Avalanche Forecast

A frontal system should spread increasing rain and snow at gradually rising freezing levels Monday along with strong ridgetop winds. This weather system should cause a significant increase in the danger through the day Monday.  Given the variety of weak surface snow conditions, including surface hoar, weak near surface faceted snow or previously deposited weak low density snow any significant storm snow should lead to sensitive slab avalanches as dense cohesive surface snow becomes effective for slab avalanche propagation.

These storm or wind slabs should develop from increasing density storm snow and also from wind transported wind slab.  Watch for more significant storm and/or wind transported snow by Monday as avalanche danger may increase rapidly if loaded by significant new snow.  

Travel in avalanche terrain is not recommended above tree-line Monday.  Manage travel in lower elevations by avoiding wind loaded lee slopes and any steep open terrain being loaded by significant storm snow. Watch for shooting cracks as a sign of instability. Even a small wind slab in the wrong terrain, i.e. above cliffs or very steep slopes, could have unintended consequences.    

Watch for areas receiving locally greater new snow amounts along with strong winds. In these areas sensitive human triggered slab avalanches should become increasingly likely.  Watch also for some east facing ridges or slopes that have have had surface snow scoured to the late Jan crust.  New storm snow may bond poorly to any exposed hard crust layers.

Very cold temperatures this week have produced strong temperature gradients in the upper portion of the snowpack. The strong temperature gradient has produced a variety of weak surface snow conditions that will cause the avalanche danger to significantly increase when loaded.  Some of the weak surface snow include surface hoar, as well as developing weak near surface faceting and preserved 3-6 inches of recent low density snow from the last few days.  In addition, the cold temperatures have produced weak faceted crystals above the buried melt-freeze crusts. The preservation of weak layers buried about a week ago near Stevens and Snoqualmie Pass is found under 25-50 cm of snow from the last storm cycle. Easy shears were reported through the week, most likely failing on near-surface facets found on or above the late Jan crust on non-solar aspects below tree-line. However, the faceting of the most recent storm snow has generally diminished the likelihood of an overlying slab (see pit profile from Stevens Pass DOT). The persistent slab concern has been removed until a more significant load able to affect these layers returns.     

During the cold weather spell this past week, moderate to strong easterly winds quickly transported loose surface snow building unstable wind slabs on many lee mainly SW thru N facing slopes.  Several skier triggered wind slab avalanches were reported earlier this week. Two different skier triggered wind slab avalanches were reported Wednesday on Snoqualmie Mt. One slide caught a skier and was classified as a hard slab with a 16 in. crown and entrained the settled storm snow down to the old crust. Alpental pro patrol was able to ski trigger shallow yet quickly reloading wind slab in their back bowls on Thursday. Stevens Pass pro patrol reported a skier triggered slide in Highland Bowl outside the area on a SE aspect near 5500 feet...underscoring pockets of wind slab forming outside the expected lee aspects. 

Further south... Mt. Rainier and White Pass received several inches of low density snow through Saturday afternoon.  NWAC observers in White Pass near treeline on Thursday and in the Tatoosh Range in MRNP below and near treeline on Friday reported easy shears and good propagation in the upper snowpack near treeline in wind loaded slopes.  Tom Curtis at White Pass observed the new wind slab to be sensitive with widespread cracking as he moved through the terrain Thursday. 

Dallas Glass in the Tatoosh Range observed natural avalanches on lee aspects extending below treeline and sensitive test results were also found Friday. https://www.youtube.com/watch?v=NfGHCXM97kI  

Video from Tom Curtis White Pass: https://www.youtube.com/watch?v=xzK8fCV2xfw

Photo by Tom Curtis near White Pass 2-6-14

Near Mt. Baker less snow from the last storm cycle and less wind transport has translated to smaller wind slab more likely to be found in the above treeline elevation band. NWAC observer Jeff Hambelton reported low density snow overlying a locally stiffer late Jan crust in the Mt. Baker area. The upper snow is easily scraped off on steeper slopes, revealing a slick crust hard to hold an edge on.  In this area where winds have been lighter, low cohesion surface snow is limiting slab potential. see video below:

https://www.youtube.com/watch?v=N_uMXEKsyvM  

Note: The forecast discussion is the same across the west slopes but the Considerable rating extends into the near treeline zone between Stevens and Snoqualmie Passes due to potentially larger wind slab in those areas.