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Archived

Avalanche Forecast

Feb 26th, 2015–Feb 27th, 2015
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
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
2: Moderate
The avalanche danger rating below treeline will be moderate

Storm related avalanche problems could escalate quickly in the near and above treeline bands Friday morning with new snow and shifting winds.  

Detailed Forecast

Light to occasionally moderate snowfall is possible for the south Washington Cascades, especially during the morning hours. Snowfall rates may be intense in the Mt. Rainier/Crystal Mt area during the morning with shallow yet unstable storm slabs developing by mid-day. 

As winds increase and back to SE Friday morning and then to ENE Friday afternoon, the potential for new wind slab and cross-loaded slopes will become more likely, especially above tree-line.  

Expect increasing storm related avalanche concerns in the near and above treeline bands on Friday if you travel into avalanche terrain.  

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. 

A few weather disturbances passed through a week ago, bringing a welcome few inches of snow near and above treeline. Winds veered to the east or northeast last Saturday, causing some new wind slab formation on non-traditional more westerly to southerly aspects mainly above treeline. A few reports of ski-triggered shallow wind slabs were reported from the west slopes over the weekend, generally found above treeline.  

Generally 1 to 3 inches of snow accumulated at NWAC sites Wednesday night through Thursday afternoon above about 4000 ft with rising snow levels in the afternoon. No significant avalanche problems were expected or reported.   

The mid and lower snowpack west of the crest snow consists of layers of stable consolidated rounded grains or melt forms and crusts from multiple warm periods this winter.

 

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1

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: Possible

Expected Size: 1 - 1