Avalanche Forecast

One of the major impacts of this storm will be the winds. Moderate to strong winds are expected at all elevations. This should easily transport the new snow and form slabs on the lee sides of ridges and mid-slope gullies. Poor visibility may make it hard to visually identify and avoid wind loaded slopes. You need to find other clues like blowing snow, scoured surfaces, and snow plastered on trees to help you anticipate which slopes could harbor wind slabs. If you suspect an area is wind-loaded, give it a wide berth and seek out lower angled terrain. 

These wind slabs will form on top of the snow from this weekend’s storm. While you’re more likely to trigger a newly forming slab, avalanches could fail down into this recent snow in isolated areas. You’re most likely to see larger and deeper avalanches on NW-N-E aspects at mid and upper elevations, especially where the terrain is steep and rocky. Two avalanches were reported Sunday from Snoqualmie Pass that failed at the base of the recent snow. Both occurred in the near treeline band in steep, rocky, and slightly wind affected areas. Observations from the Pass found weak surface hoar at the base of the recent storm snow about a foot below the snow surface. If an avalanche fails on this snow it could break widely or trigger low on the slope.

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.

Snow levels should be on the rise Monday, as warmer air moves into the region. We expect snow to change to rain during the morning at the Pass. When you feel the temperatures warm, or see rain falling on snow, anticipate loose wet avalanches. New rollerballs and wet heavy snow could be your first clue that conditions are changing. As they do, navigate around steep slopes and be leery of what’s above your head. As the rain/snow line continues to rise, you could see loose wet avalanches well into the near treeline band.

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.