Avalanche Forecast

Issued: Apr 4th, 2018 10:36AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Stormy weather with rain will maintain dangerous avalanche conditions on Thursday. Wet, heavy snow and wind have built new and reactive slabs. Avoid slopes steeper than 35 degrees where you can trigger Wind Slab avalanches. Expect that you can trigger wet avalanches on slopes receiving rain.

Summary

Detailed Forecast

Snow, wind, and rain are combining to create dangerous avalanche conditions. Warm, wet weather is building new slabs at upper elevations and soaking snow surfaces near and below treeline. Wind has drifted slabs on lee slopes above treeline. In some locations, these new slabs will overly lower density snow from earlier in the week. Visual clues such as blowing snow, fresh cornices, and cracks in the snow all indicate you could trigger a Wind Slab avalanche.

Near and below treeline, rain has set the stage for wet avalanche conditions. As the rain line moves up in elevation, expect natural Loose Wet avalanches especially near treeline. If water reaches buried crusts or softer layers of snow, Wet Slab avalanches could occur. Wet Slabs can be hard to predict and very destructive. If you see slab avalanche activity during periods of rain, avoid traveling in avalanche terrain.

Snowpack Discussion

Ongoing wet stormy weather will continue before a lull on Friday. The wind is stacking fresh slabs on older wind drifted slabs above treeline. Rain is creeping up to higher elevation levels and soaking previously dry snow laid down early this week. The April 1-2nd storm deposited a relatively even blanket of 1.5 feet of snow across the Cascades West and Passes zones. Despite the significant accumulation and snowfall rates, few avalanches were reported with the storm. Interfaces of concern were limited to reports of isolated graupel on a crust at the 4/1 interface. In most locations, this new snow fell on a firm melt-freeze crust up to 5,500ft. On Tuesday, observers easily triggered large, fresh cornices on test slopes. Meanwhile Wind Slab avalanches were harder to trigger. 

Observers have been tracking persistent weak layers deeper in the snowpack. The exact distribution of all of these layers is not well known. Where they have been found, there is good evidence that they are generally difficult to trigger. 

Mt Baker: Several avalanches last Friday and Saturday failed on one of these layers of buried surface hoar (3/25). You are most likely to find this layer as a thin grey line 2-3 feet (60-90cm) below the snow surface on shaded slopes above 5000 feet.

Crystal: A weak interface buried on 3/22 can be found 2-3 feet (60-90cm) below the snow surface. On sunny aspects, a thin layer of weak sugary facets can be found just above a firm melt-freeze crust. On shaded slopes buried surface hoar has been reported.

Below the top 2-3 feet (60-90cm), the snowpack is generally well bonded and significant layers of concern. The much older 2/8 melt-freeze layer can still be found over 6 feet (200cm) deep in the snowpack. While this layer isn't listed in our current avalanche problem set, it may reawaken if enough liquid water reaches it during the coming rain storms.

 

Observations

North

On Wednesday, Mt Baker Patrol reported a skier triggered avalanche on a steep southeast aspect at 4,900ft in the adjacent backcountry. The skier was caught and carried, but not injured. 

On Monday Mt Baker Ski Patrol reported 12 inches (30cm) of new snow moderately well bonded to the old snow surface. Loose snow conditions were observed.

Central

NWAC avalanche forecaster Josh Hirshberg was at Stevens Pass Monday and Tuesday. Josh reported 16 to 18 inches (40-45cm) of new snow in sheltered locations. Near ridgeline, winds  transported snow, forming drifts up to 22 inches (55cm) deep. No new avalanches were observed.

NWAC avalanche forecaster Dallas Glass traveled in the Snoqualmie Pass area Tuesday. Dallas found 18-24 inches (45-60cm) of settled storm snow, generally well bonded to the old 4/1 crust. Several natural and human triggered loose dry avalanches were observed. 

South

NWAC professional observer Jeremy Allyn traveled in the Crystal backcountry Monday. Jeremy found wind transported snow above 6000 feet. The new snow was moderately bonded to the old snow surface with the bond strengthening during the day. As the sun came out, sunny aspect quickly became moist to wet and produced rollerballs.

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

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.

Elevations: Treeline, Below Treeline.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Apr 5th, 2018 10:36AM