Avalanche Forecast

Issued: Mar 29th, 2018 11:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

You may still trigger Loose Wet avalanches Friday on steep, rocky, and unsupported terrain where more than 6 inches of wet surface snow exists. If you see new roller balls, observe new Loose Wet avalanches, or experience wet surface snow deeper than your ankle, stay off of slopes greater than 35 degrees and minimize your exposure these slopes above you. If you travel in at higher elevations where dry snow can still be found, use visual clues such as cornices, snow drifts, and wind textured snow to identify and avoid steep wind loaded slopes.

Summary

Detailed Forecast

You may still trigger loose wet avalanches Friday at elevations where moist to wet surface snow still exists. If you experience surface snow deeper than your ankle, see new rollerballs, or observe new loose wet avalanches stay off of nearby slopes greater than 35 degrees. You are most likely to encounter these conditions on steep, rocky, and unsupported slopes. In locations with a poor overnight refreeze, this may occur quickly with daytime warming. Rain showers and/or sun breaks during the day will also accelerate the development of wet surface snow.

Limit your exposure to steep rocky slopes above you if you see signs of wet avalanche conditions. Numerous natural wet avalanches occurred over the last several days traveling down and crossing common travel routes. Where and how you travel can minimize you and your team's exposure to this potential overhead hazard.

While not listed as a problem, we cannot rule out the potential for a wet slab avalanches. These avalanches can be difficult to assess and predict. Step back and avoid avalanche startzones if you see any new signs of slab avalanches in areas with wet snow.

Wind slabs continue to linger at higher elevations. If you travel in areas with dry surface snow expect wind slabs on slopes below cornices and near wind drifts. You are most likely to trigger an old wind slab on slopes greater than 35 degrees, on unsupported slopes, and below cornices. Use visual clues such as snow drifts, firm surface snow that cracks, and cornices to identify and avoid these steep wind loaded features.

Snowpack Discussion

Warm air temperatures and small sun breaks over the last several days created moist to wet surface snow in many locations. Generally two or more feet of melt-form snow exists at the snow surface. Wet surface snow resulted in a loose wet avalanche cycle in the Snoqualmie Pass area. This likely occurred in other locations as well.

At higher elevations drier soft snow can still be found. The elevation where this transition occurs can vary even with in a zone. Typically drier snow can be found above 6500 feet in the southern areas an near 5000 feet in the northern areas. Moderate winds formed wind slabs Monday through Wednesday near and above treeline. These wind slabs have been slowly gaining strength throughout the week.

Several avalanches 3/24-3/27 in the Crystal and Stevens areas failed on a layer of weak older snow buried on 3/22. Observations this week demonstrate this layer has been gaining strength. In some locations you may find a layer of small sugary facets above a firm melt-freeze crust. This layer has generally been reported 12-20” (30-50cm) below the snow surface.

The much older 2/13 facets layer just above the 2/8 melt-freeze layer can also still be found 4-6 feet (120-200cm) deep in the snowpack. While neither of these layers are listed in our current avalanche problem set, they may reawaken if they become wet.

Observations

North

On Wednesday Mt Baker Ski Patrol reported 8-12 inches (20-30cm) of soft snow over a recent melt-freeze crust. This snow was very rapidly effected by small sun breaks and thinning in the clouds. Winds earlier in the weak formed new wind slabs at higher elevations.

Central

On Thursday NWAC professional observer Jeremy Allyn traveled in the Snoqualmie Pass area. Jeremy reported numerous wet avalanche below and near treeline that over the last few days. He found moist to wet surface snow up to 4600 feet.

On Tuesday, Stevens Pass Pro-Patrol reported gusty west winds redistributing snow onto all aspects. Rain at Pass level in the afternoon caused small Wet Loose avalanches. A skier triggered avalanche was reported outside the ski area near Gemini Pass. This slab avalanche may have released down to the 3/22 crust. 

South

On Wednesday. NWAC professional observer Ian Nicholson traveled in the Crystal backcountry. Ian found a 1" (2-3cm) surface crust on all aspects in the morning. This crust quickly broke down on sun exposed slopes as clouds thinned mid-day. Ian did observe the 3/22 interface and found that it was still reactive in some snowpack tests. This layer was 12-18" (30-45cm) below the snow surface.

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.

Elevations: Treeline, Below Treeline.

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 30th, 2018 11:00AM