Avalanche Forecast

Issued: Apr 6th, 2015 11:00AM

The alpine rating is moderate, the treeline rating is low, 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 above treeline, however watch for local areas that have received greater recent snowfall where a locally greater danger may exist.

Summary

Detailed Forecast

A slowly changing weather pattern will maintain cool weather, light winds and isolated showers again Tuesday.  Only light amounts of new snow are expected Monday night through Tuesday.  Due to the showery nature of the expected precipitation, a fair amount of uncertainty is expected with the subsequent hazard forecast. A moderate hazard rating will be maintained above treeline for any new storm snow problems. However, in areas that receive little or no new snowfall through Tuesday afternoon... expect a lower local avalanche danger than the regional forecast. 

Shallow storm slabs may develop Tuesday in areas that receive greater shower activity. It's April, so be aware of the increased solar input reaching more aspects and affecting the snow surface quickly. Loose wet avalanches involving shallow amounts of recent storm snow will be also be possible Tuesday, especially during sunbreaks in the afternoon. 

Small loose dry avalanches are also possible for 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, however 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. 

A front moved across the Northwest Tuesday, followed by showers and rapid cooling, but caused little snow east of the crest except 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 in the Washington Pass area over the Easter weekend and found a few inches of recent snow on a thick crust that needed additional loading to become problematic. Greater recent storm snow was being preserved west of Rainy Pass where about 10 inches of great unconsolidated powder was being maintained on shaded terrain at higher elevations, providing a bit welcomed powder skiing!

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.

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.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Apr 7th, 2015 11:00AM