Avalanche Forecast

Avalanche conditions will not significantly improve Monday. Snowpack observations and one serious skier triggered avalanche from Sunday show you can still trigger dangerous Persistent Slab avalanches that are big enough to kill you on slopes steeper than 35 degrees. Persistent Slabs can surprise even very experienced travelers and can break over terrain features. Put a wide buffer of terrain between where you travel and any steep slopes. Use extra caution in areas where avalanches could come down from above. Avalanches may be surprisingly large and run farther than you expect. Make conservative terrain choices until we know more about these avalanches.

Cold temperatures will slow recent wind slabs from stabilizing. N-NW winds above treeline will transport recent snowfall onto lee slopes Monday, forming fresh wind slabs. Due to shifting winds, avoid steep wind loaded terrain especially near and above treeline. Wind sculpted features, cracks in the snow, pillows, cornices, and variable height of recent snow are all indicators that you could trigger a new or old Wind Slab avalanche. Triggering a Wind Slab could dig into deeper layers and result in a very large Persistent Slab.

The combination of Wind Slab and Persistent Slab requires very cautious decision-making and terrain selection. Persistent Slab avalanches are not common in the Cascades and the usual playbook of waiting a day or two for storm instabilities to settle out does not apply.

Storm slabs are possible in the southwest zone including Crystal, Mt. Rainier and White Pass. Small loose dry avalanches are likely on very steep slopes and could be problematic around terrain traps. Neither problem will be listed due to larger and more dangerous avalanche problems forecast for Monday.  

Persistent Slab avalanches were reported in Snoqualmie Pass area Saturday and Sunday with a skier caught, carried and injured near the Alpental ski area Sunday. The culprit weak, sugar-like snow has been observed adjacent to a crust 3-4 feet below the snow surface in almost all of the forecast zones. Professionals from around the region Sunday found this layering reactive in snowpack tests. You will still be able to trigger these large and dangerous avalanches Monday.  One regional difference in the Mt. Baker area exists where moderate E-NE winds transported new snow near and above treeline Sunday. Several skier triggered wind slab avalanches were reported outside the Mt. Baker ski area, with one partial burial. 

An extended cycle of natural and triggered avalanches occurred Wednesday through Saturday on the west slopes of the Cascades. Peak avalanche activity occurred late Friday to Saturday. 

Since February 15th, up to 3.5 feet of snow has fallen with extreme winds on Saturday. Rain was reported up to 4000 feet from Stevens Pass south Saturday. If you dig 2.5-4 feet below the surface, you will find recent, firmer snow sitting on a thin layer of weak, sugar-like facets adjacent to a crust. This strong slab over weak facets is the key recipe for Persistent Slab avalanches.

Observations

Mt Baker

On Sunday, Mt. Baker ski patrol reported 3 separate skier triggered wind slab avalanches up to 2 feet deep outside the ski area. One wind slab avalanche on Hemispheres broke 180 ft wide and caught, carried and partially buried a skier. The skier was able to self-extricate.

Details are spares but two snowmobile triggered avalanches were reported Sunday, one on a WNW aspect at 5800 feet. One was in the Canyon Creek area.  

NWAC pro-observer Lee Lazarra on Sunday found NE winds significantly redistributing snow near Artist Point with scoured surfaces near ridges. In this area the 2/8 crust was 3+ feet down in non-wind scoured areas.  A crust facet sandwich is widespread about 3-4 feet below the surface on all but steep solar slopes. Snowpack tests were limited and inconclusive regarding the potential for a persistent slab avalanche in this area. 

On Saturday, an observer in the Mt Baker backcountry reported remotely triggering an avalanche from low-angle terrain and test results indicating propagation.

Stevens and Snoqualmie Pass

A snowboarder triggered what was likely a large persistent slab avalanche Sunday outside the Alpental ski area near Powder Bowl on a NE aspect near treeline. The avalanche began in recent storm snow and stepped down to about 3 feet, likely the 2/8 crust. The rider was carried 1000 feet and suffered non-life threatening but serious injuries.  

On Sunday, NWAC staff performed snowpack tests indicating propagation (Propagation Saw Test) on the facets buried February 13th. These tests were located on W and SW aspects at Snoqualmie below treeline in the Kendall Peak and Rampart Ridge areas. A ski-supportable re-freezing rain crust was noted up to 4200 ft. The depth of the 2/8 crust varied depending on elevation, between 15-30" (40-80 cm) down.

In the Yodelin area near Stevens Pass in snowpack tests Sunday, persistent slabs were deemed likely to propagate on the facets above the 2/8 crust which was 2.5-3 ft (70-90 cm) deep. A slight rain crust was noted 6" below recent low density snow. 

On Saturday, Alpental Ski Patrol reported ski cuts triggering large avalanches that were 2 feet deep and ran on facets. These avalanches were surprising, with widely propagating crowns, and involved audible collapsing of the snowpack. One slide was roughly 300 feet wide. They were also able to trigger a small storm slabs on steep slopes within the storm snow.

Mt Rainier

On Sunday, an avalanche professional at Mt. Rainier reported 4 feet of snow above the 2/8 crust. Facets were found above the crust but were rounding. Recent wind distribution was significant from Saturday and debris from several from large avalanches that ran on Saturday were noted. Due to continued shower activity Sunday, new storm instabilities were present in the storm snow with natural small storm slab avalanches observed.