Avalanche Forecast
Issued: Apr 4th, 2017 11:58AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs, Loose Wet and Cornices.
NWAC,
Northwest Avalanche Center
Difficult and dangerous travel and avalanche conditions will develop in the back country in this area on Wednesday where careful snow pack evaluation and cautious terrain choices will be essential.
Summary
Detailed Forecast
Heads up because winter isn't done yet and the quiet weather and snowpack seen the past couple days is going to see significant snow and rain depending on location on Wednesday!
A front will stall over the Northwest on Wednesday with a series of waves moving south to north along the front. This should cause periods of moderate to heavy rain or snow on the Washington volcanoes especially Mt Baker with light to moderate rain or snow in most other areas. Precipitation should fall as snow in the above treeline and perhaps down into the near treeline zones.
New storm slab should build in the above and perhaps down into the near treeline in most areas and will be deepest where there is the most new, rapidly accumulating new snowfall such as Mt Baker. New storm slab should be very touchy especially if there is a daytime warning trend.
Potentially large and destructive wet slab avalanches are likely in the Mt Baker area near and below treeline on Wednesday where rain will be heaviest. Glide avalanches may also be possible in this area.
Loose wet avalanches should be seen in all areas in the near and below treeline. Watch for wet surface snow deeper than a few inches, pinwheels and initial natural releases that indicate an increasing loose wet avalanche danger.
Recent cornices are very large and will get new loads on Wednesday. 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.
New wind slabs won't be added to the lengthy list of avalanche problems on Wednesday but should also build in the above and perhaps down into the near treeline in most areas and will be deepest where there is the most new snowfall such as Mt Baker. New wind slab should also be very touchy especially if there is a daytime warning trend.
Snowpack Discussion
Weather and Snowpack
March was wet and wild for weather and avalanches in the Cascades.
The last major system in March was seen Tuesday and Wednesday when a strong low pressure system brought rising snow levels and locally heavy precipitation 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 ft) and Paradise (14 in) 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 Thursday and combined with daytime warming, this allowed for wind slab and storm slab to begin stabilizing.
A weak front crossed the Northwest on Saturday morning, causing light rain mainly along the Cascade west slopes. This was followed by an upper trough on that caused some light amounts of snow along the Cascade west slopes at much cooler temperatures on Sunday.
Fair but cool weather has been seen in the Olympics and Cascades Monday and Tuesday. Reports generally indicate strong surface or near surface crust layers and shallow recent snow.
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.
Lee was at Little Devil Peak in North Cascades on Monday 4/3. Near and above treeline Lee found about 4 inches of recent storm snow over the most recent crust, and wind slabs averaging about 1 foot on N-E aspects below ridges. Snowpack tests indicated triggering of wind slabs was stubborn. There was evidence of recent large cornice failures. The recent strong rain crust dominated the snowpack below treeline with shallow wet snow conditions during the afternoon. Shallow cold snow was preserved on steep shaded terrain near and above treeline.
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 Sunday the Alpental pro-patrol only reported minor amounts of new snow on the crust from last week 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
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
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
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 5th, 2017 11:58AM
