Avalanche Forecast

Note that avalanche warning conditions from Snoqualmie Pass through Mt. Baker are expected to last through Monday morning. These areas have been broken out from west slope areas further south due to more loading during the storm. 

Another weather system passing through the Pacific Northwest on Monday should bring a rapid increase in precipitation rates mid-day with predominately W to SW winds loading lee slopes. The rain line should stay near or below 3500 feet in the north and 4000-4500 feet in the central and south Washington Cascades.

In the below treeline zone that receives rainfall, wet loose avalanches will be the primary concern especially on rain soaked steeper slopes. Areas that received heavy snowfall before a switch to rain will be primed to entrain the most recent storm snow with wet-loose releases. 

New wind and storm slab formed on Monday should bond well given the warm temperatures, but intense precipitation rates may produce storm layer instabilities Monday near and above treeline.

Also, watch for lingering storm snow instabilities that have not settled from Sunday night.  Near the Cascade Passes, the switch to W-SW winds Sunday night should have locally cross loaded slopes.  

High avalanche danger is likely in the near and above treeline zones with additional wind and storm loading of lee slopes. Skier triggered slides related to storm snow instabilities are likely at all elevation bands. The possibility of deep slab avalanches with a low likelihood to trigger and high consequence has diminished, especially near and below treeline where strong surface crust layers are forming. This layer still exists however and remains a concern.  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 last Monday with an extended period of freezing rain near Snoqualmie Pass producing a locally stout crust and rain to 4500 feet in the north Cascades and 6000-7000 feet in the south Cascades Monday. 

Warm conditions Tuesday through Friday drastically changed the snowpack conditions from recent deep powder snow to wet snow conditions. Significant snow settlement of the upper snowpack occurred during this period. Colder air seeped into the region Friday night through Saturday firming up melt/freeze crusts throughout the region. Snow showers began Saturday for the west slopes and picked up Saturday night depositing the most new snow from Mt. Baker to Snoqualmie Pass. East winds began transporting the new low density snow near the passes as early as Saturday afternoon. A developing weather system passed through the area Sunday afternoon and night with moderating temperatures away from the passes. About 8 to 12 inches of new snow fell from Snoqualmie Pass through Mt. Baker with lesser amounts further south through 4 PM Sunday. Moderate east winds will switch to W-SW Sunday evening and begin to cross load slopes. 

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 as recent as Friday. The storm snow accumulated during the two week period in late February has been reported as bonded well and nearly uniform in character. 

While there were many reports from ski areas two weekends ago 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 become a huge avalanche taking out mature timber. If you haven't seen the impressive video, it's worth a viewing.

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. 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.  Many small wet loose slides were reported along the west slopes on solar aspects Wednesday through Friday releasing with the warmer temperatures and during periods of sunshine. 

Temperatures dropped rapidly by Saturday morning, with some temperatures along the crest dropping to single digits. This allowed old wet snow to refreeze and form a firm crust. New cold low cohesion snow has been reported as not bonding well in the Snoqualmie through Mt. Baker area to the most recent surface crust.  

New soft storm slabs were reported at Mt. Baker ski area Friday morning in the 8-10" range. Snoqualmie Pass DOT had up to 10 feet of loose slide debris on the interstate during planned control work Sunday afternoon.  

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. 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. 

  

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

The following provides an excellent overview of the relatively rare condition we have in our maritime climate: deep persistent slabs.  

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