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Archived

Avalanche Forecast

Apr 19th, 2014–Apr 20th, 2014
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
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Snoqualmie Pass.

Increasing winter like avalanche conditions should be seen in the near and above treeline zone by late Saturday afternoon as a front moves across the region.

Detailed Forecast

An approaching Pacific front should move to the Washington coast Saturday morning, then spread increasing precipitation over the area Saturday afternoon and evening.  Freezing levels should rise ahead of the front, then lower with the onset of precipitation midday Saturday. 

Increasing snow with strengthening winds Saturday afternoon at lowering snow levels should begin to build new wind and storm slab layers.  Depending upon the how early the significant snow begins and the intensities, most areas should only see a modest rise in danger by afternoon.  However, if heavier snowfall begins earlier in the day Saturday, watch for a more significant increase in danger by Satuday afternoon, especially higher elevations. 

The greatest danger increase is expected on lee slopes near and above treeline on mainly north through east facing lee slopes. Watch for signs of firmer wind transported snow or cracking snow.

Snowpack Discussion

A warm front spread increasing light precipitation over the region late Wednesday with a cold frontal passage moving through the region late Thursday with showers persisting into early Friday.  As of Friday afternoon, areas above 5000 feet had received about 8-12 inches of snow.  

Daytime warming and slightly rising freezing levels Thursday has pushed snow lines above 5500 feet and closer to 6000 feet.  This wet recent snow and added some shallow additional new wet snow above tree line. Cooling overnight Thursday and early Friday allowed for a good bond to form on the old snow surface while adding some shallow, colder new snow amounts. 

The greatest recent storm snow has been received over the Mt Baker area where reports from Thursday and Friday indicated new storm and wind slab were responsive to ski cuts by ski patrol Friday morning and NWAC observer Jeff Hambleton also reported reactive new storm and wind slab while testing new snow conditions.

Storm slab releases on Mt Herman in the Mt Baker backcountry Thursday, April 17th. photo: Jeff Hambelton  

Recent storm slabs and wind slab should settle quickly given the warming daytime temperatures Friday, however they may be also susceptible to becoming wet and creating a wet snow avalanche danger. 

 

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, North East, East, South East.

Elevations: Alpine.

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, Treeline.

Likelihood: Possible

Expected Size: 1 - 1

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.

Elevations: Below Treeline.

Likelihood: Possible

Expected Size: 1 - 1