Avalanche Forecast

Issued: Jan 12th, 2018 10:28AM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs, Loose Wet and Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Weather factors across the area will drive the loose wet avalanche problem Saturday. Watch for clues such as big changes in the weather, pinwheels, and roller balls to indicate wet surface snow conditions are developing. Heavy snowfall has accumulated across the area this week and that deserves respect. Wind slabs and storm slabs are gaining strength, but this takes time.

Summary

Detailed Forecast

Local weather will drive avalanche problem development Saturday. In the south warm temperatures and clearing skies will allow loose surface snow to develop on sunny slopes. In the north, light rain will continue to create moist surface snow conditions near and below treeline. In the central Cascades and passes, rain in the morning and clearing in the afternoon will cause moist surface snow conditions. Pay attention to the weather. Note changes in precipitation and sky conditions, and anticipate how this will affect potential loose wet avalanches.

Near and above treeline lingering wind slabs and storms slabs area expected. Mild weather will continue to allow these problems to gain strength, but this takes time.

The west slopes and passes have received a substantial amount of snow this week. Any avalanche may entrain significant new snow or step-down to deeper layers resulting in avalanches that area larger than anticipated.

Ease into terrain on Saturday. Significant precipitation, recent natural avalanche cycles, and limited observations all warrant stepping out with caution.

Snowpack Discussion

Light rain Friday fell across the west slopes of the Cascades. Rain reached elevations of 5500 feet in the south and 4800 feet in the north. Moist to wet surface snow below treeline resulted in a natural loose wet avalanche cycle Thursday night and Friday morning.

Above the rain line, a series of storms Wednesday through Friday has deposited 2 or more feet of cold snow. Multiple weaklayers were found within the storm snow. A widespread natural storm slab cycle from Thursday has been reported from the Stevens, Snoqualmie, and Crystal backcountry areas. Observations show these weaknesses gaining strength, but this takes time.

Winds redistributed snow throughout the storm, depositing snow onto a variety of aspects. Limited information about above treeline terrain has been received due to poor visibility and difficult travel conditions. A high degree of uncertainty exists in the above treeline areas.

The recent storm snow sits on a thin 1/9 crust found up into the near treeline elevation band in most areas. A more supportable and thicker 1/5 crust from rain or freezing rain is easily identifiable in the upper snowpack.

Below the 1/5 interface, observations continue to indicate a strong snowpack with no notable layers of concern.

Observations:

North

Mt Baker Ski Patrol reported rain to 4800 feet at the ski area Friday. Recent loose wet avalanche activity was observed below treeline in surrounding terrain. Low visibility limited observations into the near and above treeline terrain outside the ski area.

Central

Snoqualmie Pass DOT reported rain reaching up to 5400 feet in the Alpental area Friday. A loose wet avalanche cycle occurred Thursday night below treeline. Some loose wet avalanches ran for 1000 feet and entrained significant snow.

Thursday was a very touchy and active avalanche day at Stevens Pass and Snoqualmie Pass. NWAC forecaster Dallas Glass observed numerous natural storm slab avalanches on all aspects near and below treeline in around the immediate Stevens Pass backcountry. Storm slabs released within new storm layers, occasionally stepping down deeper to a storm layer from Wednesday 1/10. Pro-patrol reported natural storm slabs released in the Alpental Back Bowls Thursday.  Storm slabs were very touchy during control work at Alpental throughout the day with paths reloading quickly during heavy snowfall. 

South

NWAC professional observer Jeremy Allyn traveled in the Crystal Mountain backcountry Friday. He observed rain to 5400 feet. In near treeline areas observations showed storm snow weaknesses gaining strength.  Evidence from Thursday's natural storm slab cycle was evident on all aspects in the surrounding terrain.  

Problems

Wind Slabs

An icon showing 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

Likely

Expected Size

1 - 1

Loose Wet

An icon showing 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.

Aspects: East, South East, South, South West, West.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Storm Slabs

An icon showing 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

Possible

Expected Size

1 - 1

Valid until: Jan 13th, 2018 10:28AM