Avalanche Forecast
Issued: Feb 27th, 2015 10:29AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Winds slabs will be the primarily concern Saturday and should generally be found on non-traditional westerly aspects with significant wind loading. Careful route finding and attention to local loading patterns will be necessary to have a safe and enjoyable day in avalanche terrain.
Summary
Detailed Forecast
Moderate east winds will become northeasterly Friday night and continue significant loading of westerly aspects through mid-day Saturday for the central and southeast Cascades. Less snow is available for transport near Washington Pass.
Winds slabs will be the primarily concern Saturday, so pay attention to the local loading pattern in your area, but generally expect non-traditional westerly aspects to have significant wind loading. Wind slabs may form further downslope of areas of stronger winds, a fact that may not be apparent until you've already committed to a slope.
Lesser avalanche problems that are unlikely but still possible are storm slabs where storm instabilities persist in wind sheltered terrain and loose wet avalanches on steeper solar slopes. The sun is strong enough at the end of February to activate loose wet avalanches even on cooler days.
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 warm 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.
6 inches of new snow accumulated Thursday night and Friday at the NWAC mid-station at Mission Ridge. These new snowfall amounts were likely representative of most of east slope locations above 4000 ft except in the northeast Cascades where only an inch or two of new snow likely accumulated. By early Friday, east winds were beginning significant snow transport near treeline and fresh wind slabs were likely developing on westerly aspects.
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: North, South, South West, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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: All elevations.
Likelihood
Expected Size
Valid until: Feb 28th, 2015 10:29AM
