Avalanche Forecast

Significant cooling is expected Friday night and Saturday with mostly cloudy conditions and a few generally light snow showers expected late Saturday.  This weather should cause the previously wet surface and upper snow layer to re-freeze, forming a hard and supportable crust.  The surface or near surface crust should limit the avalanche potential, especially near and below tree line.  Any new snow received Saturday is not expected to cause a notable increase in the danger Saturday.  

The current surface snow has been reported to be very rough where a thin surface crust existed Friday in the back country in the Mt Baker area.  In areas where the forming surface crust has significant surface roughness, this may help new snow bonding.  However the rapidly dropping temperatures expected Friday night ahead of the onset of precipitation Saturday may counter that in many areas where poor bonding may develop.

In addition there have been reports of surface hoar formation earlier this week that may be an additional factor if buried by the expected snowfall over the weekend.  Much of this may have been destroyed by solar radiation and warm temperatures, especially near and below treeline on solar aspects, but should be something to tuck in your memory as we begin to build new snow accumulations later in the weekend and next week.   

The strengthening new surface crust expected Saturday should cause generally safe avalanche conditions.  However continue to watch for unstable snow on isolated terrain features, especially above treeline.  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 could provide a large enough load to trigger a large avalanche. 

Finally the threat of deep slab avalanches with a low likelihood to trigger and high consequence (read deadly) has diminished, especially near and below treeline where strong surface crust layers are forming. This layer still exists however and remains a concern even though the overall danger rating has dropped.  The avalanche danger has been lowered to Moderate above treeline to reflect the unlikely nature but means that large avalanches are possible in isolated areas. Moderate danger does not mean the persistent deep slab threat has abated or is of any less consequence, only that it is becoming harder to trigger. 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. 

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 Thursday have drastically changed the snowpack conditions from recent deep powder snow to wet snow conditions. Significant snow settlement of the upper snowpack has occurred over the last several days.

Friday was still mild but beginning to cool with abundant solar input.  Fewer and fewer loose wet slides have occurred over the past few days. 

Observations for Washington Cascades near and west of the crest

The late January crust layer and associated weak layers are now deeply buried, with many observers in the Snoqualmie Area reporting facet/crust layers of interest 1.5-2 m deep.  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' that released on a steep and open convex slope.  For the few of you that may have missed the show, take a look at these mightily impressive results. 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.

Observations by NWAC forecaster Dennis D'Amico Friday 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 paired with the frequent and large results from ski patrol should continue to steer the discussion to terrain management of low probability and high consequence slides that release down to weak layers on or near the late Jan crust. 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.