Avalanche Forecast

Competing factors on Wednesday will keep dangerous avalanche conditions in play despite a relative break in the storm cycle and a favorable temperature trend.  A cooling trend overnight Tuesday and continuing on Wednesday should build lower density snow on top of Tuesday's storm slab and allow new storm instabilities to slowly settle. An exception may be found locally in persistent heavy showers near a convergence zone expected to form near Stevens Pass tonight and eventually shift south toward Snoqualmie Pass mid-day where new loading may stress the most recent storm layer instabilities. In addition moderate westerly winds will continue to load lee easterly aspects throughout the day Wednesday. South to southeast winds ahead of Tuesday's frontal system near the passes may have cross loaded slopes before winds switched to westerly so NW aspects have been added to the wind slab concern.  

Expect snow conditions to still be touchy on Wednesday and human triggered avalanches will be likely mainly on lee NW thru SE aspects at all elevations. Triggered avalanches are likely to start and stay in the new snow from yesterday...but the potential exists for very large avalanches in specific areas that release down to older storm and stiffer wind slab layers or even the late Jan crust. 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 conservative terrain choices for another day and let all the recent snow and associated water weight received over the last week a chance 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 8 days are 6-11 inches with snowfall in the 5-7 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 12" of generally moist new snow fell during daylight hours Tuesday along the west slopes and strong winds loaded NW thru SE aspects.  

Observations for Washington Cascades near and west of the crest

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.