Avalanche Forecast
Issued: Apr 8th, 2018 11:44AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Cornices.
NWAC,
Northwest Avalanche Center
The avalanche danger will rise Monday as sunshine and warm temperatures will quickly make recent snow unstable. You can encounter a variety of avalanche dangers, including Wind Slabs, Loose Wet avalanches, Cornices and Glide avalanches. Use visual clues such as fresh cornices, wind drifted pillows and cracks in the snow all indicating that you could trigger a Wind Slab avalanche. Give fresh Cornices a wide margin of safety and limit exposure on steep sun exposed slopes or where the surface snow is becoming wet.
Summary
Detailed Forecast
Rapid clearing and warming is expected Monday. This should create dangerous avalanche conditions following the weekend's strong winter-like storm cycle.
Fresh wind slabs will be easy to trigger on steep lee slopes below ridges and features where wind drifted snow exists.
Recent storm snow, ranging from 8 to over 24 inches, will become wet, making Loose Wet avalanches likely, especially on steep sun exposed slopes during the late morning and afternoon. Watch for signs of wet snow conditions such as sluffing, roller-balling and any natural Loose Wet avalanches.
In areas that received the greatest storm snow this past weekend, such as the central areas, including higher elevations around Snoqualmie Pass and on Mt Rainier, there is the potential for some large and potentially destructive wet snow avalanches. It will be best to get off steep slopes and run out zones at the first sign of wet snow conditions.
Watch for and avoid exposure to overhead hazards,such as cornices and large unsupported slopes.
Large fresh cornices will become fragile with the sun and warming. Give cornices a large margin of safety and avoid travel on slopes below, as cornice failures may trigger large avalanches on slopes below.
Minimize or avoid exposure below unsupported slopes as the potential for Glide Avalanches will increase during prolonged sunshine and warm temperatures.
Snowpack Discussion
A strong storm over the weekend caused rain Saturday to high elevations followed by cooling and new storm snow of 1 to over 2 ft by Sunday afternoon. There may be a strong difference of new snow amounts with elevation as most lower elevations warmed to near or above freezing Sunday. The new snow was easily transported by periods strong or very strong winds that built fresh Wind Slabs on a variety of aspects, especially below ridges.
The recent storm increased fresh cornices along exposed ridges.
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 rain 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 or gradually refreezing old wet snow should form the bed surface for 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.
Central
Pro patrol at Alpental Sunday reported about 1 ft of new storm snow that was sensitive to ski trigger, releasing soft slab avalanches of 8-12" and running long distances, entraining all the recent snow, running on the old wet snow surface. At lower elevations the new snow was easily releasing as Loose Wet avalanches by skis.
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
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
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: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 9th, 2018 11:44AM
