Avalanche Forecast
Issued: Jan 15th, 2016 10:16AM
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
Back country travel in avalanche terrain along the Cascade west slopes is not recommended on Saturday. An avalanche warning has been issued for the Cascade west slopes for Saturday.
Summary
Detailed Forecast
Active weather and avalanches should be seen this weekend!
A front will approach the Northwest Saturday morning and should cross the Cascades Saturday midday. This will cause increasing winds and moderate to heavy rain or snow with warming on Saturday. A change to rain or snow showers should be seen following the front Saturday afternoon and night.
This weather will build generally upside down snow layers of increasing density near the surface and snow may also change to rain in some areas. Up to about a foot of upside down snow should be seen along the west slopes in especially in the higher terrain bands by late Saturday.
This snow will load the January 3rd and January 11th persistent slab layers were they exist along the Cascade west slopes. This could produce natural or triggered avalanches that could be remotely triggered, propagate around terrain features and bury, injure or kill.
New wind slab will also be very likely mainly on lee north to east slopes but on more varied slopes in the Cascade passes.
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 Saturday may entrain previous snow producing large avalanches.
It has been hard to decide whether or not to extend the high danger to below tree line along the west slopes on Saturday. This issue was discussed with several pro patrol members at ski areas along the west slopes today. There was general agreement that natural avalanches due to persistent slab, wind slab and storm slab will be likely to below treeline on Saturday.
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 4-15 inches of snowfall along the west slopes from about January 3-6th.
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.
The latest period of snowfall has been from about January 11-15th with about 7-20 inches along the west slopes.
There has been unexpected dangerous avalanche activity along the west slopes Thursday and today. It looks like persistent layers buried about January 3rd and and January 11th are beginning to get active due to loading from the past couple storm cycles.
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 today 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.
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.
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 16th, 2016 10:16AM
