Avalanche Forecast
Issued: Apr 6th, 2018 1:05PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wet Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Heavy rain, snow, and wind are creating dangerous avalanche conditions. Dangerous and challenging Wet Slabs could run naturally. Avoid avalanche terrain if you see slab avalanches in areas of wet snow. Minimize your exposure to large avalanche terrain from above. Steer around fresh drifts and pillows of Wind Slabs near and above treeline, as they may build later in the day.
Summary
Detailed Forecast
Heavy rain, snow, and wind will combined to create dangerous avalanche conditions. You can trigger a dangerous Wet Slab avalanche at most elevations through midday. These avalanches may even run naturally. Watch for liquid water reaching buried crusts or softer layers of snow well-below the snow surface as a sign that Wet Slab avalanches could occur. Wet Slabs are dangerous because they are hard to predict, give little warning, and can be very destructive. If you see slab avalanches in areas of wet snowpack, avoid traveling in avalanche terrain. You may be able to trigger smaller Loose Wet avalanches more easily in the same types of terrain where the Wet Slab problem exists.
There's a chance that large Glide avalanches could release naturally. Use extra caution and avoid stopping near and below snow covered rock slabs where Glide avalanches could occur. With poor visibility and dangerous avalanche conditions, it's critical that you limit your exposure to large avalanche terrain from above. Many areas, such as Bagley Lakes, are exposed to significant overhead avalanche terrain that cannot always be seen from below. Use maps, satellite imagery, and expert local knowledge to limit your exposure.
You may see a dramatic shift to snow at mid to upper elevations later Saturday and into Saturday night. Wind and new snow will form Wind Slabs on lee slopes near and above treeline. Visual clues such as blowing snow, fresh cornices, and cracks in the snow all indicate that you could trigger a Wind Slab avalanche. Snow falling on recently wet surfaces could result in a problem weak layer.
Snowpack Discussion
Vigorous stormy weather resumes Saturday. The Mount Baker and Rainier areas may see more significant snow accumulations at lower elevations that the other West Slopes and Passes zones. This snow will be falling a wet surfaces on most slopes. Rain has moved into higher elevations and soaking previously dry snow laid down early this week. 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 weekends snow fell on a firm melt-freeze crust (4/1) up to 5,500ft. This could be a problematic weak layer for Wet Slabs this weekend. Other potential layers of concern for Wet Slabs include:
-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 wet loose and slab avalanches on north, east, and south aspects around 5,000ft. Some were big enough to bury a person. He also found very wet snow in the upper 2 feet of the snowpack and challenging, wet travel conditions.
On Wednesday, observers reported a skier triggered avalanche on a steep northeast aspect at 5,300ft 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
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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
Expected Size
Valid until: Apr 7th, 2018 1:05PM
