Avalanche Forecast

Issued: Dec 16th, 2017 11:56PM

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 - West.

Sunday promises to be a tricky and potentially dangerous day after a long period of low avalanche danger. Building storm slabs may release on a variety of interfaces on all aspects. Choose conservative terrain and look for increasing signs of instability as snow levels rise and winds and precipitation increase during the afternoon and evening.

Summary

Detailed Forecast

A complex storm system will bring increasing rain and snow, rising snow levels and W-SW winds on Sunday. See the mountain weather forecast tab for more details, but in general more warming is expected in the SW Cascades (Crystal, Paradise, and White Pass) with a slower transition to rain at Stevens and Snoqualmie Passes and at Mt. Baker. The heaviest precipitation is forecast for the Stevens and Snoqualmie Pass areas through the afternoon on Sunday, with further moderate to locally heavy precipitation expected in all areas Sunday night.

New snow will build upon the light amounts of new snow received Friday night. New storm and wind slab may fail at a few different interfaces; within unstable new storm layers caused by the rising temperatures and increasing precipitation rates, at the recent snow/new snow interface or down to the previous snow surface comprised of various crusts or weak persistent grain types. We want to highlight that storm slabs may be touchiest in the Stevens Pass area where persistent weak layers have been preserved by Friday night's snow and exist at a variety of elevations and aspects. The later transition to rain forecast for the Stevens Pass area on Sunday will also build locally deeper slabs.

Strong winds will quickly build unstable wind slab on lee slopes at higher elevations Sunday.

Natural loose avalanches will release on steep slopes of all aspects in areas/elevations that see a switch to rain. In areas that receive more snowfall before the transition, expect the potential for loose wet avalanches to entrain more snow. Be especially careful of loose wet avalanches around terrain traps.

Remember that beneath any new snow received during this storm, a myriad of early season terrain hazards exist, especially at lower elevations. Expect terrain hazards with poorly covered rocks, vegetation and creeks, particularly on south-facing terrain, at lower elevations, and on exposed ridges where wind events have stripped much of the seasons snowcover.

Snowpack Discussion

After 12 long days of high pressure, the first frontal system in awhile crossed the Cascades Friday morning. It wasn't much of a front as the high pressure weakened it significantly, with only a brief period of light rain and snow. Localized snow showers continued Friday night. New snow accumulations ranged from 2-5 inches along the west slopes of the Cascades with a local maximum at the top of Alpental with 8 inches of new snow.

Below this most recent storm snow, a highly varied array of surface snow conditions exists, including melt-freeze crusts, settled old storm snow, and a mix of recently formed surface hoar or near surface facets (sugary snow) at lower elevations and some shaded terrain.

The overall snowpack of 2-5 feet along the west slopes is stable with settled old snow sitting over the strong Thanksgiving rain crust, buried about 1.5-2.5 feet.

Observations

North

On Wednesday, NWAC Pro Observer Jeremy Allyn was on Green Mountain and found settled, stable snow conditions. At the summit, northeast slopes were wind affected, but there was no snow available for transport. At peak solar warming, spring-like conditions were found on steep solar aspects. There was little to no snow below 5000 ft.

Central

There is a pertinent observation from the Yodelin area of Stevens Pass from Friday detailing recently buried surface hoar in an open location below treeline on the NWAC observations page.

Alpental pro-patrol reported sensitive but shallow 4-6" storm slabs during Saturday morning's avalanche mitigation. The storm slabs were easy to trigger but did not run far. Wind transported snow was noted near-treeline from N-NW winds Friday night.

On Wednesday, NWAC Pro Observer Jeff Ward was on Jove Peak where he found little to no avalanche concerns. He found a mix of conditions, ranging from corn-snow on solar slopes, to powder on lower angle slopes at lower elevation, and large (1 cm+) surface hoar in the valleys.

South

Professional observer Jeremy Allyn was in the Crystal backcountry on Saturday. He found variable snow with pockets of enjoyable dust on crust in smoother terrain. Very localized pockets of soft wind slab were unreactive. Plenty of early season terrain hazards exist on solar aspects, wind scoured terrain and at lower elevations.

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: North, North East, East, South East, North West.

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: Treeline, Below Treeline.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Dec 17th, 2017 11:56PM