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RegisterJan 19th, 2020–Jan 20th, 2020
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Although avalanche danger is slowly decreasing, large slab avalanche activity Saturday night into Sunday has us slightly concerned. Slab avalanche potential lingers in dry snow on steep slopes above treeline, while in sticky, wet snow, smaller wet avalanches may occur naturally or by human trigger on slopes steeper than 35 degrees. Avoid exposure to large, recently formed cornices that are sagging, failing, and may entrain wet or dry snow on the slopes below.
Discussion
The West North zone experienced a dramatic change in the snowpack over the last several days. Cold, deep, dry snow conditions developed last week and have transitioned rapidly over the last 48 hours as snow changed to rain. The temperature warmed at Mt Baker Heather Meadows from 20 degrees Friday evening to 38 degrees. And although the precipitation has decreased, it has been raining much of the day on Sunday.
The warming and rain on snow created large avalanches, particularly Saturday night into Sunday morning. 3 large, natural avalanches with crowns released below 4000 ft on Shuksan arm (either storm slabs or wet slabs). 3 crowns at multiple depths were visible on Mt. Herman’s east chutes.
The largest avalanche reported Saturday: A cornice failure triggered a large to very large (D2-2.5) avalanche 2 ft deep, 75-100 ft wide, that ran from 4900 ft to 4100 ft elevation. Photo Credit: Pete Durr
Cornices have grown large and threatening with a very active storm cycle over the last two weeks. Warm temperatures have destabilized them and sagging cornices may continue to fail, entraining large amounts of dry or wet snow to produce avalanches on Monday. Give cornices a wide berth on ridgelines and mitigate your overhead hazard by respecting potential runout zones.
January 16th, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
In the past week and a half, there have been five avalanche fatalities in three separate accidents in the US. One occurred near Kellog, ID and another outside of Baker City, OR. Local avalanche centers will perform accident investigations including final reports. You can find preliminary accident information at avalanche.org.
From January 9th to 16th the Pacific Northwest slid into deep winter. A cold and snowy regime brought a nearly continuous barrage of storms through the area. Temperatures bottomed out as modified arctic air made its way south from interior Canada, and many stations recorded the lowest temperatures of the season so far. A snowpack has been growing at lower elevations due to some lowland snow on both sides of the Cascades. NWAC’s snow depth climatology report shows most stations have surpassed average depths on the ground for this time of year. Quite the comeback from two weeks ago, when most were at 25-64% of normal.
Location
Total Snow Depth (in) 1/8/20
Total Snow Depth (in) 1/16/20
Hurricane Ridge
51
91
Heather Meadows Mt Baker
95
126
Stevens Pass
63
85
Snoqualmie Pass
33
77
Mission Ridge Mid Mtn
18
28
Crystal Mt Green Valley
66
92
Paradise Mt Rainier
105
138
White Pass Upper
69
110
Timberline
57
118
Mt Hood Meadows
53
98
Snow depths continued to rise. Total snow depths doubled in some locations.
The mountains went through a period of prolonged dangerous to very dangerous conditions as the snow kept coming. Many locations picked up over a foot of new snow per day for a number of days in a row, and storm slab instability was widely experienced across the region. At times, instabilities within new snow layers were very reactive, and you didn’t have to do much to provoke an avalanche. Many people triggered small to large soft slab avalanches, even well below treeline. The cold temperatures tended to preserve these instabilities longer than usual during this time.
Small ski triggered storm slab near Mt Hood Meadows. January 11, 2020. Scott Norton photo.
This cold, low density snow was also susceptible to wind drifting as westerly winds buffeted the alpine zone from the 8th to the 15th. On the 15th the mean winds shifted, and a south and east wind event disturbed the powder on open, exposed terrain near the passes and at upper elevations throughout the region. This created wind slab problems in some unusual locations.
Wind slabs formed over the low density powder snow. Mt Baker Backcountry. January 15, 2020. Zack McGill photo.
Trailbreaking in undisturbed snow was often very deep and difficult. In most places at any point in the week you could step off your skis or machine and sink in up to your chest in deep powder snow. The deep snow presented hazards of its own such as tree wells, and made it very easy to get stuck on a machine or lose a ski. Many folks experienced excellent, deep powder conditions and stuck to conservative terrain choices.
-MP
A cold winter’s day over the Chiwaukum Range, from Stevens Pass. Matt Primomo photo.