Avalanche Forecast
Issued: Mar 30th, 2017 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Cornices.
NWAC,
Northwest Avalanche Center
Loose wet avalanches will be increasingly likely Friday, where small wet snow avalanches may entrain deeper wet snow layers and become large. Use caution in steep sun exposed terrain during the warmest part of the day, avoiding steep slopes above terrain hazards such as trees or cliffs.
Summary
Detailed Forecast
High pressure is expected to rebuild over the area Thursday night through Friday. This will cause clearing overnight and allow for wet surface snow to begin refreezing and strengthening.
Partly to mostly sunny conditions Friday and warming temperatures should cause a gradual increase in danger from wet snow avalanches, especially on slopes receiving direct sun and during the warmest part of the day.
Recent shallow wind slabs should continue to settle and stabilize where formed on lee slopes, mainly above treeline and on NW-SE aspects.
The danger will be rated higher in the Mt. Baker area where once again, more snow fell with a cooling trend Wednesday afternoon through Thursday. Snow levels were regionally lower in the northwest zone during this event, maintaining the low probability/high consequence potential for very large avalanches above treeline and large wet snow avalanches below treeline.
The expected clearing and cooling overnight Thursday should cause a strengthening surface crust by Friday. However, if previously wet surface snow has not refrozen, be suspicious of loose wet avalanches that may begin small but entrain older snow and become dangerous and difficult to manage. Pay particular attention to steep solar facing slopes above terrain traps such as trees, cliffs or gullies where being caught and carried, even in a small slide, could prove consequential.
Recent cornices are very large. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes directly below large cornices. See a blog post regarding cornices here.
Snowpack Discussion
Special Note: For more information on the massive natural cornice triggered avalanche on the north side of Ruby Mountain on Sunday 3/19 and general thoughts about low-likelihood/high consequence avalanches, please see NWAC's blog post issued, Sunday, March 26.
Weather and Snowpack
Let's just say it's been a wet and wild few weeks regarding weather and avalanches in the Cascades.
This past week has also been active weather-wise, but water amounts/snowfall totals have been slightly lower relative to the extreme wetness of the past few weeks. In the last 5 days ending Monday morning NWAC stations near and west of the Cascade crest have picked up 1.5-4 ft of snow with the most at Mt. Baker and above the Pass levels.
A strong low pressure system brought rising snow levels and locally heavy precipitation Tuesday night through Wednesday along the west slopes of the Cascades. Most ski areas and DOT programs checked in reporting natural and explosive controlled avalanches in their area of responsibility. The most snow was received at Mt. Baker (2 feet) and Paradise (14 inches) with more moderate amounts elsewhere before changing to rain on Wednesday.
Cooling and showery weather Thursday allowed wet snow to begin refreezing with an additional 3-6 inches accumulating in most areas. The winds diminished by Thursday and combined with daytime warming, this has allowed for recent wind slabs to begin stabilizing.
Recent Observations
North
On Wednesday Mt. Baker pro-patrol reported widespread storm slab and loose wet avalanches below 4500 feet during morning control work. 1 large natural avalanche was observed off Shuskan Arm. Start zones above 5000' were stubborn to trigger during early morning control work but likely became more sensitive as warming pushed the snow-line higher during the day.
Central
On Wednesday Alpental pro-patrol reported widespread natural and explosive triggered storm slab avalanches late morning. Loose wet avalanches on the lower half of the mountain were beginning to entrain deeper layers. Snoqualmie DOT reported large natural avalanches (up to size D2.5) running in start zones above 4500 feet late Wednesday morning. Stevens Pass DOT reported large slides during control work Tuesday night with avalanches gouging down to deeper layers.
South
Crystal pro-patrol reported sensitive 4-8" storm slab on the upper half of the mountain Wednesday morning. A widespread but shallow natural loose wet cycle was observed in the surrounding backcountry later in the day with continued warming.
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.
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.
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
Valid until: Mar 31st, 2017 11:00AM
