Avalanche Forecast

Issued: Mar 12th, 2018 11:28AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Loose Wet and Deep Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cooling Tuesday will limit wet snow avalanches to steep slopes at lower elevations. A Cornice failure could trigger a large and destructive Deep Persistent Slab avalanche. Limit your exposure to overhead hazard Tuesday, staying away from large avalanche paths. 

Summary

Detailed Forecast

Clouds and mild temperatures Monday night will maintain shallow wet surface snow and limit a hard re-freeze. A weak front should spread light rain or wet snow over the area Tuesday but amounts should be light. The precipitation should come with cooling through the day. This should allow for decreasing chance of Loose-Wet avalanches Tuesday and limit any Loose-Wet activity to below treeline. Stay away from slopes where even a small Loose-Wet avalanche may have higher consequences such as above cliffs, gullies, and creeks.

Several older deeper persistent weak layers exist in the snowpack. This is a classic low likelihood-high consequence scenario. Just because you see tracks on a slope does not mean it is safe. While it may be very difficult to trigger these deeper layers, any avalanche failing on them will be large and destructive. The avalanche that resulted in a fatality Saturday on Park Butte near Mt. Baker is believed to have released down to the 2/8 crust - 2/13 facet interface. This is a recent example of the low likelihood-high consequence structure that remains in our snowpack.

Persistent slab problems in the upper snowpack have recently been reported in the Steven Pass area and the East Slopes of the Cascades. Without any recent avalanche activity or snowpack observations confirming their presence for the other West Slope Zones, we are removing the Persistent Slab problem moving forward. 

Limit your exposure to overhead cornices as you travel. They may fail without warning due to daytime heating and the sun.

Snowpack Discussion

Sunshine and temperatures in the 40's to 50's over the past few days has allowed for significant snowpack settlement and for a variety of surface conditions to develop. Wet surface snow in the daytime and firm melt-freeze crusts are forming overnight on all but true north facing slopes which are barely hanging on to some settled old snow that fell Thursday.

Clear weather over the past few nights has allowed for surface hoar or near surface faceted snow to develop on many slopes. These surfaces are easily destroyed by wind or sun and warming, so may end up being widely variable when future snowfall occurs. 

Less Loose-Wet avalanche activity was reported Monday with more widespread activity reported Sunday across the west slopes of the Cascades on steep sunny slopes. Several natural slab avalanches were reported Monday around the Mt Baker area, including Shuksan. These slab avalanches released from starting zones above treeline on SW-W-N aspects.  A snowmobile rider triggered a fatal deep persistent slab avalanche Saturday in the Mt. Baker backcountry.  

New snow fell across the western regions of the Cascades Thursday night and Friday. Significant changes in snow totals were experienced with elevation. Wet heavy snow and rain was observed in up to 5500’ at Crystal, 4400’ at Snoqualmie, and 4300’ farther north. Above these elevations, 12-20 inches of new storm snow accumulated.

Generally SW winds transported the snow in all regions forming wind slabs on lee slopes and cross-loaded mid-slope features. These wind slabs have mostly settled and stabilized over the past few days of warm weather. 

This recent snow fell on a variety of old snow surfaces including settled cold snow and new melt-freeze crusts. There is potential in some locations that surface hoar and/or near surface facets were buried.

Several older persistent weak layers exist within the snowpack. On E-S-W aspects a thin facet-crust combo has not been widely found or reactive in quite some time. An older, deeper and more widespread persistent weak layer has been observed for several weeks. Weak sugary facets (2/13) sit just above a firm crust formed and buried in early February (2/8). This crust is generally found about 3-4 feet below the snow surface.

There are no other significant layers of concern below the 2/8 crust.

Observations

Baker

Pro patrol at Mt Baker reported seeing several large natural slab avalanches releasing Monday above treeline on a variety of aspects ranging from SW-W-N, likely a result of the very warm temperatures.

On Sunday, NWAC professionals near the Park Butte accident site observed widespread wet loose avalanche activity on steep solar aspects, including a long running wet loose in the Sisters Range that became very large (D3). In this area on a NE aspect at 5200', the 2/8 crust is down 4 ft (1.2 m) with a layer of facets above the crust. Recent wind effect was noted throughout the immediate terrain. 

Mt Baker ski patrol reported widespread wet loose activity on solar aspects (E-W) up to size 1.5 Sunday afternoon. A snowpit on a NNE aspect at 5200' Sunday showed recent storm snow well settled and generally right-side up in the upper snowpack. 

Snoqualmie

Backcountry skiers triggered and were caught in a wind slab avalanche on a NW aspect just below the summit of Humpback Mountain Saturday. They were not buried or injured but did lose some gear. Generally small wet loose avalanches were reported by Snoqualmie DOT Saturday on steep solar slopes. 

An avalanche professional at Alpental reported rain up to 4400 feet Thursday night. Two new natural wind slabs were observed in steep NE facing terrain.

South

NWAC professional observer Jeremy Allyn traveled in the Crystal Backcountry Monday, finding older wind slabs to have stabilized. Easterly winds were keeping surfaces cool and limited wet snow avalanches. No wet snow avalanches were seen Monday, despite the warm temperatures. 

On Saturday, Crystal Mt Ski Patrol reported two new natural avalanches on NW slopes around 6000'. These D1.5 avalanches occurred mid-day. They also reported large natural slab avalanches on NE aspects on the Cowlitz-Chimneys across from the ski area in Mt. Rainier National Park. These avalanches likely occurred Saturday afternoon with this photo taken on Sunday by Peter Dale.  

NWAC professional observer Jeremy Allyn traveled in the Crystal Backcountry Friday. He found significant wind transportation of the new snow above 6000’. A cornice failure triggered a widely propagating wind slab 2 feet deep on an E aspect of East Peak. Observations generally demonstrated the new snow was bonding well to the old snow surface.

Also Friday, Crystal Mountain ski patrol reported several new large crowns seen across the White River valley in the Sourdough Mountains. Avalanches occurred on NE aspects and were estimated to be 4-5 feet deep. Wind transported snow was observed on the upper mountain.

Problems

Loose Wet

An icon showing 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

Deep Persistent Slabs

An icon showing 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

Valid until: Mar 13th, 2018 11:28AM