Avalanche Forecast
Issued: Apr 3rd, 2018 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Stormy weather with rising snow levels will increase the avalanche danger Wednesday creating dangerous avalanche conditions at all elevations. Avoid all open slopes greater than 35 degrees Wednesday as new avalanche problems grow and develop. Wet heavy snow and wind will build new and reactive wind and storm slabs. At lower elevations receiving rain, expect wet avalanche conditions to develop.
Summary
Detailed Forecast
A warm and wet weather system will increase avalanche hazard during the day Wednesday creating dangerous avalanche conditions. Avoid all open slopes greater than 35 degrees and limit your exposure to overhead avalanche terrain during periods of increased rain and snow. With all the recent snow from early in the week, any resulting avalanche may grow large enough to bury, injure, and kill you.
New and reactive wind slabs will build on lee slopes throughout the day. In some locations, these new slabs will overly lower density snow from earlier in the week. Visual clues such as blowing snow, snow drifts, and fresh cornices all indicate wind slabs are likely on nearby slopes.
Wet heavy snow is expected as snow levels rise during the storm. This will create stronger storm snow over weaker drier storm snow. Storm slabs will exist on all aspects. You are most likely to see storm slab conditions develop in areas receiving more new snow such as the Passes and Mt Rainier.
Below treeline, rain will create wet avalanche conditions. Expect new rollerballs, pinwheels, and loose wet avalanche to occur. These conditions will develop during the initial rain events and during periods of prolonged or higher rainfall intensity. If water reaches the most recent buried crust, wet slab avalanche could occur. If you see evidence of slab avalanche activity during periods of rain, avoid traveling in avalanche terrain.
Snowpack Discussion
Tuesday afforded the snowpack time to gain strength. The most recent storm Sunday and Monday deposited 12-18 inches (30-45cm) of snow in most of the Cascade West and Pass zones. Locally higher amounts were reported. Numerous natural and human triggered loose dry avalanches were reported Monday and Tuesday. While subtle mid-storm weak layers were observed, these appeared to gain strength rapidly and no slab avalanches were reported in backcountry areas. Moderate to strong winds transported snow near and above treeline forming wind slabs on lee slopes. Snow drifts of up to 2 feet (60cm) deep were observed.
In most locations, this new snow fell on a firm melt-freeze crust. Observations late Monday and Tuesday demonstrated that the new snow was generally bonding well to the old (4/1) crust.
Two old weak persistent layers have been reported over the last week. The exact distribution of these layers is not known.
Mt Baker: Several avalanches last Friday and Saturday failed on a layer of buried surface hoar (3/25). You are most likely to find this layer as a thin grey line 2-3 feet (60-90cm) below the snow surface on shaded slopes above 5000 feet.
Crystal: A weak interface buried on 3/22 can be found 2-3 feet (60-90cm) below the snow surface. On sunny aspects, a thin layer of weak sugary facets can be found just above a firm melt-freeze crust. On shaded slopes buried surface hoar has been reported.
Below the top 2-3 feet (60-90cm), the snowpack is generally well bonded and significant layers of concern. The much older 2/8 melt-freeze layer can still be found over 6 feet (200cm) deep in the snowpack. While this layer isn't listed in our current avalanche problem set, it may reawaken if it becomes wet.
North
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.
On Friday, NWAC Observers Lee Lazzara and Simon Trautman traveled in the Bagley Lakes area. They reported 3 triggered or natural avalanches about 1 foot deep on north aspects near treeline on slopes with recently drifted snow. The observers found a thin weak layer of surface hoar buried 3/25 at the interface of concern.
Central
NWAC avalanche forecaster Josh Hirshberg was at Stevens Pass Monday and Tuesday. Josh reported 16 to 18 inches (40-45cm) of new snow in sheltered locations. Near ridgeline, winds transportied snow, forming drifts up to 22 inches (55cm) deep. No new avalanches were observed.
NWAC avalanche forecaster Dallas Glass traveled in the Snoqualmie Pass area Tuesday. Dallas found 18-24 inches (45-60cm) of settled storm snow, generally well bonded to the old 4/1 crust. Several natural and human triggered loose dry avalanches 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
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
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
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
Valid until: Apr 4th, 2018 11:00AM
