Avalanche Forecast
Issued: Mar 10th, 2016 11:02AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Cornices and Loose Wet.
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.
The persistent slab avalanche problem will only be listed in the northeast zone and is most likely to be found on non-solar aspects in the above and near treeline band and stretching into the upper portion of the below treeline band. This interface is likely getting harder for a human to trigger but if triggered is capable of producing large avalanches. Be aware that at depths approaching the 1 m mark the extended column test becomes a less reliable indicator of propagation across a column. Deeper tests like the propagation saw test or deep tap test may help but layer identification and terrain selection are your best friends for managing a persistent slab danger.
Snowpack Discussion
Weather and Snowpack
During a period of fair weather in late February, widespread surface hoar formed in the northeast zone mainly surviving outside of steeper solar aspects and wind affected terrain. A weak front buried the surface hoar layer in the Washington Pass area about 2/27 and to a lesser extent the central-east zone.
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. Generally 1 to 1.5 inches of water accumulated along the east slopes in the 24 hours ending 4 am Thursday with minimal shower activity in westerly flow during the day Thursday. Snow levels rose above 5000 feet in the Mission Ridge area Wednesday night but stayed lower closer to the Cascade crest and in the northeast Cascades. About 6-16 inches of snow accumulated through Thursday morning with the higher tallies of over a foot seen at Holden, Washington Pass NWAC and Lyman Lake and Hart's Pass NRCS stations.
We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack due to lack of recent activity and confirming field observations.
Recent Observations
Strong winds on March 3rd caused widespread natural wind slab avalanches in the NE zone. While many were contained to the recent storm snow, one larger slide on a N-NE aspect at Windy Pass likely released down to the February 27th PWL.
Mission Ridge pro-patrol reported a very active day of control on Thursday, March 10th with 1-4 foot slides produced during control work on lee slopes below ridges. Wind slabs were sensitive and ran far. Widespread cornice growth was also noted along ridgelines.
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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
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
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
Expected Size
Valid until: Mar 11th, 2016 11:02AM
