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Archived

Avalanche Forecast

Feb 28th, 2015–Mar 1st, 2015
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
1: Low
The avalanche danger rating at treeline will be low
Below Treeline
1: Low
The avalanche danger rating below treeline will be low

Winds slabs will be less likely, but remain the primary concern Sunday and should generally be found on non-traditional westerly aspects, especially below ridges. Careful route finding and attention to local loading patterns will be necessary to have a safe and enjoyable day in avalanche terrain.

Detailed Forecast

Mostly sunny Sunday with increasing high clouds late, remaining cool with light winds. This should allow for further settlement of any recently formed wind slabs. Winds slabs will remain the primary concern Sunday, so pay attention to the recent local loading pattern in your area, but generally expect non-traditional westerly aspects to be the most suspect. Wind slabs may have formed further downslope of areas of stronger winds, a fact that may not be apparent until you've already committed to a slope.  

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

A warm and wet start to February was followed by mostly fair weather through mid-February, leading to generally thick surface crusts, as well as further deterioration of the snow cover below treeline. 

By early Saturday, NWAC snowdepth sensor readings increased about 3-7 inches from the new snow that accumulated Thursday night and Friday at Mission Ridge. These new snowfall amounts were likely representative of most of east slope locations above 4000 ft.

From early Friday through Saturday morning, moderate to strong east winds were beginning significant snow transport near treeline and fresh wind slabs were likely developing, especially on westerly aspects. NE winds were very strong on Mission Ridge through Saturday, so watch for evidence of recent wind loading, such as exposed windward slopes and windward sides of trees stripped of recent snow.

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.

Avalanche 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: Possible

Expected Size: 1 - 1