Avalanche Forecast
Issued: Feb 23rd, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
Small loose wet avalanches and lingering but isolated recently formed wind slab will most likely be found above treeline Tuesday.
Summary
Detailed Forecast
Generally light winds and cooling temperatures are expected Tuesday.
As a result of the cooling, loose wet avalanches should be less likely, but possible in the late morning or near midday, on steeper solar slopes near and above treeline.
Recent shallow wind slab formed on non-traditional westerly aspects above treeline may still be reactive on specific terrain features.
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 (new or existing) to pose an avalanche hazard.
Snowpack Discussion
Since the mild and wet start to February, fair weather through mid-February has led to generally thick surface crusts, as well as further deterioration of the snow cover below treeline especially further from the crest.
A few weak weather disturbances passed through under NW flow aloft last Thursday and Friday, bringing an inch or two of new snow near and above treeline and near the Cascade crest.
Winds veered to the east or northeast on Saturday, likely causing some very shallow new wind slab formation onto non-traditional westerly to more southerly aspects, mainly above treeline.
Observations in the Rainy Pass area over the weekend noted shallow wind slabs of 15-25 cm (6-10 inch) forming on SE-S-SW facing slopes, with cracking but no releases observed. Challenging wind crusts were noted on wind exposed northerly aspects above tree line. Good skiing was found on more sheltered terrain near tree line.
While the January 15th facet/crust layers can still be identified in parts of the NE Cascades, this persistent weak layer has been stabilizing and become unlikely to trigger. As a result the Persistent Slab problem has been removed from the northeast zone.
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: South East, South, South West, West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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: South, South West, West, North West.
Elevations: Alpine.
Likelihood
Expected Size
Valid until: Feb 24th, 2015 10:00AM
