Avalanche Forecast

A weather system passing into Oregon should only bring light amounts of new snow from Snoqualmie Pass and south.  Moderate to strong east winds are expected to peak Thursday evening and ease on Friday with temperatures remaining quite cold. 

Until we receive more significant load...the increasing avalanche danger on Friday across the west slopes will be tied to new wind slab formed mainly on SW thru N aspects, although potentially cross-loaded on other aspects, by east to southeast winds at Cascade Pass to crest level seen Tuesday through Thursday.  East facing ridges may be scoured down to the late Jan crust.

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. 

Continue to test for the presence of recently buried weak layers in lower north facing terrain near Stevens and Snoqualmie Passes, mainly below tree-line and choose conservative slope angles and avoid terrain traps if unsure about stability.    

 

Very cold temperatures this week have produced strong temperature gradients in the upper portion of the snowpack.  See WSDOT Snoqualmie pit profile below for an example of the steep temperature gradient in the upper 10 cm and a more moderate temperature gradient (~ 10C/m) over the top meter.  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 from the last storm cycle in and near the Cascade Passes continues during this cold spell as well.  Public and NWAC observers continue to report buried surface hoar on sheltered non-solar aspects in the below tree-line zone, most widely reported near the Cascade passes and just east of the crest under about 25-50 cm of the most recent and settled storm snow.   However, near surface facets on or just above the late Jan crust show the most sensitivity in snowpack tests with high quality shears but low sensitivity to ski triggering and have not been linked to any recent natural avalanches.  These layers are not likely on solar aspects and the most recent storm snow has been reported as better bonded to the old crust on solar aspects.  A thin sun crust has formed on some steeper solar slopes.

  

Click to enlarge pit profile by Mark O'Geen, Snoqualmie DOT 2-5-14

Outside the passes these layers are not widely found south of Snoqualmie Pass and only to be reactive as of last weekend in isolated areas near Mt. Baker on Shuskan Arm.  Not to be overlooked...the cold temperatures have also preserved good surface snow conditions this week (read powder or recycled powder!). 

Newly formed wind slabs on lee slopes have been sensitive near Snoqualmie Mountain including 2 different skier triggered wind slab avalanches from Wednesday. 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. Shooting cracks and Watch for recent wind transport even into lee below tree-line zone to put additional load on recently buried weak layers.      

Less snow was available for transport near Crystal, Mt. Rainier and Mt. Baker and should make for smaller wind slabs. 

 

 

On Thursday NWAC observer Tom Curtis (see picture) reported easy shears and good propagation in the upper snowpack above treeline on a WSW aspect near White Pass. He observed the new wind slab to be sensitive with widespread cracking as he moved through the terrain. 

 

 

 

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

Note: The forecast discussion is the same for the west slopes but the persistent slab concern is relegated to near Stevens and Snoqualmie Passes and the Considerable rating extends into the near treeline zone due to potentially larger wind slab.