Avalanche Forecast

A pattern change will begin to take place over the Northwest on Friday. Dry weather and strong cold E-NE winds seen Wednesday and Thursday will gradually get replaced by increasing southwest flow aloft, increasing rain or snow and milder temperatures Friday and Saturday. You will need to pay lots of attention to conditions and forecasts during the transition Friday and Saturday if you decide to venture into the back country.

Wind slab formed by strong E-NE winds on Wednesday and Thursday will linger on Friday. The primary aspects for these wind slabs should be any W facing slopes but could be found on other aspects.

New sensitive wind slab is likely to begin forming as shower bands of snow begin to move across the Olympics and Cascades on Friday with a slight warming trend. The primary aspects for these wind slabs should be NW to SE facing slopes except for W facing slopes in the Cascade Passes.

Wind slab may be more touchy in areas where poorly bonded poorly to an underlying crust or firm old layer. Watch for firmer wind transported snow in all elevation bands especially in open terrain in the upper elevations of the below treeline band. All aspects are listed for wind slabs to account for the strength of recent winds and for any lingering older wind slabs formed over the weekend.  

Older wind slabs should be stubborn to trigger at this point, but in the Mt. Baker area, be aware of a higher likelihood of triggering wind slab where it overlies recently buried surface hoar. Field observations suggest this setup exists in isolated areas on northerly aspects near treeline in the Mt. Baker backcountry.

New storm slab is also likely to begin forming on Friday on more sheltered slopes in areas that see more than a few inches of rapidly accumulating snow. The warming trend will help build upside down layers and promote storm slab formation.

Weather and Snowpack

An atmospheric river arrived over the Northwest on 1/17 forming a rain crust or freezing rain in the Passes in most locations below 6000-7000 feet. A storm cycle from Thursday 1/19 through Sunday, 1/22 deposited 1-2 feet of snow along the west slopes of the Cascades with the greatest totals seen at Mt. Baker.

A large upper ridge and warm air mass built over the Northwest last Thursday to Saturday 1/26-1/28 with temperatures reaching the 40s in many places along the west slopes away from the lower parts of Stevens and Snoqualmie. Strong southwest winds were seen Saturday in the Northwest Cascades, with moderate southwest winds in the central west Cascades and light winds in the southwest Cascades. East winds were seen at Stevens and Snoqualmie. Sun crusts formed on most solar aspects during this period.

A weak front crossed the PNW Sunday afternoon and night, initially starting as rain below 6000 feet before depositing 1-4 inches of snow with the most at Mt. Baker. A generally ski supportable crust was reported in the Stevens and Snoqualmie Pass areas up to 5500 feet Monday and Tuesday. Light snow fell during a cooling trend with light winds Monday night and Tuesday morning, with 2-6 inches of snow accumulating along the central and south Cascades, with the most at Stevens Pass.

Very strong NE-E winds have been seen Wednesday and Thursday with very cold temperatures. Fresh, touchy wind slabs formed with large plumes of snow visibly transporting loose snow from exposed terrain.   

Recent Observations

North

NWAC received an observation Saturday describing 2 groups of skiers who triggered 3 separate wind slab avalanches with depths averaging about 2 feet in the Mt. Baker backcountry. The wind slab avalanches were triggered on N through E slopes between 5000-5800 feet on Mt Herman with one slide burying a skier waist deep. Fortunately the skier involved was not injured.

NWAC pro-observer Lee Lazzara was in the Bagley Lakes area on Saturday and reported wind slab formation. He also reported a large cornice failure on Saturday night onto a north slope of Table Mountain near Mt Baker at about 5000 feet. 

Lee was back out in the Mt. Baker backcountry Monday and through a fair number of column tests found wind slab reactive in isolated pockets near treeline where buried surface hoar sat below recently formed hard wind slab and above the old snow surface from 1/28. Wind slabs were found on a variety of aspects above treeline, but did not show signs of likely propagation. About 10-15 cm of new snow was not well bonded to the previous snow surface but was too shallow to be a significant avalanche problem.

Lee was on the south side of Mt Baker Wednesday and experienced the rapidly forming touchy wind slabs on a variety of aspects. One natural wind slab released Wednesday on the eastern side of the Metcalf Glacier. The strong winds were predominantly easterly but wind slabs were forming on a variety of aspects, including cross loaded slopes and other specific terrain features. 

Central

NWAC pro-observer Ian Nicholson was at Stevens Pass on Sunday and observed small loose wet activity in the morning before cloud cover and cooling quickly diminished the loose wet potential. A layer of small faceted crystals above the 1/17 crust is becoming rounded at about 40 cm down and were not reactive in column tests or on test slopes.

Ian was back out on Tuesday in the Alpental Valley and found 8-10 cm of new snow poorly bonded to 1/29 rain crust. The crust was thicker on solar aspects, but ski supportable on most aspects up to about 6000 feet.

Both Tom Curtis and Jeff Ward traveled independently in terrain east of Stevens Pass, covering the areas of Rock Mountain, Jove and Union Peaks. Both reported rapidly forming, very touchy wind slabs by mid-morning Wednesday. Plumes of wind transported snow were seen along all ridges and exposed terrain Wednesday. Wind slabs were building much further downslope due to the strength of the winds. Wind slabs up to 12" were seen and noted forming well below treeline. Any small feature or convexity would produce shooting cracks or release small wind slabs, even on relatively shallow angled terrain. Both avoided travel on any steep terrain.

South

NWAC pro-observer Dallas Glass was at Paradise on Sunday and traveled up to about 6400 feet. He found a surface melt-freeze crust from Saturday over about 15 inches of settled recent snow on the thick 1/17 crust on solar aspects. True northerly aspects still held some settled powder. During the day some wind transport was seen at higher elevations above treeline. Evidence of numerous small loose wet and size 1-2 wet slabs from Saturday were observed on solar slopes.