Avalanche Forecast

Issued: Jan 13th, 2018 10:13AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Monitor changes in surface snow as you travel Sunday. Identify and avoid areas where avalanche are likely to trigger such as unsupported slopes, convex rolls, and steep rocky terrain. Surface crust forming overnight Saturday may delay, but will not eliminate the development of loose wet avalanches on sunny slopes.

Summary

Detailed Forecast

Wind slabs will continue to linger in areas above Friday’s rain line. While the recent mild weather has allowed the likelihood of triggering to decrease, wind slabs still remain possible. Identify and avoid locations where avalanches are more likely to trigger, such as unsupported slopes, convex roles, below cornices, and other shallow locations within the snowpack.

Clear skies overnight Saturday should allow moist snow to refreeze creating surface crusts on most slopes, especially near and below treeline. While this new crust will delay and reduce the development of a loose wet avalanche hazard Sunday, it will not eliminate it. Warm air temperatures and mostly clear skies will allow the surface crust to break down during the day. Pay particular attention to sunny, rocky slopes where the snowpack warming will be accelerated. Monitor changes in surface snow conditions throughout the day. It only takes a few inches of wet surface snow to avalanche.

Snowpack Discussion

Very light rain and snow showers tapered off Saturday morning giving way to clearing skies and warm temperatures. Mild weather provided yet another day for lingering storm and wind slabs from last week’s storms to gain strength.

A combination of recent rain, warm air temperatures, and the sunshine has developed moist to wet surface snow conditions near and below treeline. Clear skies Saturday night should allow snow to refreeze, creating a surface crust on most slopes.

Rain Friday resulted in a natural loose wet avalanche cycle observed in the Mt Baker and Snoqualmie Pass areas.

Above the rain line, a series of storms last week deposited 2 or more feet of cold snow. Winds redistributed snow throughout the storms, 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 crust (Jan. 9) Found in the near treeline elevation band in most areas. A more supportable and thicker crust (Jan. 5) from rain or freezing rain is easily identifiable in the upper snowpack.

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

Observations

North

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

Central

Snoqualmie Pass WSDOT and Alpental Pro Patrol Reported rain to 5000 feet Friday with ice pellets above. Several natural loose wet avalanches were observed from Thursday night, some running 1000 feet. No new avalanches were observed on Saturday.

An avalanche professional in the Alpental valley on Saturday observed wet surface snow down 4 inches and moist snow all the way to the 1/5 crust below treeline. He reported very difficult travel conditions due to the wet heavy snow. 

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. 

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

Possible

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

Valid until: Jan 14th, 2018 10:13AM