Avalanche Forecast
Regions: Cascades - West.
Watch for new wind or storm slab layers above treeline Thursday afternoon in the northwest Cascades. Watch for previous shallow wind slab and loose wet avalanches in all areas west of the crest.
Detailed Forecast
A southwest-northeast oriented front will move slowly to the International border area on Thursday with a series of waves beginning to move along the front. This should cause new snow above treeline and rain near and below treeline in the north Cascades Thursday afternoon. But the central and south Cascades should remain dry through the daylight hours on Thursday.
So the main change to watch for will be in the northwest Cascades Thursday afternoon. In these areas above treeline watch for new wind and new storm slab to develop if there is more than several inches of new snow by the end of the daylight hours. Less new snow will mean less of an increase in new wind and storm slab avalanche danger.
Further snow above treeline and rain near and below treeline should be seen in the northwest Cascades Thursday night.
In all areas west of the crest Thursday watch for previous shallow wind slab on previous lee slopes. Loose wet avalanches should also remain possible especially on solar slopes.
Less recent snow and ample terrain anchors should greatly limit the avalanche danger below treeline.Â
Remember the NWAC forecast applies to elevations up to the Cascade crest (~7000 to 8000 ft). Significant new snowfall was received above these elevations on the volcanoes over the weekend and this likely poses a higher avalanche danger. Â
Due to the low snowpack, especially below treeline, watch for terrain hazards such as open creeks, partially covered rocks and vegetation. Many areas below treeline do not have enough snow to cause an avalanche danger.
Snowpack Discussion
Last weekend another warm and wet system brought a one-two punch to the Pacific Northwest. The initial system Saturday brought 1.5 - 2.5 inches of rain with the snow line around 6000-7000 ft. On Sunday a second low pressure system deposited about 8-16 inches of snow above 4500 feet in the north Cascades and above about 6000 feet in the south Cascades. Significantly less new snow was received below these elevations.
A weak front then caused about 1-3 inches of new snow above about 5-6000 feet in the south Cascades on Tuesday.
NWAC observer Dallas Glass was at Paradise Sunday and saw a climber trigger a loose wet avalanche, size 2, on a steep slope at 6500 feet near treeline (no injuries). He also witnessed skier triggered moist storm slabs and loose wet avalanches in the near treeline band involving the 10-12 inches of new storm snow.
NWAC observer Jeff Hambleton was at Mt Baker on Monday and reported 12-18 inches of storm snow with recent and witnessed natural and triggered storm slab and wet loose size 1-1.5 avalanches.
The Chinook DOT crew on Monday found debris from an natural cycle of loose or storm slab avalanches from Sunday.
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Evidence Monday, March 16th of storm avalanches on Sunday on Naches Peak. Photo J. Stimberis.
NWAC observer Tom Curtis was on Nason Ridge near Stevens Pass on Wednesday and found ongoing possible loose wet avalanche conditions on solar slopes and lingering shallow wind slab on northwest to northeast slopes.Â
The snowpack at low elevations remains meager to non-existent with the average snow-line around 4500 feet along the west slopes. The mid and lower snowpack west of the crest consists of layers of stable consolidated rounded grains or melt forms and crusts from multiple warm periods this winter.
Avalanche 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: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
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.
Elevations: Alpine.
Likelihood: Unlikely
Expected Size: 1 - 1