Avalanche Forecast

Updated for avalanche warning this afternoon through Monday morning, no other changes made. 

Increasing snow Saturday night with gradual warming Sunday and further increasing moderate rain and snow is expected.  The initial cold low cohesion snow Saturday should form a poor bond to the surface crust in most areas.  There is also the possibility that some existing surface hoar formed and survived as of early Saturday, though there have only been spotty reports of its presence. 

Increasing strong winds, gradually rising temperatures with heavier precipitation rates Sunday should all combine to cause a significant increase in the danger Sunday.  

New unstable storm layers should form on most terrain near or below treeline with unstable wind slab forming on open lee terrain near and above treeline. Sensitive triggered slab avalanches should become likely by later Sunday and potentially run far and fast on a smooth hard crust layer.  Therefore terrain choices should be very conservative as dangerous avalanche conditions are expected.  Avoid steep open slopes, slopes connected to steep open slopes above and lee slopes receiving wind deposited snow. The best choices should be lower angled terrain supported by denser trees.  Also watch for recently formed large cornices along ridges and give them a wide birth and safety margin.  Some of these cornices have grown large over the last 2 weeks and a cornice failure has the potential to trigger a large avalanche on the persistent weak layer from late January where present. 

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 Monday night. An extended period of freezing rain occurred near Snoqualmie Pass Monday morning and evening producing a locally stout crust. Outside the passes, rain fell up to roughly 4500 feet in the north and 6000-7000 feet in the south Cascades Monday. Moderate east winds reloaded avalanche paths near Stevens Pass Sunday through Wednesday.  

Warm conditions from Tuesday through Friday have drastically changed the snowpack conditions from recent deep powder snow to wet snow conditions. Significant snow settlement of the upper snowpack has occurred through the workweek.

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 or more.  While there were many reports from ski areas last weekend 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 released on a steep and open convex slope becoming a raging avalanche taking out mature timber.  For the few of you that may have missed the show, take a look at this mightily impressive avalanche. https://www.youtube.com/watch?v=tJkZgjrzJ40

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. The slides were aided by rapid day-time warming and solar affects on recent storm snow and released on a variety of elevations and aspects with some cornice induced or loose-wet avalanche triggered slab releases as well. 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.  View this photo by Ian Meyer of a skier triggered soft slab on Shuskan Arm Wednesday. 

Several small wet loose slides were reported along the west slopes on solar aspects Wednesday in the Snoqualmie Pass area and in the Stevens Pass area on Thursday. Above freezing temperatures Thursday softened the freezing rain crust around Alpental despite the cloud cover.

Warm temperatures and solar input maintained wet snow conditions Thursday and Friday but there were fewer avalanches reported.

Temperatures dropped rapidly by Saturday, with some temperatures along the crest dropping to single digits.  This has allowed for old wet snow to refreeze and form a new hardening crust.  The crust likely formed prior to snowfall beginning Saturday morning.  The new cold low cohesion snow is not likely to bond well with the new surface crust and this should lead to more sensitive triggered slides as greater and greater snow loading occurs. 

Regarding the Deep Persistent Slab concern: Observations Friday by NWAC forecaster Dennis D'Amico in Commonwealth Basin, Snoqualmie Pass found the late January facet/crust layer down about 1.5 meters near 4100 ft on a west aspect and still producing clean shears on tests. 

Deep Persistent layer of 1-2 mm facets down 155 cm over late January crust layer Snoqualmie Pass 4100 ft, west aspect, D. D'Amico

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.  While 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. These slides could also possibly be human triggered at a thinner spot in the slab such as near a rocky outcrop.    

  

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

More natural loose and slab releases on Mt Herman near Mt Baker, Tuesday 2/25. Photo by Patrick Kennedy

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