Avalanche Forecast
Regions: Snoqualmie Pass.
As we transition from stormy to sunny this week, Snoqualmie Pass may present avalanche danger in a number of ways. Don’t get surprised by unusual wind loading patterns or loose avalanches in consequential terrain. If you travel in the eastern part of the zone, you may be able to trigger deeper avalanches.
Discussion
Observations indicate that around 5†of low-density new snow fell during the storm on a variety of snow surfaces. The old snow surface changes based on elevation and aspect, but the most common are crusts of varying thicknesses and density. We believe the snow to be fairly well bonded to the old surface, but you should use hand-pits and test slopes to check the bond before stepping into any consequential terrain. These crusts will make an excellent bed surface for any potential avalanche to run on. In the adjacent East Central zone, the danger is elevated due to higher snow totals falling on weaker snow surfaces. Avalanches and travel conditions were surprising a number of people on Saturday, so don’t get lulled to sleep by the low snow numbers.
Snowpack Discussion
March 12, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
From March 6th to the 12th the Pacific Northwest experienced a few small storms and a few notable days for people and avalanches. Light to moderate snow accumulated slowly over a period of a few days, and avalanche danger increased from Low to Moderate over the weekend of March 8th. The exception was the West North zone, which won the snowfall competition this week. 18-24†of new snow was observed in the Mount Baker area on the 6th, and 10†of new snow on the 11th. Avalanche danger reached Considerable there for storm slab, wind slab, and loose wet avalanche problems.Â
People, complicated avalanche terrain, and lots of avalanches. Bagley Lakes Basin, West North zone. Photo by Nathan Resick, March 8, 2020.
Loose dry slides initiated by skiers in big terrain. No one was caught or carried. East Central zone. March 8, 2020. Matt Primomo photo.
The snow fell with minimal wind, and accumulated slowly over firm suncrusts or wind stiffened surfaces. Thicker cloud cover inhibited the sun in areas where 4†to 12†of snow accumulated over the course of a few days. Cold overnight temperatures allowed the low density snow to weaken. By the 8th, the clouds broke up enough to allow strong radiation to change snow surfaces. When the sun finally did hit the snow, it quickly caused loose avalanches in steep terrain.Â
An active weekend for people and avalanches:Â
Many areas experienced some form of loose avalanche activity from March 6th to 9th. Sunday, March 8th was the day that recorded the most widespread avalanche activity across the region. The size of these avalanches were directly related to the amount of new snow that had recently accumulated in that area, and the size of the terrain.Â
West North zone on March 8, and West South zone on March 8th. Uphill and downhill tracks got covered by debris. Photos by Zack McGill and Dallas Glass.
We as a community can look back at, and learn from these few days. Numerous human triggered slides occurred, some were slabs but the majority were loose. A number of catch and carries were observed, and many more likely went undocumented. Luckily, no injuries or burials were reported. Observers across the region reported lots of instances where previous tracks got hit with debris, either from human triggered or natural avalanches.Â
As the days get longer and folks begin to step into the alpine zone and bigger terrain more frequently, it is important to keep a humble attitude. Continuously evaluate route selection with potential for loose wet and cornice falls in mind. Check the mountain weather and avalanche forecasts for the most up to date info. Enjoy the backcountry, be safe, and let us know what you see out there!
-MP
Solo snowboarder aired into this slope, and went for a ride but was not buried or injured. Slide took out the corner of a skin track. Bagley Lakes Basin, March 8, 2020, Zack McGill photo.
Wind slab triggered by a skier at 7,500ft next to the popular route on Mt St Helens. The skier wasn’t caught or carried. Andy Goodwin photo, March 8, 2020.
Avalanche Problems
Wind Slabs
Winds during the storm were strong enough to blow the new low-density snow around, so small Wind Slabs may exist at upper elevations. With east wind events like we just experienced, different loading patterns such as cross loading and mid-slope loading may be more prevalent. The result is that dangerous pockets of snow may exist in features not generally on your radar. You can look for textured snow surfaces or freshly built cornices to help you determine wind-loaded features that you should avoid.
In areas where the snow hasn’t been blown into a cohesive slab, loose snow should be sitting on a crust, which should put Dry Loose avalanches on your radar. Often referred to as sluffs, these slides can run fast and for far distances. If the sun comes out, it could initiate a loose avalanche by knocking snow off trees and rocks, running as either a Dry Loose or Wet Loose. Regardless of the moisture content of the snow, be mindful of your location in terrain and don’t let one of these avalanches catch you by surprise by pushing you into a terrain trap or other consequential 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.
Likelihood: Possible
Expected Size: 1 - 1