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Archived

Avalanche Forecast

Mar 2nd, 2014–Mar 3rd, 2014
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
4: High
The avalanche danger rating at treeline will be high
Below Treeline
4: High
The avalanche danger rating below treeline will be high
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
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Stevens Pass.

Avoid avalanche terrain on Monday.

Detailed Forecast

Note that avalanche warning conditions from Snoqualmie Pass through Mt. Baker are expected to last through Monday morning. These areas have been broken out from west slope areas further south due to more loading during the storm. 

Another weather system passing through the Pacific Northwest on Monday should bring a rapid increase in precipitation rates mid-day with predominately W to SW winds loading lee slopes. The rain line should stay near or below 3500 feet in the north and 4000-4500 feet in the central and south Washington Cascades.

In the below treeline zone that receives rainfall, wet loose avalanches will be the primary concern especially on rain soaked steeper slopes. Areas that received heavy snowfall before a switch to rain will be primed to entrain the most recent storm snow with wet-loose releases. 

New wind and storm slab formed on Monday should bond well given the warm temperatures, but intense precipitation rates may produce storm layer instabilities Monday near and above treeline.

Also, watch for lingering storm snow instabilities that have not settled from Sunday night.  Near the Cascade Passes, the switch to W-SW winds Sunday night should have locally cross loaded slopes.  

High avalanche danger is likely in the near and above treeline zones with additional wind and storm loading of lee slopes. Skier triggered slides related to storm snow instabilities are likely at all elevation bands. The possibility of deep slab avalanches with a low likelihood to trigger and high consequence has diminished, especially near and below treeline where strong surface crust layers are forming. This layer still exists however and remains a concern.  Stay conservative and avoid open and exposed avalanche terrain and also be aware of the terrain and parties above your location if you want to minimize this threat. 

Snowpack Discussion

Recent Northwest Weather

An extended mild and dry period occurred the latter half of January generally forming a crust in all areas. Very cold weather followed in early February which produced light low density snowfall, near surface faceting, and faceting near and just above the crust.

Our impressive 2 week storm cycle ended last Monday with an extended period of freezing rain near Snoqualmie Pass producing a locally stout crust and rain to 4500 feet in the north Cascades and 6000-7000 feet in the south Cascades Monday. 

Warm conditions Tuesday through Friday drastically changed the snowpack conditions from recent deep powder snow to wet snow conditions. Significant snow settlement of the upper snowpack occurred during this period. Colder air seeped into the region Friday night through Saturday firming up melt/freeze crusts throughout the region. Snow showers began Saturday for the west slopes and picked up Saturday night depositing the most new snow from Mt. Baker to Snoqualmie Pass. East winds began transporting the new low density snow near the passes as early as Saturday afternoon. A developing weather system passed through the area Sunday afternoon and night with moderating temperatures away from the passes. About 8 to 12 inches of new snow fell from Snoqualmie Pass through Mt. Baker with lesser amounts further south through 4 PM Sunday. Moderate east winds will switch to W-SW Sunday evening and begin to cross load slopes. 

Observations for Washington Cascades near and west of the crest

The late January crust layer and overlying faceted weak layer are now deeply buried, with many observers in the Snoqualmie Area reporting facet/crust layers of interest 1.5-2 m deep as recent as Friday. The storm snow accumulated during the two week period in late February has been reported as bonded well and nearly uniform in character. 

While there were many reports from ski areas two weekends ago of deep slabs down to the crust, the most recent ski patrol reports of explosive triggered slides were from Stevens Pass ski area Wednesday. A heli-bombing mission in the N-NE facing terrain of Rooster Comb outside the ski area produced one very large deep slab of 10-15 ft that become a huge avalanche taking out mature timber. If you haven't seen the impressive video, it's worth a viewing.

Warm temperatures and periods of sunny weather lead to an avalanche cycle in the Mt Baker backcountry Tuesday with more skier triggered slides reported on Wednesday. On Tuesday, one very large deep slab on the SE aspect of Goat Mountain likely released down to the early February facet/crust critical layer as the crown was estimated at 6-8 ft.  Many small wet loose slides were reported along the west slopes on solar aspects Wednesday through Friday releasing with the warmer temperatures and during periods of sunshine. 

Temperatures dropped rapidly by Saturday morning, with some temperatures along the crest dropping to single digits. This allowed old wet snow to refreeze and form a firm crust. New cold low cohesion snow has been reported as not bonding well in the Snoqualmie through Mt. Baker area to the most recent surface crust.  

New soft storm slabs were reported at Mt. Baker ski area Friday morning in the 8-10" range. Snoqualmie Pass DOT had up to 10 feet of loose slide debris on the interstate during planned control work Sunday afternoon.  

A large natural hard slab avalanche released off of steep north facing terrain of Chair Peak near Alpental Saturday and partially caught 3 skiers Saturday. Luckily no on was injured in this potentially deadly avalanche with a 10' crown.

This avalanche in conjunction with the previous frequent and large results produced by ski patrol should continue to steer the discussion of mitigation to terrain management choices. This concern is of the low frequency, low probability but high consequence variety and there really are not practical tests to perform in the field to eliminate that potential, so prior trip planning making a choice to eliminate large open avalanche terrain is the best response. These deep weak layers are unlikely to release by the weight of a skier/rider and likely need a much larger trigger, however as this picture below shows; a collapsing cornice, warming and sunshine all contributed to tip that balance. The best way to deal with these conditions remains avoiding large open avalanche terrain. 

  

Natural deep slab release (center left) on SE flank of Goat Mtn 2/25 est.crown 6-8 ft.plus numerous fresh loose-wet slides. Photo by Patrick Kennedy

The following provides an excellent overview of the relatively rare condition we have in our maritime climate: deep persistent slabs.  

The mid and base pack should still consist of mostly stable crusts and melt form layers from periods of warm weather earlier this winter.

Avalanche Problems

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: Likely

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, West, North West.

Elevations: Alpine, Treeline.

Likelihood: Likely

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: Likely

Expected Size: 1 - 1

Deep Persistent Slabs

Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.

 

Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.

 

A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.

Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Unlikely

Expected Size: 1 - 2