Avalanche Forecast
Issued: Dec 1st, 2016 10:15AM
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
New wind and storm slab layers are likely on Friday due to winds, new snow and a warming trend.
Summary
Detailed Forecast
A frontal system will cross the Northwest on Friday. This will cause increasing southwest to west alpine winds, increasing rain or snow and a rise in snow levels.
About 5-10 inches of snow seems likely along the west slopes generally from the upper part of the below tree line band to above treeline on Friday with a warming trend.
This weather should build new upside down wind slab on lee slopes which should be mainly southeast to northwest slopes near and above treeline. Watch for firm wind transported or hollow sounding snow.
New storm slab is also likely in areas that see rapid accumulations of new snow. The warming trend will also aid in the development of new upside down storm slab.
The danger level in this forecast has been nudged up a notch from the outlook issued the previous day. Small human triggered avalanches in many areas along the west slopes in the near and above treeline seems likely on Friday due to the alpine winds, the new snow and the warming trend.
In most of the lower part of the below treeline band there is still insufficient snow for avalanches but watch for early season hazards such as terrain traps, rocks and creeks. The shallowest snow is at pass level at Stevens, Snoqualmie and White Passes.
Snowpack Discussion
Weather Discussion for the Cascade West Slopes
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 with snow levels around 3500-4000 feet.
Tuesday was a break between weather systems.
Another front passed over the Northwest on Wednesday. NWAC stations along the crest had winds generally in the 20-30 mph range with gusts places to the 40-50 mph range. New snow on Thursday morning was around 10-12 inches at NWAC stations along the west slopes.
A bit of a break has been seen on Thursday with snow showers west of the crest generally tapering with a couple more inches in places such as at Stevens and Paradise.
Recent Reports for the Cascade West Slopes
On Wednesday NWAC pro observer Dallas Glass in the backcountry of Crystal Mountain found a right side up snowpack with 4-6 inches of storm snow well bonded to the older settled snow. No avalanches were seen and no evidence of wind effects or layers of concern was noted.
Also Wednesday the Mt Baker pro patrol reported only minimal recent storm snow avalanches from control work. These avalanches ranged up to about 5 inches and all within the most recent storm snow since midnight. No significant storm layer or older layers were noted.
A couple reports for Wednesday are available via the NWAC Observations tab. A skier in the Bagley Lakes area near the ski area reported some small triggered wind slab on northwest slopes within the new storm snow. Another skier near the Stevens Pass ski area found generally right side up snow and good skiing on east to southeast slopes.
NWAC pro observer Lee Lazzara was on the west side of Mt Baker on Thursday up to about 5300 feet. He saw no signs instability and compression tests were negative for a wind layer within the recent snow. He could see some wind effects at higher elevations.
Reports from the Stevens pro patrol and from Dallas at Stevens on Thursday also indicated generally stable, well bonded, right side up snow conditions.
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 2nd, 2016 10:15AM
