Avalanche Forecast
Regions: Cascades - East.
A winter storm will impact the area on Saturday, creating dangerous avalanche conditions. Slab avalanches will become likely at upper elevations due to snowfall and strong winds. Choose your routes carefully, planning to stay off and out from underneath steep slopes during the day.
Discussion
Expect dynamic and dangerous conditions to develop on Saturday. On Friday the sun poked out for a bit, giving a brief respite to the stormy pattern. At Mission Peak, we found some lingering but stubborn wind slab problems. We found a couple layers of note, both which involved weak snow above or below thin crusts on an ENE aspect at 6,600ft. These produced some planar, though inconsistent results. One of these was likely responsible for the remotely triggered slide reported here on the 12th. On Thursday, an observer reported fresh debris from a large (D2.5) slide in one of the Berne Camp Chutes above Hwy 2 in the neighboring Stevens Pass zone. Anywhere from 5" to 10" of snow fell the evening prior, with very low water content and moderate to strong south and east ridgetop winds.Â
Mount Stuart and the eastern Stuart Range from Mission Peak. January 17, 2020. Matt Primomo 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
Storm Slabs
Most storm slab avalanches happen during times of heavy snowfall. All it takes is a slight density change within the new snow, or a poor bond with the weak old snow surface. Look for signs of instability like shooting cracks within the upper layers of recent snow, and new soft slab avalanches. You can use small test slopes to check for these, however avoid putting yourself in a place where even a small avalanche may be consequential. If snow moves downhill easily, either in the form of a slab or loose snow, I'd be leery of terrain traps such as the bottoms of gullies and steep slopes above stands of thick trees.
If you find over a foot of new snow, expect that storm slabs could be large and dangerous. You are most likely to find these conditions at high elevations and closer to the cascade crest, where more snow will accumulate. Observers have reported weak snow from an old interface dating back to last weekend (January 10). If an avalanche ran on this layer it could be surprisingly deep and large.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
Wind Slabs
Large wind slabs may become likely to trigger on steep slopes in upper elevation terrain. You are most likely to trigger these near corniced ridgelines and within thick drifts on open slopes. Look for visual clues such as duning, and listen for hollow drum-like snow as you transition into wind affected areas. If you find strong over weak, slabby snow, avoid slopes steeper than 35 degrees.
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: Likely
Expected Size: 1 - 1