Avalanche Forecast
Issued: Nov 28th, 2016 9:58AM
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
Fresh wind slab found in the near and above treeline bands will be the primary avalanche problem on Tuesday. Look for signs of recent wind loading and don't assume a lack of natural avalanche activity means a slope can't be human triggered!
Summary
Detailed Forecast
A weak front will approach the area Tuesday spreading high clouds over the area in the afternoon. After a cool start Tuesday morning freezing levels should moderate a bit into the afternoon.
Fresh wind slabs in the near and above treeline bands found mainly on lee easterly aspects will be the main avalanche problem Tuesday. Watch for signs of firmer wind transported snow that indicate wind slab. Storm slabs should be more difficult to trigger and isolated on Tuesday since storm snow instabilities have had time to settle.
In much of the below treeline band there is not enough snow yet for avalanches. Avalanche problems should be limited to the upper part of this elevation band.
Snowpack Discussion
Weather Discussion for the Cascade West Slopes
After a stormy Thanksgiving weekend which especially hammered the Mt. Baker area, a frontal passage Sunday night was followed by a period of NW flow that dropped 4-12 inches of snow along the west slopes of the Cascades through Monday morning. A few additional inches accumulated during the day Monday as light to moderate showers continued. Snow levels bumped up a bit this afternoon to mix rain with the snow showers at Snoqualmie Pass.
Recent Reports for the Cascade West Slopes
NWAC pro observer Lee Lazzara was touring up to about 5000 feet near the Mt Baker ski area on Sunday. Overall the snowpack was found to be generally right side up with less dense snow nearer the surface and with no alarming layers.
Mt. Baker pro-patrol generally triggered 6-8" storm slabs during their Monday AM control work, but one larger 12" slab released on a SE aspect @ 5500 ft during control, breaking 150 feet across and running several hundred vertical feet.
Alpental pro-patrol found a homogeneous 2 ft of recent storm snow on top of older snow at the top of their hill with an average snowdepth of 3 ft (1 m) above 5000 ft although snowdepths varied widely depending on wind affect. Low quality shears were found in snowpack tests and there were no layers of concern within the new storm snow.
In general the below tree-line elevation band has plenty of terrain anchors and offers lower avalanche potential outside of the Mt. Baker zone. The near and above treeline bands have been filling in throughout the west slopes of the Cascades but are the most shallow in the Pass zones including Stevens, Snoqualmie and White.
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: Nov 29th, 2016 9:58AM
