Avalanche Forecast

Issued: Apr 5th, 2015 11:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Due to a fair amount of uncertainty with the weather forecast, a moderate hazard rating will be forecast near and above treeline across the Cascades for new storm snow problems.

Summary

Detailed Forecast

An upper level weather disturbance should bring generally light to locally moderate new snow Sunday night through the first half of Monday followed by scattered afternoon/evening showers. Due to a fair amount of uncertainty with the weather forecast, a moderate hazard rating will be maintained near and above treeline across the Cascades for new storm snow problems. In areas that receive little new snowfall through Monday afternoon... expect a lower avalanche danger than the regional forecast. 

Shallow storm slabs may develop Monday with new snow bonding poorly to aspects with surface crusts. It's April, so be aware of increased solar input reaching more aspects and affecting the snow surface more quickly than winter-time. Loose wet avalanches involving recent storm snow will be possible as well Monday, especially if there are sunbreaks in the afternoon. 

Small loose dry avalanches are also possible on non-solar aspects at higher elevations. Loose dry avalanches won't be listed as a top avalanche problem, but be aware of fast moving sluffs knocking you off your feet and into unintended terrain traps on steeper slopes. 

Due to the continued cool weather, cornices won't be listed as an avalanche problem either but use caution on slopes below cornices and on ridges where it can be hard to know if a cornice is present.

Snowpack Discussion

The recent active weather since about mid March onward has brought periodic light snowfalls east of the crest and allowed general snowpack consolidation and stabilization. 

During the fair and mild weather Monday Mar 30th, DOT avalanche professionals working in the Washington Pass area triggered slides using explosives. Wet slab avalanches entrained moist snow in the Liberty Bell paths and buried the closed highway with about 8 ft of debris. Little natural avalanche activity was observed in the area during this time period. 

A front moved across the Northwest Tuesday followed by showers and rapid cooling but caused little snow east of the crest expect locally downwind of a convergence zone centered on Snoqualmie Pass.

A TAY report from Mt. Stuart Thursday reported 6-12" of unconsolidated snow even on solar aspects above treeline, likely a benefactor of Tuesday night's convergence zone. NWAC observer Jeremy Allyn was at Washington Pass on Friday and found about an inch of recent snow on a thick crust that needed additional loading to become problematic, however about an inch or less fell Friday night along the east slopes. 

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

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, Treeline.

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 6th, 2015 11:00AM