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Archived

Avalanche Forecast

Mar 19th, 2015–Mar 20th, 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
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
1: Low
The avalanche danger rating below treeline will be low

Shallow storm slabs and loose wet avalanches are possible near and above treeline Friday, and will mostly likely involve the new snowfall received Thursday and Thursday night above 5-6000 ft. 

Detailed Forecast

An approaching frontal system should cause cloudy skies, moderate S-SW winds and periodic light rain and snow Friday in the north Cascades.  

Storm slabs and loose wet avalanches are possible near and above treeline Friday, and will mostly likely involve the new snowfall received Thursday and Thursday night above 5-6000 ft.  Also, watch for isolated pockets of newly minted wind slab on lee slopes above treeline. 

Less recent snow and ample terrain anchors should greatly limit the avalanche danger below treeline. 

Remember the NWAC forecast applies to elevations up to the Cascade crest (~7000 to 8000 ft). Significant new snowfall was received above these elevations on the volcanoes over the weekend and this likely poses a higher avalanche danger.  

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 to cause an avalanche danger.

Snowpack Discussion

Last weekend another warm and wet system brought a one-two punch to the Pacific Northwest. The initial system Saturday brought 1.5 - 2.5 inches of rain with the snow line around 6000-7000 ft. On Sunday a second low pressure system deposited about 8-16 inches of snow above 4500 ft in the north Cascades and above about 6000 ft in the south Cascades. Significantly less new snow was received below these elevations. On Tuesday, a weak front dropped 1-3 inches of snow above 5-6000 ft in the south Cascades. Light to occasionally moderate snowfall fell in the north Cascades above 5000 ft with moderate southwest transport winds. 

Natural or skier triggered slides that occurred Sunday or Monday were generally loose wet size D1-D2, and to a lesser extend storm slabs, and reported throughout the west slopes of the Cascades relative to last weekend's snowline. The few inches of new snow from Tuesday were reactive to daytime warming/sunbreaks on Wednesday allowing for easy ski-triggering or natural point releases on steeper solar slopes.  

The Chinook DOT crew on Monday found debris from an natural cycle of loose or storm slab avalanches from Sunday.

 

Evidence Monday, March 16th of storm avalanches on Sunday on Naches Peak. Photo J. Stimberis.

The snowpack at low elevations remains meager to non-existent with the average snow-line around 4500 feet along the west slopes. The mid and lower snowpack west of the crest consists of layers of stable consolidated rounded grains or melt forms and crusts from multiple warm periods this winter.

Avalanche Problems

Loose Wet

Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

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: Alpine.

Likelihood: Possible

Expected Size: 1 - 1

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, North East, East, South East.

Elevations: Alpine.

Likelihood: Possible

Expected Size: 1 - 1