Avalanche Forecast
Issued: Mar 21st, 2015 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
There may be some shallow wind slab above treeline Sunday. Any new snow that does fall will be susceptible to sunbreaks later Sunday afternoon, so use extra caution on steeper solar slopes near and above treeline.
Summary
Detailed Forecast
A frontal system will bring some light rain and snow to the Cascades Sunday, but mainly to the west slope areas, followed by showers Sunday afternoon. Increasing southwest ridge top winds may build new shallow wind slabs on lee slopes, mainly northerly to easterly facing near and above treeline.
Any new snow that does fall will be susceptible to sunbreaks later Sunday afternoon, so use extra caution on steeper solar slopes above treeline.
Less recent snow and ample terrain anchors should greatly limit the avalanche danger below treeline especially in the south and central east Cascades.
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
During the storm cycle the weekend of March 14-15th, 1 to 2 inches of water fell along the east slopes, with the snow level lowering to around 4500-5000 feet in the north and about 6000 feet in the south last Sunday. 15-18 inches of snow fell at the NWAC Washington Pass station and the Hart's Pass Snotel by Monday morning.
North Cascades Heli operations in the Washington Pass area reported small some loose dry avalanches near and above treeline earlier in the week along with some sensitive but shallow wind slab on lee aspects. 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.
Benign weather and milder temperatures have aided storm snow settlement and promoted a general stabilizing trend this week.
The latest frontal system Friday night and showers Saturday only deposited very light amounts of new snow to the east slope areas, so there is not a great deal of new snow instabilities to deal with.
Snowdepths vary greatly across the east slopes with a regionally healthy snowpack in the northeast Cascades to bare solar and low elevation slopes for the central and southeast Cascades.
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, 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: South East, South, South West, West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Mar 22nd, 2015 11:00AM
