Avalanche Forecast
Issued: Jan 16th, 2016 10:38AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs, Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Backcountry travel in avalanche terrain along the Cascade west slopes is not recommended on Sunday due to a variety of avalanche problems. An avalanche warning has been issued for the Cascade west slopes for Sunday with High danger forecast for all three elevation bands.
Summary
Detailed Forecast
Due to the high likelihood and variety of avalanche problems expected, the high avalanche danger forecast will continue along the west slopes of the Cascades Sunday.
Another front will bring more moderate rain and snow to the Cascades Sunday, beginning in most areas by mid-morning and turning to moderate showers in the afternoon. The heaviest precipitation will likely focus on the volcanoes and areas favored by southerly flow. A confusing mix of wandering snow levels should be seen again Sunday, with areas west of the crest warming initially during the morning, and the Passes only slightly warming in the evening with a subtle switch to westerly winds.
This weather will generally cause upside down snow layers of increasing density near the surface and snow may also change to rain in some areas west of the crest and at lower elevations. Up to about a foot of upside down snow should be seen along the west slopes by late Sunday afternoon.
This snow will load the January 3rd and January 11th persistent weak layers were they exist along the Cascade west slopes. This could produce natural or human triggered avalanches that could be remotely triggered, propagate around terrain features and bury, injure or kill.
New wind slab will continue to build mainly on lee northwest to east slopes near and above treeline but should be more varied aspects in the Cascade passes. As the same lee aspects continue to load Sunday, larger avalanches will become possible.
New storm slab due to the warming trend is also very likely on varied aspects where winds are lighter and snow rapidly accumulates to deeper than a few inches Avalanches releasing in near surface layers on Sunday may entrain previous snow producing large and powerful avalanches.
Loose wet avalanche will again be possible below treeline in areas that see initial snow turn to rain Sunday, mainly on steeper slopes below treeline, although this will not be listed as a primary avalanche problem.
Snowpack Discussion
Deep storm snow that fell during mid-late December is well settled, homogeneous and has stabilized so the current avalanche danger focus will be on the upper snowpack. Fair weather over the New Year caused extensive surface hoar and near surface faceted snow. This was buried by about light to moderate snowfall along the west slopes from about January 3rd-6th. Another fair weather period was seen from about January 7-10th. Surface hoar and near surface faceted snow formed again in many areas during this period.
A parade of weather systems this week has added moderate amounts of new snowfall along the west slopes. The latest frontal system on Saturday brought 8-12 inches along the west slopes through 5 pm Saturday, with rain mixing up to roughly 5000 feet outside the Cascade Passes, from Mt. Baker down to Mt. Rainier.
Avalanche and snowpack observations
From a combination of reports from the Mt Baker pro patrol and the NWAC Observations page, a 2-3 foot slab was remotely triggered Thursday at about 5500 feet on the north side of Shuksan Arm. A phone report from the Mt Baker pro patrol to the NWAC indicated the persistent January 11th layer.
NWAC Pro observer Lee Lazzara was near Mt Baker on Thursday and above 5000 feet on W-N-E aspects found 30 cm of snow over a 2 cm think layer of surface hoar buried on January 11th. He was able to trigger this persistent slab with a cornice test and snow pit tests gave easy results indicating propagation. He also noted pockets of wind slab over the same persistent January 11th layer.
A solo ski tourer in Silver Basin near Crystal Mountain Friday triggered a 40 cm slab on a west aspect at about 6600 feet that released on the persistent January 11th layer. This skier was caught and and sustained minor injuries.
NWAC Pro observer Dallas Glass found the buried surface hoar layer well intact 45-50 cm down on a west aspect of Kendall Peak Saturday. Natural avalanches he observed were limited to recent storm snow, with storm slabs averaging around 8-12 inches (20-30 cm). Mt. Baker pro-patrol found storm slabs to reactive with ski cuts this morning in the 8-10 inch range before a change to rain. They also reported an 18" storm slab that caught but did not bury a snowshoer on a steep but short NW slope outside the ski area.
Make sure to check out the Professional observer avalanche and snowpack photos in the film strip!
Problems
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
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
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 17th, 2016 10:38AM
