Avalanche Forecast
Issued: Dec 3rd, 2016 10:00AM
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
Despite a favorable cooling trend with Saturday night's storm, westerly winds will build fresh wind slab on lee aspects near and above treeline. Look for wind slab development further downslope than you might expect due to locally strong winds and persistent loading.
Summary
Detailed Forecast
A strong frontal system will quickly sweep through the Cascades Saturday night and Sunday morning. This system should deliver a nice shot of snow to the west slope of the Cascades with an overall cooling trend continuing on Sunday even as shower activity wanes heading into the afternoon.
Despite a favorable cooling trend with this storm, westerly winds will build fresh wind slab on lee aspects near and above treeline. Look for wind slab development further downslope than you might expect due to locally strong winds and persistent loading. Watch for firm wind transported or hollow sounding snow.
Storm slab should be a lesser avalanche concern on Sunday. However, in wind protected areas that experience persistent and intense snowfall rates, locally sensitive storm slab may develop.
In much of the lower part of the below treeline band there is still insufficient snow for avalanches, so watch for early season travel hazards such as terrain traps, rocks and open creeks. The shallowest snow is at pass level at Stevens, Snoqualmie and White Passes.
Snowpack Discussion
Weather and Snowpack
A front crossed the Cascades Friday with 2-8 inches of new snow reported at NWAC stations through early Saturday morning. The new snow was generally on the heavier side and was accompanied by a slight warming trend Friday. Post-frontal showers were generally light on Saturday and ridgetop winds were moderate out of the west. With the mild snow levels seen so far this season, most avalanche problems have been confined to the most recent storm layers found in the upper snowpack.
Recent Observations
Observations from mid-week indicated a generally stable snowpack.
NWAC pro-observer Simon Trautman was in the Bagley Lakes area near Mt Baker on Friday and at about 5000 feet found dangerous new wind slab conditions due to winds, new snow and the warming trend. He reported a very reactive easy to trigger 15-25 cm new wind slab giving cracking to 2-4 feet and up to 20-30 feet where the new wind slab was deeper. This is a great picture to keep in mind when thinking about the potential wind slab hazard.
Reactive wind slab in a snowpit on Blueberry Hill in the Mt Baker backcountry on Friday. Photo by Simon Trautman.
The Alpental pro-patrol on Friday reported rain soaking into the snowpack in the below treeline band. They reported a small 2 foot climax avalanche on a rock slab due to the rain and warm temperatures.
Professional observations from the Stevens Pass area (pro-patrol) and Crystal backcountry (Dallas Glass) described generally stable conditions Saturday except for pockets of shallow wind slab found near-treeline. Dallas found shallow wind slab generally just below ridges but observed wind sculpted snow much further downslope.
Mt. Baker pro-patrol reported two climax slides at 3800 ft below treeline that released in steep terrain. These events were likely isolated and related to the extra water content found in the snowpack below treeline.
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, South.
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: Dec 4th, 2016 10:00AM
