Avalanche Forecast

Additional loading in showers Wednesday will transition into increased frontal snowfall and strengthening winds Wednesday night and Thursday morning while remaining cool.  This should add additional loading from storm layers to new wind slab over existing deep storm slabs and older wind slab layers.  Strong westerly winds through early Thursday will build large wind slab layers on a variety of lee aspects ranging from NW-S with many slopes or features being cross loaded by wind transport. 

Expect snow conditions to be touchy on Thursday and very sensitive to human triggers.  The most likely slopes to trigger avalanches should be mainly on lee NW thru S aspects. Triggered avalanches are likely to start and stay in the new snow as of Thursday morning, however the potential exists for some very large avalanches in specific areas that release down to older storm and stiffer wind slab layers or the rain crust from last week. Recently formed and sensitive cornices along ridgelines are an example of a natural trigger that could be large enough to trigger a much deeper and very destructive slide.

Stick with very conservative terrain choices and let all the recent snow and associated water weight received over the last 9 days have time to settle.  

Recent Northwest Weather

An extended mild and mostly dry period occurred over the latter half of January which allowed a variety of crusts to form. Very cold weather followed in early February which produced low density snowfall, periods of moderate east winds, and strong temperature gradients in the upper snowpack that allowed near surface faceting and broke down previously strong crusts buried in late January. During the last week the Pacific Northwest has been pummeled almost daily by frontal systems producing periods of strong winds and intense snowfall accumulation at fluctuating and generally moderate snow levels.

Water equivalents in the Cascades near and west of the crest over the 9 days are 6-12 inches with snowfall in the 5-8 ft range at NWAC weather stations. Expect wind loaded slopes to hold much more new snow from the past week than the generally sheltered weather stations.  

About 24" of new snow fell over the past 36 hours as of 5 pm Wednesday along the west slopes and strong winds loaded NW thru SE aspects.  

Observations for Washington Cascades near and west of the crest

Another strong frontal passage Tuesday evening caused very heavy loading rates at gradually warming temperatures.  This new load caused a widespread natural avalanche cycle that was observed in many areas by Wednesday morning.  Numerous storm and wind slab avalanches were noted from Mt Baker area, to Snoqualmie Pass and Crystal Mountain.  It was believed these recent slides remained within layers of the past few days snow and it is unknown whether any slides released to the persistent crust/facet layer from late January.  Even so, just recent storm snow avalanches have been up to 3-5 feet deep in some areas! 

Check out the latest video observation from NWAC observer Dallas Glass in the Snoqualmie Pass area Wednesday: 

Earlier in the week long storm cycle avalanches were releasing down to weaker snow over the old January crust in addition to new storm snow instabilities. As weaker layers became buried more deeply and more compressed, natural, skier and explosive triggered slides were more closely tied to most recent storm snow and wind loaded slopes. Even with warmer periods often producing denser snow, the reported wind and storm slab avalanches have been characterized as soft slabs.   

On Monday, heavy amounts of new snow produced numerous results with explosives and ski cuts. Most ski areas reported triggered soft slab releases in the 12 -18" range with ski cuts and explosives releasing within the new storm snow.  Mt. Baker patrol was able to produce slides estimated at 5-7' off of Shuskan Arm Monday morning on a lee aspect that has been continually wind loaded during this storm cycle. Crystal mountain patrol was able to produce slides of similar depths with explosives off of their Rockface path releasing down to the late January crust as well as a large natural in a closed area off their ridgeline late in the day. While unlikely that a skier's weight would trigger avalanches this deep, cornices have grown very large over the last week and could be the natural trigger needed to release deeper slides. NWAC observer Dallas Glass reported clean failures with stability tests at Yodelin nr Stevens Pass Monday at 35 and 60 cm that were likely involved in natural avalanches in that area. He also noted clean shears on facets still found above a crust 120 cm down on a N aspect below treeline - hard to trigger with a skier's weight, but a big slab. 

Similar sensitivity to ski cutting/and explosives was reported Tuesday by professional ski patrol.  The storm slabs were shallower relative to Monday since control work was performed as snow accumulated during the day. Limited backcountry observations were available due to the stormy conditions and limited visibility.  

A skier triggered a 12 inch (30 cm) D2 wind slab on an ENE facing slope on Gunsight Peak near treeline, elev 6400 ft outside the White Pass ski area Sunday. Photos by D'Amico 

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

Note that persistent slab has been added as a concern east of the Cascade crest. Be sure to read the forecast for that area if you venture even slightly east of the crest.

Keep an eye on your riding partner in the deep snow until the new snow settles and the risk of SIS subsides.