Avalanche Forecast

A weather system off the Oregon coast late Saturday spread northeastwards into the southern WA Cascades late Saturday and should produce light to moderate snowfall totals from Snoqualmie Pass and south through Sunday morning.  Increasing wind transport should be seen Sunday shifting to more widespread terrain as winds shift and increase.  Another disturbance moving onshore later Sunday should spread increasing snowfall and strong ridgetop winds to most areas.  This should build more widespread storm slabs and begin to load a variety of weak critical surface snow layers such as surface hoar, near surface faceted snow or weak low density snow deposited with light winds and cold temperatures.     

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.

Manage the terrain by avoiding freshly wind loaded lee slopes near and above tree-line. 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. 

Wind transport has occurred down into the below treeline zone.  Moderate avalanche danger at this level is specifically tied to the likely size found at this level...with small human triggered avalanches possible in specific areas or large avalanches in isolated areas. 

 

Very cold temperatures this week have produced strong temperature gradients in the upper portion of the snowpack. The strong temperature gradient has lead to near surface faceting and a breakdown on recently buried 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.     

Not to be overlooked...the cold temperatures have also preserved good surface snow conditions this week especially on wind sheltered slopes(read powder or recycled powder!). 

The greatest avalanche concern entering this weekend will be wind slabs formed during the work week. Moderate east to southeast winds transported snow to lee westerly aspects, building denser wind slab on top of weaker snow. While the SW thru N aspects are most likely to contain wind slab...local terrain channeling has distributed wind transported snow to a variety of aspects.  Wind slab will likely fail with the density change between these layers but the resulting avalanche may entrain snow down to the late Jan crust. Recent cold temperatures will continue to limit good bonding between wind slab and the underlying surface. 

Several skier triggered wind slab avalanches were reported over the last few days. 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.