Avalanche Forecast
Regions: Stevens Pass.
Large avalanches are possible near and above treeline, where wind has created thick slabs on a variety of aspects. Approach all steep slopes with caution, keeping in mind the fact that lots of new snow has fallen over the past week.
Discussion
Another 5" of snow with very low water content fell at Stevens Pass on Wednesday night/Thursday. Over 6 feet of snow has fallen at Stevens Pass since the 10th, accumulating to over 4 feet of snow on the ground. East winds blew through the pass on Wednesday night. On Thursday, an observer saw fresh debris from a large (D2.5) slide in one of the Berne Camp Chutes above Hwy 2. On Wednesday, a small (D1.5) recent storm slab was noted near Stevens Pass Mountain Resort on a southeast aspect at 5,600ft. The crown was roughly 8" deep and 50ft wide.Â
With all the new snow, travel conditions are deep and challenging. Remember that even away from avalanche slopes, hazards exist like tree well falls and snow immersion suffocation. Make sure you’re informed and travel with a partner (www.deepsnowsafety.org).Â
Skyline Ridge on January 16, 2020. Note some subtle drifts behind the trees down low near the rd. Josh Hirshberg photo.
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
Large avalanches are possible on steep slopes in upper elevation terrain. Expect to find a few different layers of wind slab from the past few days as winds blew from the west and the east, creating slab on a variety of aspects. Even with light winds, it won't take much to redistribute the very low density snow into shallower reactive slabs on the highest ridges.
You are most likely to trigger wind slabs in recently formed drifts in open terrain. Look for hollow, drum like snow as you transition onto exposed slopes. If you see cracking, feel slabby snow, or suspect wind slab disguised by newer snow, avoid slopes steeper than 35 degrees. Excellent riding can still be found in more sheltered areas, outside of the effect of the wind. Earlier in the week, snowpack tests highlighted potential instability at an old storm interface, buried on January 10th. This layer is now at least 4ft below the surface, and bears consideration before venturing into larger terrain.
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: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1