Avalanche Forecast
Issued: Dec 28th, 2017 11:27PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs, Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Travel in avalanche terrain is not recommended along the west slopes of the Washington Cascades on Friday.
Summary
Detailed Forecast
The active changeable pattern will continue on Friday. Another moist system arrives on a SW flow. A surface low will track across across NW Washington and a cold front will move across the Cascades Friday evening. Crest level winds ahead of the low will be moderate to strong in the central Cascades, with strong winds in the south Washington Cascades and at Mt Hood. Snow and rain amounts should moderate to heavy along the west slopes of the Washington Cascades on Friday especially in the central to south Cascades with warming trend in all areas.
Travel in avalanche terrain is not recommended along the west slopes of the Washington Cascades on Friday.
Storm slab should be the most widespread avalanche problem on Friday due to rapidly accumulating snow and the warming trend.
But wind slab won't be far behind in extent due to winds, and also to the rapidly accumulating snow and the warming trend.
Loose wet snow avalanches will also become a problem in the lower terrain bands Friday due to snow followed by moderate to heavy rain in those areas.
If you decide to travel in non-avalanche terrain on Friday also remember that unconsolidated snow, particularly around small trees, can present a non-avalanche, snow immersion hazard. Keep visual and verbal communication with your travel partners when traveling in treed terrain.
Also despite all this new snow, early season hazards still exist at some lower elevation locales and especially around creek beds that are not filled in.
Snowpack Discussion
A very big change is underway compared to the past 5 days of quiet weather. The Northwest will experience a very active changeable weather pattern Thursday and Friday with moist systems crossing the Pacific Northwest.
On Thursday strong westerly flow aloft is carrying a very moist occluded front across the NW. Along the west slopes of the Washington Cascades this is bringing increasing crest levels winds, increasing moderate to heavy rain and snow, and rising snow levels. Stevens Pass should remain snow Thursday while Snoqualmie will continue to see see mixed precipitation or freezing rain at pass level.
Initial new snow by the end of the day Thursday mostly ranges from about 5-10 inches along the west slopes of the Washington Cascades.
The frontal zone will remain over the South Cascades and Hood zones Thursday night with the strongest crest level winds and heaviest rain or snow possibly in the south Cascades.
This incoming weather is in contrast to the quiet weather of the past 5 days when weak weather systems deposited about 3-8 in of low density snow along the west slopes of the Washington Cascades. There was mainly gradual snowpack consolidation and stabilizing during this period. The few recent avalanches reported over the last week have involved wind slabs located in higher terrain and on steep features such as terrain convexities.
The upper snowpack along the west slopes of the Washington Cascades has been widely reported as well bonded to the December 15th interface with tests unreactive. Where found, the December 15th layer varies from about 2-3 ft in depth throughout the west slopes of the Cascades.
Observations
North
The Mt Baker pro patrol reported some initial wind loading and building of new wind slab as the winds and snow began to ramp up on Thursday.
On Tuesday 12/26, NWAC Observer Lee Lazzara traveled between 2000-6000 feet in the Swamp Creek drainage. Snowpack tests failed to produce results within the upper snowpack and the 12/15 layer was not identifiable in this area.
Central
The WSDOT crew at Stevens Pass reported heavy snow and some loose wet D1-1.5 avalanches along the highway below the pass level.
The Alpental pro-patrol on Thursday reported that new shallow 6 in storm slab was forming mid-mountain under cliffs as the winds and snow began to ramp up on Thursday.
NWAC pro observer Ian Nicholson was in the Commonwealth Basin area on Wednesday and found an overall right side up snow pack and EC and CT tests did not give results. Wind transport was seen on Red Mountain but any wind slab was seemed limited to specific terrain features. The 12/15 layer was seen at 50-55 cm.
The Stevens pro patrol on Saturday found the 12/15 layer down 3 ft (85-90 cm) and unreactive in large column tests performed on north aspects. No wind slab was observed.
South
A professional in the Crystal backcountry Wednesday observed a settled snowpack well bonded to the 12/15 interface and no obvious layers of concern.
Problems
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
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Dec 29th, 2017 11:27PM
