Use caution in steep terrain especially if the surface snow is wet more than a few inches, avoiding steep slopes above terrain hazards such as trees or cliffs. Avoid travel on or below cornices.
Summary
Detailed Forecast
Showers should remain light in most areas on Sunday except possibly in convergence in the central west Cascades with cooler temperatures.
This should not greatly change the regional avalanche danger on Sunday.
Expect shallow wet snow conditions over one or more strong near surface crust layers mainly on solar slopes near and below treeline.
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 below large cornices. See a blog post regarding cornices here.
Recent large wind slabs should have mostly stabilized where formed on lee slopes, mainly above treeline and on NW-SE aspects, but will remain listed as an avalanche problem in the Northwest zone for higher terrain that received heavy storm snow Wednesday.
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.
Last week was also active weather-wise, but water amounts/snowfall totals were slightly lower relative to the extreme wetness of the past few weeks. In the 5 days ending Monday morning NWAC stations near and west of the Cascade crest 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 light amounts of new snow in most areas. The winds diminished by Thursday and combined with daytime warming, this allowed for wind slab and storm slab to begin stabilizing.
Very mild temperatures and increased solar radiation Friday allowed for wet surface snow conditions in most terrain, even northerly facing slopes. Shallow loose-wet snow avalanches occurred Friday on many steep slopes but remained small, along with snowpack consolidation.
A weak front crossed the Northwest on Saturday morning causing light rain mainly along the Cascade west slopes.
Recent Observations
North
On Friday, NWAC observer Lee Lazzara was in the Mt Baker backcountry and reported a large number of loose-wet avalanches in many areas had released recently, likely during rain Wednesday or warming Thursday. Several large slab avalanches likely released Wednesday as well. Deep rain runnels were throughout the terrain to about 4800 feet with about a 4 inch supportable surface crust as of Friday morning. By afternoon shallow wet surface snow was making loose wet avalanches possible on steeper terrain.
Central
On Wednesday the 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.
By Saturday the Alpental pro-patrol only reported some surface softening and no avalanches.
South
The 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.
NWAC observer Jeremy Allyn traveled above Paradise Friday to well above the forecast elevations. A strong, supportable near surface crust was encountered at least to about 8000 feet with about 4-5 inches of recent storm snow above. This shallow surface snow shed from the steeper slopes as loose wet avalanches by midday under sunny, warm weather. Several larger slab releases were noted in the Tatoosh and Mt Rainier proper, likely releasing during the rain event this past Wednesday.
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.
Elevations: Treeline, Below 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
Valid until: Apr 2nd, 2017 11:00AM
