Avalanche Forecast

Issued: Apr 3rd, 2015 11:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

New snow should require careful snow and terrain evaluation at higher elevations east of the crest Saturday.

Summary

Detailed Forecast

Weak flow and mostly light showers should move out of a low or form over the Northwest in a cool slightly unstable air mass Saturday afternoon. These showers may be more likely during the afternoon hours and at higher elevations east of the crest.

Watch for possible new small wind and storm slab from Friday. These layers would be deeper if you find an area that had more snowfall.

It is April and the sun is getting much stronger. So possible small loose wet avalanches will remain in the forecast as well. Solar slopes in any area with significant new snow from Friday will be very susceptible on Saturday.

Cornices won't be listed as an avalanche problem but use caution on slopes below cornices and on ridges where it can be hard to know if a cornice is present.

Snowpack Discussion

Active weather has been seen since about mid March but there has been little snowfall overall east of the crest. So consolidation and stabilizing has been mainly seen east of the crest the past couple weeks.

During the fair and mild weather Monday, DOT avalanche professionals working in the Washington Pass area triggered significant slides Monday using explosives. Wet slab avalanches entrained moist snow in the Liberty Bell paths and buried the closed highway with about 8 feet of debris. But little natural avalanche activity was observed in the area. 

A front moved across the Northwest Tuesday followed by showers and rapid cooling. But this caused little snow east of the crest.

NWAC observer Jeremy Allyn was at Washington Pass on Friday morning and only found about an inch of recent snow on a thick crust. He commented this would be a good set up for wind and storm slab if there is significant new snowfall.

Another front and short wave trough are crossing the Northwest Friday afternoon. This should be accompanied by moderate southwest winds and mostly light rain or snow east of the crest. Snow showers at higher elevations should decrease pretty rapidly Friday evening. 

Snowdepths vary greatly across the east slopes with a regionally healthy snowpack in the northeast Cascades to bare solar and lower elevation slopes in the central and southeast Cascades. Many areas at lower elevations and further away from the Cascade crest do not have enough snow to cause an avalanche danger.

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: North, North East, East, South East.

Elevations: Alpine.

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.

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: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Apr 4th, 2015 11:00AM