Avalanche Forecast
Regions: Olympics.
You can trigger large and dangerous wind slabs near ridgelines and particularly above treeline, where strong winds on Wednesday stripped exposed areas and deposited large amounts of cold, dry snow onto a variety of aspects. Near and below treeline, recent snow has layering but shows a trend toward increasing stability. Avoid steep rollovers such as (Sunrise Bowl and Mustang), particularly under recently corniced terrain and all slopes steeper than 35 degrees above treeline.
Discussion
The snowpack nearly doubled at Hurricane Ridge since the last crust formed on January 7th. And below treeline, we now have a snowpack capable of producing avalanches with around a meter of snow sitting on the crust at most elevations. Several storm systems dropped 2.3†of snow water equivalent last Friday through Monday morning with significant additions of lower density snow on Tuesday night (when up to two ft of snow accumulated in Port Angeles, corresponding to 0.31†of snow water equivalent picked up at Hurricane Ridge). Snow started up again Wednesday afternoon with winds increasing and averaging 25-30 mph with gusts to 50 mph on Wednesday evening. The storm snow has been capped by a thin layer of lower-density snow.
With all the recent snow, deep snow hazards are real. Before heading out, learn about tree well or snow immersion suffocation risks and travel with a partner (www.deepsnowsafety.org).
Forecast Schedule
For the 2019-20 winter season, avalanche danger ratings will be issued for the Olympics every Friday through Sunday and during mid-week holidays. During the week, No Rating will be issued but forecasts will include expected conditions and relevant travel advice. If you are out in the Olympics, share your backcountry observations with us and the greater community.
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
NWAC and NPS observers on Thursday (1/16) found enough evidence of wind transport near treeline that they are quite concerned about the potential for large wind slabs above treeline and were unwilling to engage with that terrain themselves. These wind slabs are only 2 days old on Friday and should heal very slowly at current cold temperatures. They might be hidden by 4 inches or more of less wind-affected snow. Give the above treeline more time to heal before negotiating slopes steeper than 35 degrees. Near treeline look for recently formed cornices to identify wind-loaded slopes. Stiffer snow near the surface confirms wind slab concern. Use trees and lower angle slopes to negotiate this problem.
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
Storm Slabs
The snowpack at Hurricane Ridge has nearly doubled in the last 9 days and with limited observations on these changes, we remain with uncertainty about what you will encounter when you travel. Observations on Thursday found that snow was generally bonding well to a gradually refreezing crust down 1 meter. Several density interfaces create the potential to trigger small or large avalanches, and you are most likely to trigger them on steep 40-degree convex rollovers. Travel in supported terrain (steeper slopes with lower angle slopes below) to maintain your margin of safety.
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.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1