Avalanche Forecast
Regions: Snoqualmie Pass.
Approach any steep slope where you anticipate the wind drifted snow into firmer and thicker slabs with suspicion. Some of these recent wind-loaded areas may hide under a few inches of soft snow. Use large clues such as cornices and drifts to help you identify and avoid locations where the wind deposited snow.
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
A strong wind east to southeast wind event formed slabs in the Snoqualmie Pass and I90 corridor Wednesday at all elevations. Now, a few inches of new snow hides easy to see clues that often help us locate winds slabs. Finding and avoiding wind loaded slopes could be a challenge Friday and may lead to a surprise avalanche for an unwary traveler. You’ll need to look and feel for wind slabs. Large features such as cornices and drifts can point you toward wind loaded terrain. As you move, the snow underneath you may change, become firm, or even feel hollow. These are great signs you encountered a wind-loaded feature. Steer around any slope greater than 35 degrees where you suspect the wind drifted snow into slabs. This isn’t our normal loading pattern, and you could find wind slabs well below ridgeline, in clear-cuts or other exposed features at low elevations, and along the sides of gullies.
Snoqualmie Pass received an impressive amount of snow over the last week. Several travel concerns should be on your mind including deep snow immersion, treewells, and opening in creeks. Stay informed and travel with a partner (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: North, North East, South, South West, West, North West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1