Avalanche Forecast

Issued: Apr 7th, 2018 11:39AM

The alpine rating is high, the treeline rating is high, and the below treeline rating is considerable. Known problems include Wind Slabs and Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Winter-like stormy weather will create very dangerous avalanche conditions by Sunday. Wind and Storm Slabs will grow on a variety of terrain features and lee slopes, possibly forming lower on slopes than typical, due to very strong winds. Natural or triggered avalanches are likely Sunday so stay well away from avalanche start zones or areas where avalanches could travel to. Use visual clues such as blowing snow, fresh cornices, and cracks in the snow all indicating that you could trigger a Wind Slab avalanche.  Give fresh Cornices a wide margin of safety. 

Summary

Detailed Forecast

Heavy rain Saturday morning transitioned to snow by late Saturday afternoon following a strong frontal passage. Very strong winds and further snowfall in moderate showers will continue Saturday night and Sunday.

Continued winter-like stormy weather will create very dangerous avalanche conditions at mid and higher elevations by Sunday. The greatest impact of the current storm is affecting the Mt Baker and Mt Rainier volcanic peak areas where the greatest precipitation and hence danger increase is expected.  

Wind Slabs will build on lee slopes and a variety of cross loaded terrain features, possibly well below ridges due to the strength of the winds where natural or triggered avalanches are likely to very likely. Use visual clues such as blowing snow, fresh Cornices, and cracks in the snow all indicating that you could trigger a Wind Slab avalanche.  

While the bonding of new snow to the old wet snow surface should be favorable, unstable Storm Slabs will build on a variety of steep terrain, especially in those areas receiving the greatest new snow amounts. Watch for deeper stiffened slabs developing Sunday on a variety of aspects.

Fresh Cornices are likely to build along ridges, so give Cornices a wide berth if traveling along ridgelines as they often break back further than expected. Be aware of overhead hazard if traveling on slopes with Cornices above as they may fail naturally or be human-triggered. 

Snowpack Discussion

A vigorous storm Saturday caused periods of heavy rain and warm temperatures early Saturday that transitioned to snow or wet heavy snow by Saturday afternoon with shallow new snow beginning to accumulate by Saturday afternoon.  The old rain soaked snowpack has begun to refreeze and form a crust under continued cooling. The Mount Baker and Rainier areas may see more significant snow accumulations at lower elevations than other West Slope areas and Passes zones. New snow has fallen on rain wet surfaces on most slopes.

The April 1st-2nd storm deposited a relatively even blanket of 1.5 feet of snow across the Cascades West and Passes zones and has now been soaked at most elevations. In most locations, last weekend's snow fell on a firm melt-freeze crust (4/1) up to 5,500 ft. 

The newly forming rain crust should form the bed surface for any expected avalanches, however there remain deeper buried layers that continue to be tracked, including:

-A weak layer (3/22) down 2-3 feet (60-90cm) below the snow surface. On sunny aspects, this could show up as a thin layer of firm crust.

-A series of crusts and softer snow about 3 feet below the snow surface.

-The 2/8 crust, now 5-7 feet below the surface. This was associated with the 2/13 facets. Though it's unlikely, if significant water pools on the crust, it could result in very large and dangerous Wet Slabs.

Observations

North

On Friday, NWAC observer Jeremy Allyn traveled in the Bagley Lakes area. He reported many Loose Wet and Slab avalanches on north, east, and south aspects around 5,000 ft. Some were big enough to bury a person. He also found very wet snow in the upper 2 ft of the snowpack and challenging, wet travel conditions.

On Wednesday, observers reported a skier triggered avalanche on a steep northeast aspect at 5,300 ft in the adjacent backcountry. The skier was partially buried, 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.

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

Very Likely

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: All elevations.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Apr 8th, 2018 11:39AM