Avalanche Forecast
Issued: Mar 13th, 2020 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Windy conditions should form new and reactive slabs in unusual locations Saturday. Choose more conservative travel plans and remember, this wind loading pattern may not match what you normally find. You’ll need to look for subtle clues like thin cornices and small new drifts to locate and avoid wind loaded slopes.
Summary
Discussion
This weather system appears to impact mountain locations south of Hwy 12 (White Pass) the most Friday night and Saturday, creating some uncertainty with the avalanche forecast. You could experience locally lower avalanche danger if you travel in a location that receives less snow. As of Friday afternoon, most areas in the West-South had accumulated 4-6” of low density and unconsolidated new snow. We’re expecting an additional 6-8” overnight in the snowier locations.
The new snow fell on a variety of old surfaces including firm crusts, wind packed snow, old settled powder, and possibly weak facets. Now that this interface is buried, it may be hard to anticipate what lies below the storm snow. You could see the slopes react differently from one location to the next. In general, firm crusts were seen on S-W aspects, while softer and weaker snow was observed on N-E aspects.
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.
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.
An active weekend for people and avalanches:
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.
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.
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.
Problems
Wind Slabs
Moderate to strong easterly winds should peak overnight and in the morning hours around the West-South. This will easily transport the storm snow and form new, reactive wind slabs. While you may find wind affected snow on any aspect, most slabs will exist on S-W-E facing slopes. That means you could see wind slabs in some unusual locations. Slopes you may normally consider safe could be more dangerous Saturday. You’ll need to look for subtle clues to help you find and avoid wind drifted areas. Small new cornices, textured snow surfaces, and thin drifts behind trees can help point toward wind loaded slopes. These wind slabs will be easiest to trigger in the morning hours, during periods of blowing snow, and at upper elevations.
Just because you’re out of the wind doesn’t mean you’re out of danger. Observations from the Crystal backcountry Friday found loose dry avalanches running far in the shallow new snow. This could occur again on Saturday, especially in areas where the sluffs can run on a firm bed-surface. Don’t let these loose avalanches surprise you, especially if you travel above cliffs, gullies, or creeks.
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
Expected Size
Valid until: Mar 14th, 2020 11:00AM
