Avalanche Forecast

Issued: Mar 19th, 2015 11:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Loose wet avalanches and shallow new and older wind slabs are possible near and above treeline Friday.  Loose wet avalanches will mostly likely involve the new snowfall received Thursday and Thursday night above 6000 ft.    

Summary

Detailed Forecast

An approaching frontal system should cause a general increase in clouds and winds throughout the day, but significant precipitation is unlikely until Friday night for the east slopes. 

Loose wet avalanches and shallow wind slabs are possible near and above treeline Friday, and will mostly likely involve the new snowfall received Thursday and Thursday night above 6000 ft.    

Due to the low snowpack, especially below treeline, watch for terrain hazards such as open creeks, partially covered rocks and vegetation. Many areas below treeline do not have enough snow to cause an avalanche danger.

Snowpack Discussion

The latest installment of warm and dry weather lasted from early to mid-March and led to more snowpack consolidation and a scarcity of avalanche activity. Snowdepths vary greatly across the east slopes with a regionally healthy snowpack in the northeast Cascades to bare solar and low elevation slopes for the central and southeast Cascades.   

Last weekend, another warm wet SW flow brought a one-two punch to the Pacific Northwest. Saturday's event brought up to 1.0 inches of water with a snow level around 6500 feet through Saturday afternoon, with much lower totals in the Blewett-Mission Ridge area. 

Another inch or more of water fell more evenly along the east slopes Sunday and Sunday night, with the snow level around 4500-5000 feet in the north and about 6000 feet in the south. Around 15-18 inches of snow fell at the NWAC Washington Pass station and the Hart's Pass Snotel by Monday morning.

NWAC observer Jeff Ward was in the Hart's Pass area over the weekend and reported growing storm and wind slab concerns by late Saturday afternoon. He saw easily triggered small storm slab and dry loose near the 6000 feet elevation.   

NWAC observer Tom Curtis was on Nason Ridge near Stevens Pass on Wednesday and found ongoing possible loose wet avalanche conditions on solar slopes and lingering shallow wind slab on northwest to northeast slopes.

Light snowfall fell in the north Cascades above 6000 ft Thursday with moderate southwest transport winds. 

Problems

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: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

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, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 20th, 2015 11:00AM