Avalanche Forecast
Issued: Mar 22nd, 2016 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Cornices and Wind Slabs.
NWAC,
Northwest Avalanche Center
Spring is springing! Watch for rapid changes especially if the sun comes out. Continue to evaluate snow, terrain and cornices carefully on Wednesday.
Summary
Detailed Forecast
A bit of a break in the weather should be seen Tuesday night and Wednesday morning. Southwest flow should begin to carry another front to the Northwest beginning in the Olympics and northwest Cascades Wednesday afternoon and to the rest of the Cascades Wednesday night. Significant new snow layers are generally not expected in the Cascades through most of the daylight hours on Wednesday. Remember that you may need to adjust your plans if the next front arrives with wind and rain or snow sooner than expected.
Continued loose wet snow conditions can't be ruled out on Wednesday especially if there are sun or solar effects. Surface crusts in the morning don't mean that loose wet snow conditions won't develop by the afternoon. This is mostly likely on solar slopes but can't be ruled out on other aspects. Watch for wet snow deeper than a few inches and pinwheels or natural releases.
Recently formed cornices have grown large and with another release reported at Mt Hood on Tuesday. Avoid areas on ridges or summits where a cornice may be present and avoid slopes below cornices. Cornice releases can be unpredictable during the spring.
Small areas of previous wind slab may remain present on lee slopes. This should be mainly northwest to southeast aspects near ridges and summits. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.
Although not listed as an avalanche problem, glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces. See the recent observations above for a good example in the Mt Baker backcountry.
Snowpack Discussion
Weather and Snowpack
About 2-4 feet of snowfall accumulated along the west slopes March 9-15th.
A combination of sun, mild temperatures and then some rain was seen for several days ending about Sunday 3/20. This weather caused about 10-20 inches of consolidation of the recent snow along the west slopes. Surface snow conditions became highly variable, ranging from cold snow or wind buffed surfaces on non-solar slopes to morning crusts and wet snow on solar slopes.
Older winter avalanche problems of wind and storm slab along the west slopes will have mostly settled and stabilized during the mild weather.
Recent storms did build large cornices along ridges in many areas.
A low pressure system crossed the Northwest on Monday and Tuesday. NWAC stations along the west slopes had about 0-7 inches of new snowfall ending on Tuesday morning with the most at Paradise.
The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
The Alpental pro-patrol reported on Tuesday afternoon 3/22 that sun breaks were giving widespread, point release, small loose wet avalanches with 1-2 feet of debris.
NWAC pro-observer Dallas Glass was on Skyline Ridge at Stevens Pass on Tuesday 3/22 and found 6 inches of new snow at 5000 feet. Wind had formed small isolated wind slab which was not reactive with no releases. Ski cuts on specific steep test slopes were giving loose wet avalanches which were running fast and good distances but staying within the cut and not propagating.
The Mt Baker ski area reported small, natural loose wet avalanches releasing from steep rocks or walls on Monday 3/21.
NWAC pro-observer Lee Lazzara was out near the Mt Baker ski area on Sunday 3/20 and found the top 30 cm of snow moist to wet and no significant layers in the top 1 m of snow. He saw debris from recent small loose wet avalanche and glide avalanches and big hanging cornices.
A report via the NWAC observations page for the Alpental Valley for Sunday 3/20 indicated a wet snowpack and that a large loose wet avalanche had run off Chair Peak.
The Mt. Baker pro-patrol reported a glide avalanche to the ground (5-6 feet deep!) on a steep rock face around 3500 feet in the below treeline band off of Shuskan Arm on Tuesday 3/15.
Problems
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: East, South East, South, South West, West.
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
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
Valid until: Mar 23rd, 2016 11:00AM
