Avalanche Forecast
Regions: Cascades - North West.
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
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:Â 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.
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
Loose Wet
Light rain should once again add additional water to a recently dry snowpack. Gradually cooling temperatures should limit loose wet activity to elevations near treeline and below. We don’t know how deeply the water saturated the snowpack on Sunday, leaving some uncertainty in our minds. We think that most avalanche activity will be in the upper 6-12” of the snowpack. Check your foot penetration and if you sink up to your boot top, observe rollerball activity, or see recent debris piles, avoid exposure to slopes steeper than 35 degrees. With possible recent wet slab activity in the Mt Baker area, there is a very slight possibility that these larger avalanches may linger another day, but we think this is unlikely.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1
Storm Slabs
Warm temperatures have pushed this problem to the highest elevations (most likely above 6000 ft), where dry snow comprises the majority of the upper half of the snowpack. Moderate winds gusting strong combined with additional snowfall to load heavy, wet layers onto the existing lower density snowpack on Sunday. This up-side-down layering configuration created a prime recipe for large and dangerous avalanches at lower elevations on Sunday. While we are concerned about this problem moving up the hill, we think storm slabs should be less likely to trigger as precipitation intensities decrease and the snowpack has more time by Monday to consolidate under the weight of slab. Watch for signs of recent avalanche activity and mitigate your travel on or below slopes and corniced slopes steeper than 35 degrees where the consequences of slab initiation will be a deeply gouging and serious avalanche.
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: Alpine.
Likelihood: Possible
Expected Size: 1 - 1