Avalanche Forecast

Another fast moving and powerful storm system will impact the Pacific Northwest on Tuesday.  Snowfall will increase first in the Olympics Tuesday morning and spread into the Cascades by late morning.  A more southerly flow out ahead of frontal boundary should bring rapid loading rates first to the volcanoes and then during and following the frontal passage to all the west slopes as winds veer to more westerly by Tuesday afternoon. A warmup preceding the frontal passage should cause rain at lower elevations outside the Cascade passes and denser wind and storm slab elsewhere during the storm.  A switch to westerly flow on early Tuesday afternoon should allow for a period of rain at Snoqualmie and White Pass before snow levels fall again by late Tuesday afternoon. 

New storm slab will form on all sheltered aspects. New potentially deep wind slab will develop on lee N thru SE slopes. Natural or triggered avalanches will be likely at all elevations. Direct action avalanches that release initially in storm layers seem likely to step down to deeper layers. Wet loose avalanches should become more likely in the afternoon at lower elevations and may entrain large amounts of the most recent storm snow. 

Travel in avalanche terrain is not recommended on Tuesday.

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 last week are 6-10 inches with snowfall in the 4-6 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 18-36" of new snow has fallen in last 36 hours ending 4 pm Monday.  

Observations for Washington Cascades near and west of the crest

Earlier in the 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 Sunday, NWAC observer Dallas Glass reported sensitive storm and wind slab layers below treeline at Stevens Pass. The failure layer is lower density snow was 40-60 cm but is now likely a meter or more deep. 

A skier triggered a 12 inch (30 cm) D2 wind slab on an ENE facing slope on Gunsight Peak near treeline, elev roughly 6400 feet outside the White Pass ski area Sunday. Images below.

On Monday, heavy amounts of new snow received Sunday night into Monday produced numerous results with explosives.  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 in the 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.  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.  

Skier triggered wind slab formed Sat night/Sunday, no one caught, released in new snow. Photo by D'Amico

Another view of the crown and flank of White Pass skier triggered slab. Photo 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.