Avalanche Forecast
Issued: Feb 24th, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Any light snowfall received Tuesday night should not be significant enough to cause an increase in the generally low avalanche danger in all areas.
Summary
Detailed Forecast
Mostly cloudy and cool Wednesday with an increasing chance of light showers, mainly after daylight hours.
As a result of further cooling and cloud cover, little threat of avalanches is expected.
The isolated shallow wind slab that may have formed over the weekend above treeline, on non-traditional westerly to southerly aspects, may persist 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. With mild daytime temperatures Monday and Tuesday these isolated slabs have likely further stabilized. Wind crusts provided challenging travel conditions for wind exposed northerly aspects above tree line. Decent 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
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 25th, 2015 10:00AM
