Avalanche Forecast

Issued: Jan 23rd, 2018 12:35PM

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

NWAC, Northwest Avalanche Center

Cascades - South West.

The avalanche danger will lower slightly during the day on Wednesday, but the potential for triggering a large avalanche remains. Recent storm and wind slabs require time to heal, so be patient and match your terrain selection appropriately. Tree-well and snow immersion hazards are very dangerous, so keep communication with your partner at all times.

Summary

Detailed Forecast

A frontal system will continue to bring significant snowfall to the west slopes of the Cascades Tuesday night, with moderate to occasionally strong crest-level winds and a slight warming trend. On Wednesday, rain and snow showers will be light to occasionally moderate along with lighter crest-level winds.

Storm slabs developing in many areas Tuesday afternoon will build to 1-2' in many areas by Wednesday morning. Storm slabs will remain touchy Wednesday, with the potential to entrain the deeper lower density snow from the past week that exists in all areas, allowing these slabs to run farther and become larger than you expect. Stick to lower-angle terrain that is well-supported or heavily terrain anchored.

Moderate to strong winds are transporting the new snow, particularly near and above treeline. These will continue to build touchy wind slabs near and above treeline on a variety of aspects. New or recent wind slabs may also produce large avalanches, entraining significant amounts of deeper loose dry snow. Avoid wind-loaded terrain and consider any aspect to be fair game for wind slabs on Wednesday.

Light to moderate rain will likely mix up to 4000 feet in the central Cascades (Stevens,Snoqualmie) and 4500 feet in the southwest Cascades (Paradise, Crystal, White Pass), creating the potential for small to large loose wet avalanches at lower elevations. Small loose wet avalanches may become large and powerful if there is enough snow to gouge and entrain. Avoid steep terrain with exposure to terrain traps at lower elevations.

Snow immersion and tree-well hazards will pose a very real danger on Wednesday. Maintain constant communication with your travel partners.

Snowpack Discussion

A snowy pattern that began 1/16 continues to bring new snow nearly every day to the west slopes of the Cascades. Since the rain event on 1/16, 3 to 4 feet of snow has fallen at NWAC stations from Snoqualmie Pass and north, with 6 feet at Mt. Baker. 1.5 to 3.5 feet of snow accumulated at stations in the Mt. Rainier and White Pass areas.

A storm on Tuesday is building touchy and generally shallow storm slabs in many locations within the 6-12" of higher density storm snow. E or SE winds were moving snow on Tuesday. Below the recent storm snow, the snowpack is generally right-side up with some lingering storm snow weaknesses. This upper snowpack is well-bonded to the 1/16 melt-freeze crust throughout the Cascade west slopes.

Moderate to strong winds transported snow forming wind slabs on a variety of aspects at Snoqualmie Pass and the southwest zone including Crystal, Paradise and White Pass on Sunday. Less wind transport occurred in the Stevens Pass area Sunday.

Observations

Central

On Tuesday, NWAC avalanche forecasters in the Skyline ridge area (near treeline and below) reported 3' of snow received during this storm cycle sitting above and well bonded to the 1/16 crust. Higher intensity snowfall began creating shallow storm slabs in the afternoon, with cracking observed generally in the new shallow snow, but occasionally breaking down to an older storm instability about 1' deep. East winds were transporting snow. A professional in the Smithbrook area also reported increasing wind transported snow near treeline and one skier triggered soft slab avalanche on a south aspect near 5000'.

On Tuesday, an avalanche professional in the Alpental Valley/Source Lake area observed slabs becoming reactive 10" down by early afternoon and still building. Moderate to strong winds were transporting new snow throughout the day.

South

Professionals in the Crystal area Monday reported 1 to nearly 2 ft of snow above the 1/16 crust in wind sheltered terrain generally well bonded. In areas stripped of snow during Sunday's storm, less than 8" sits above the most recent crust and is poorly bonded. Loose dry avalanches were the biggest avalanche problem encountered Monday, found in steep terrain at higher elevations and easily ski triggered with the potential to run long distances.

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: All elevations.

Likelihood

Likely

Expected Size

1 - 1

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

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: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 24th, 2018 12:35PM