Avalanche Forecast
Regions: Snoqualmie Pass.
Snoqualmie Pass should experience all the pieces need to create dangerous avalanche conditions Saturday; warming, precipitation, and wind. Avoid traveling on any steep open slope, especially when you notice the wind drifted the snow deeper. You could encounter wind affected snow in many locations including clear-cuts, areas well below ridgeline, and along the sides of gullies.
Discussion
A significant winter storm will impact the Snoqualmie Pass area Friday night and through the day Saturday causing avalanche danger to increase over that time period. Danger should peak mid to late morning when the strongest winds and highest snowfall rates combine with a slight warming trend to form unstable snow.Â
Avalanche concerns should be limited to the new snow and how it bonds with Friday’s very light surface. It’s hard to say exactly how the new snow will respond to the lighter snow underneath, but all the puzzle pieces are there. Wind, snowfall, and warming will likely push the snowpack past its breaking point.
Snowpack Discussion
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.Â
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.Â
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.Â
-MP
A cold winter’s day over the Chiwaukum Range, from Stevens Pass. Matt Primomo photo.
Avalanche Problems
Wind Slabs
While Saturday looks stormy, the moderate to strong winds at all elevations seem to define the day, especially along the I90 corridor. Identify and avoid wind loaded slopes greater than 35 degrees at all elevations. Look for blowing snow, drifts behind trees and rocks, and cornices to point out areas the wind formed new and reactive slabs. You could find wind slabs in some unusual spots as SE winds near the Pass battle with SW near the ridgecrest. Pay attention to mid-slope break-overs, the sides of gullies, and any exposed feature below treeline where the wind could affect the snow.
The strongest winds below treeline seem to focus near I90. If you travel to other more sheltered connected valleys, you may not be out of danger. As more snow accumulates, you could encounter storm slabs. When you see cracks in the snow or feel heavy strong snow over soft weak snow, you found storm slabs. Avoid any open slope greater than 35 degrees. With all the recent soft snow, treewells, snow immersion, and open creeks are all travel concerns. Get informed and travel with a partner. https://www.deepsnowsafety.org/
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1