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Avalanche Forecast

Archived

Feb 27th, 2015–Feb 28th, 2015

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

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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.

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

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.