Avalanche Forecast

Issued: Mar 10th, 2016 11:02AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Wind slabs and cornices along ridges should be less sensitive Friday but still capable of producing large avalanches near and above treeline. The loose wet potential should be relegated to steeper slopes primarily below treeline but will have the potential to entrain heavy wet snow. Avoid terrain traps where even a small yet heavy loose wet avalanche would have serious consequences. You may not see natural avalanche activity Friday, but the human triggered potential remains elevated. 

Summary

Detailed Forecast

Clouds should spread over the Cascades from south to north on Friday as moisture rotates northward and a weak frontal band offshore approaches. Light precipitation should develop during the afternoon hours, specifically in the south Cascades. Snow levels should remain moderate Friday, rising along with daytime warming. 

All the travel advice associated with Considerable danger will be pertinent on Friday. Strong winds and the cross-loading of slopes Wednesday night and Thursday will require cautious route-finding and conservative terrain selection Friday. Wind slabs and cornices along ridges should be less sensitive Friday but still capable of producing large avalanches in specific areas near and above treeline.

The loose wet potential should be relegated to steeper slopes primarily below treeline Friday but will have the potential to entrain heavy wet snow. Avoid terrain traps where even a small yet heavy loose wet avalanche would have serious consequences. There continues to be a lot of water in our snowpack, so don't underestimate loose wet avalanches in the wrong terrain. 

Powerful glide avalanches remain possible in isolated terrain features such as on steep slopes with smooth bed surfaces and especially where the slabs are unsupported from below.

 

 

Snowpack Discussion

Weather and Snowpack

The nonstop active weather pattern continues to push a storm system through the PNW almost every day or two with fluctuating but generally moderate snow levels. The last storm impacted the area Wednesday through Thursday with a warming trend that peaked Wednesday night with the frontal passage in the early morning hours and also featured very strong winds seen throughout the range. Winds decreased Thursday afternoon along with decreasing shower activity. Generally 1.5 to 3 inches of water accumulated along the west slopes in the 36 hours ending 4 pm Thursday. Outside the Cascade Passes, rain reached up to 5000' feet in the north and 6500 feet in the south with snow levels only rising to about 4000 feet at Stevens and 4500 feet at Snoqualmie Pass.  About 4-14 inches of snow accumulated at NWAC stations through Thursday morning. 

There remains a significant amount of water in the snowpack. There are several crusts in the upper snowpack interspersed with moist or wet grains. The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

Stevens Pass and Alpental Pro Patrol reported an extensive natural cycle Wednesday night. Control results were sensitive and widespread Thursday AM involving new storm snow. As the sun poked out and temperatures rose Thursday, large loose wet avalanches, both natural and skier triggered were reported by the pro patrol in the Stevens area with smaller loose wet noted in the Alpental area. Crystal patrol generally had 1-2 ft slab releases during control work Thursday with large and sensitive cornices along ridgelines. 

In areas that experienced rain Wednesday night, mainly outside the Passes and in the below and near treeline bands, a breakable crust made for tough skiing Thursday. 

Problems

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: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Cornices

An icon showing 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

Possible

Expected Size

1 - 1

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

Valid until: Mar 11th, 2016 11:02AM