Avalanche Forecast
Regions: Snoqualmie Pass.
Increasing avalanche danger from wet snow is expected Wednesday, especially during the late morning and afternoon on steeper slopes receiving direct sun. If snow is beginning to sink to about boot top levels, it's probably time to head to shallower angled terrain well away from large open slopes above.
Detailed Forecast
Another mostly sunny day is expected Wednesday with significantly warmer temperatures expected. Freezing levels are expected to near 9000 feet in the Cascades Wednesday and this should quickly melt the morning melt freeze surface crust and begin again to create wet snow conditions.
The pre-spring sun angles and power are increasing and combined with rising freezing levels Wednesday, this should increase the possibility of triggered wet loose avalanches especially on direct sun exposed slopes near and below treeline. Watch for pinwheels and surface wet snow deeper than a few inches.
Areas of new wind slab are also possible above treeline at higher elevations, watch for signs of earlier wind transported snow.
While the likelihood or triggering a wet slab on a deeper layer is unlikely, this will be a continued concern moving forward into the spring, especially in areas that have received significant recent rain, allowing water to penetrate to these deeper weak layers from mid-winter.Â
Snowpack Discussion
Over the past month two major storm cycles moved through the Pacific Northwest, both depositing significant snow and precipitation and producing numerous avalanche cycles in the near and west of the Cascade crest and passes.
The first two week storm cycle ended February 25th and deposited about 7-12 feet of mostly wet heavy snow at most sites near and west of the crest and caused numerous storm avalanches.
The latest storm cycle wound down this past Sunday morning producing similar water equivalents as the February cycle.  Water equivalents and snowfall for the past week are about 9-13 inches and 2-6 feet at NWAC sites.  This produced many avalanches this past week including reports from Holden Village, Stevens Pass, Tumwater Canyon, Alpental and Snoqualmie with pass closures at times last week.
Wet loose avalanche on 5 March on Spiral Butte at White Pass. Photo John Stimberis/WSDOT.
The latest front moved across the area on Sunday morning and a few more avalanches were seen in some areas. The Mt Baker ski patrol reported 2 explosively triggered large wet loose avalanches that destroyed trees but nothing human triggered. The Alpental ski patrol Snoqualmie DOT reported saturated surface snow layers but limited effects from all the rain Saturday night. A snow cat triggered a large wet loose avalanche at White Pass on Sunday morning at 5000 feet.
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(Left)Looking down on a snow cat triggered large avalanche at White Pass on Sunday morning 9 March. Photo by Chris Talbot/White Pass Ski Area. (Right) Starting zone.
The ski patrol at Crystal Mountain both Sunday and again Monday morning produced deep wet slab releases with large explosive charges.  The slides are releasing to deep layers from early this winter. These slides were generally on east to northeast slopes at about 6000-6800 feet with crowns of 4 to 6 feet and up to 10 feet in spots with some trees destroyed.  One slide released Monday was a wet slab following more rainfall and propagated across the well skied Powder Bowl run.  Further control work Monday afternoon off the Throne produced a slide that destroyed the High Campbell chair lift, photos at Crystal Mountain web site. Check this impressive slide out from KIRO chopper video.Â
On Tuesday, NWAC pro observer came upon similar recent very large wet slab releases on Kendall Peak in the Snoqualmie Pass area.  It is thought that some five separate avalanches released in a similar area.  These slides began at about the 5500 ft elevation on primarily west aspects on slopes of about 35-40 degrees.  They were classified as D 2.5, R 2/3 Natural Wet Slabs and ran about 1000 feet.  These slides likely released Saturday when the area received over 3 inches of rain to elevations above 5500 feet. A video of the slide area and debris fields can be seen on the NWAC channel. Additional snowpack investigation in this area revealed that free water had drained at least to 2 meters below the surface, meaning at least that much snow may quickly become available to be entrained in a slide given ample warming and or additional rain.
While it is unlikely these deep slides could be human triggered, it remains a possibility, especially if initiated by a thin spot in the slab such as near rocks or trees.  As liquid water has now penetrated to deep in the snowpack in places that have received significant recent rain these concerns should remain with us into the spring, especially during times of rain or extended warm periods.  It may also be possible to initiate a wet slab release as a result of loose wet slides triggering a deeper layer.  As a result of this recent activity, wet slab avalanches will now be listed as a concern near and west of the crest. These avalanches may continue to release at the January crust and faceted layers from early February.  We will continue to track these layers as we move into the spring.  Here is a link to more information about wet slabs.
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: South East, South, South West, West.
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.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 2