cause widespread wet snow avalanches on an already beat up and saturated snowpack. This warm, wet and windy period melted significant snow with total snow depths decreasing by 50% or more from their mid-November peak depths at many NWAC stations.
A front and a splitting upper trough crossed the US west coast Sunday and Sunday night. Along the Cascade West slopes, the passage of a sharp cold front on Sunday brought SW-W winds of 40-50 mph with gusts into the 70's and 80's at some higher elevation NWAC stations which decreased into the 10-20 mph range. Temperatures generally dropped from the 30's and 40's into the 20's on Sunday afternoon. New snow amounts were fairly light ending Monday morning, ranging from 0-5".
Lingering light snow showers were seen on Monday at continued cooler temperatures, with lighter winds.
Observations
On Monday morning, Mt. Baker Ski Area reported 8-10" of right side up low density snow, well bonded to previous wet snow. Avalanche control yielded only small loose dry snow avalanches. Wind loading was not significant.
On Monday, Professional Observers Dallas Glass and Jeremy Allyn traveled into the Crystal backcountry. They also found 3-4" of new snow moderately well bonded to the old moist to wet snow interface. The old wet snow is beginning to refreeze. It is supportive to skis, but not boots. They ski triggered a small loose wet avalanche on a directly sun-exposed south-facing slope. They also noted minimal wind effects.
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, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Nov 28th, 2017 11:30PM
