Avalanche Forecast
Issued: Apr 12th, 2018 11:01AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Storm Slabs.
NWAC,
Northwest Avalanche Center
Warming air temperatures and additional precipitation will increase avalanche danger throughout the day Friday creating dangerous avalanche conditions. Avoid all avalanche startzones greater than 35 degrees and limit your exposure to overhead terrain.
Summary
Detailed Forecast
Avalanche danger will increase throughout the day Friday as additional precipitation and warming temperatures impact the area. Expect avalanche problems to grow and become easier to trigger throughout the day. Avoid all slopes greater than 35 degrees Friday as this weather system creates dangerous avalanche conditions.
Moderate to strong winds will form new wind slabs on lee slopes at higher elevations. Use visual clues such as blowing snow, fresh cornices, and snow drifts to identify and avoid steep wind loaded terrain.
Warming air temperatures during the day will create “up-side-down” surface snow conditions where heavier stronger storm snow sits atop lighter weaker storm snow. You will find storm slabs in wind sheltered areas throughout the terrain. Storm slabs are easiest to trigger on convex rollovers, unsupported slopes, and on steeper slopes.
As the day warms and rain begins to fall, storm slab conditions will transition to loose wet snow below treeline. Rain on new snow is an obvious sign unstable snow exists in the terrain. When you see new roller balls, observe fan shaped avalanche debris, or experience rain on snow, limit your exposure to steep overhead terrain where avalanches may initiate.
Other spring time hazards exist in the mountains. Watch for large overhead cornices, glide cracks and opening creeks.
Snowpack Discussion
In general 4-6” (10-15cm) of new snow fell in the area Wednesday night into Thursday. Locally higher amounts were observed. Winds during the snow likely formed soft wind slabs on lee slopes in exposed terrain. This recent snow fell on a new melt-freeze crust formed and buried on 4/11. The thickness and strength of this crust depends on location and elevations.
Below the 4/11 melt-freeze crust the snowpack consist of various melt-freeze crusts, refrozen snow, and strong rounded grains. This has created a strong spring-like snowpack. Avalanches should be limited to the snow above the most recent (4/11) crust layer.
Out of the weak layers that we've tracked through the season, the 2/8 crust can still be identified in many areas 5-7 feet below the surface. This interface 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
Central
NWAC forecaster Dallas Glass traveled in the Stevens Pass area Wednesday. The upper snowpack remained wet allowing for widespread triggered Loose Wet avalanches, beginning small but entraining significant wet snow and becoming large. There was evidence of recent widespread large wet avalanches following the weekends strong storm cycle, however there were many slopes that did not avalanche, holding deep wet snow.
Stevens Pass DOT and Stevens Pass Pro-patrol reported large and destructive avalanches during control work that released as soft slabs with explosives and quickly entrained wet snow at lower elevations on Monday. In a section of the ski area with minimal skier traffic on the east end of Cowboy Ridge, this explosive triggered slide (SS-AB-D2.5-R3-O) on a north aspect produced 4-5 ft of debris as it entrained old saturated snow below.
South
NWAC professional observer Jeremy Allyn traveled in the Crystal backcountry Monday 4/9. Through mid-day, Jeremy found poor travel conditions as the surface crust quickly broke down with moist or wet snow in the upper snowpack. Recent large Wet Loose and one Wind Slab avalanche from the weekend were observed in surrounding terrain.
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.
Elevations: Below Treeline.
Likelihood
Expected Size
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 13th, 2018 11:01AM
