Avalanche Forecast
Regions: Cascades - East.
Dangerous avalanche conditions continue following heavy snowfall and strong northeasterly wind. You can trigger large avalanches on open, wind loaded slopes 35 degrees and steeper. If skies clear for long enough, expect loose avalanches on steep, sun-exposed slopes.
Discussion
Saturday was a very active day with numerous reports throughout the zone at most elevations of natural and triggered avalanches as well as widespread cracking and collapsing in the snow. In the Icicle drainage, an observer reported over 1 foot of new snow, a size D2 natural wind slab avalanche on a NW aspect at 6,200ft, natural dry loose avalanches in steep terrain at 7,000ft, and a large collapse. Many avalanches were reported from the Wenatchee Mountains including a partial burial, intentionally cornice triggered wind slabs, and a natural cornice fall triggering a size D1 avalanche. Mission Ridge reported extensive avalanches with control work including crowns up to 300 feet wide. Reported avalanches typically occurred between 5,200-6,500ft on N-NW aspects and involved only storm snow. Many avalanches were reported to have run on a mid-storm interface just an inch or two above a crust buried by the storm on the 13th.
The unique character of this storm focused on the Wenatchee Mountains. Prior to this storm, snow cover was very shallow there. Stumps, logs, and rocks may be shallowly buried under the new snow. Use caution not to run into these obstacles. In areas closer to the Cascade Crest like the Teanaway, Icicle, and Entiat drainages, as well as the area around Holden Village, the avalanche problems will be similar but some of those locations received less recent snow.
A skier triggered avalanche, size D1, near Diamond Head, Blewett Pass, NW, 5200ft. 3/14/2020. Photo: Adam Butterfield
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
Expect reactive avalanche conditions in open wind effected terrain at all elevations. Put a buffer of terrain between where you travel and slopes over 35 degrees, especially where you see deeper drifts and pillows of snow. Triggering avalanches is most likely on leeward, cross-loaded terrain. You may also be able to trigger avalanches in the new snow on wind-sheltered slopes below treeline. Steer around obviously loaded slopes and pillowed wind drifts. Avoid typical trigger points like convexed rollovers and unsupported slopes. Watch for natural avalanches, cracking in the snow, and whumpfing collapses as indicators of the danger. Keep your terrain decisions conservative and simple. If you don't see obvious signs of instability, use small, inconsequential test slopes to see how easily the new snow can slide.
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: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
Loose Wet
If skies clear for long enough on Sunday, expect loose avalanche on steep, sun-exposed slopes. Watch for rollerballs or other natural loose avalanches as indicators. Don't linger under on or under steep sunny slopes. Use extra caution around terrain traps, such as cliffs, gullies, or rocks, where these avalanches could be dangerous.
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.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1