Avalanche Forecast
Issued: Mar 14th, 2020 11:36PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Dry.
NWAC,
Northwest Avalanche Center
UPDATE: Added wind slabs to forecast. More than 8 hours of moderate to strong winds overnight at Mt Hood likely built new and reactive slabs at mid and upper elevations. Take time to identify and avoid wind loaded features. Since the winds blew from the NE, you may find these slabs in some unusual locations.
Previous: Precipitation, light winds, and very cold temperatures gave Mt Hood a nice blanket of fluffy unconsolidated snow. Don’t let the cold smoke distract you. Be leery of steep slopes where you may trigger loose avalanches that run far and fast on the firm surface underneath.
Summary
Discussion
UPTDATE: Added wind slabs, but we do not believe this will change the danger rating. Only the primary avalanche problem
This unique storm has been a tricky forecast for Mt Hood. Despite the close proximity of heavier precipitation, and expected strong easterly winds, Hood remained pretty quiet, with modest new snow amounts and generally light winds. That changed late Saturday night as the winds finally returned to Mt Hood. Sustained strong winds quickly took hold of the area just before midnight, altering the snow surface near and above treeline.
As of Saturday afternoon observations from the area reported 8-10” of very low-density snow with nearly no wind affect. This storm snow accumulated over a wide variety of surfaces including blue ice, wind packed snow, old settled powder, and weak sugary facets. Now that these old surfaces are buried it’s hard to anticipate what lies beneath. If the new snow rest over blue ice, it could make for difficult, if not dangerous, travel conditions.
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
New wind slabs likely built overnight and will continue to form into the morning hours. You'll want to take time to look for and avoid wind loaded pockets on steep slopes below small thin cornices, with smooth pillow-like drifts, or that hold deeper, firmer deposits of snow. Due to the NE winds, you may find these slabs in some odd locations. Old large cornices will not be a good guide to where wind slabs may exist. You'll have to look for more subtle clues like small drifts, blowing snow, or the current direction of the wind. Some of these slabs may for over pockets of bare rock or thin snowpacks on previously wind-scoured ridges. If the wind continues to blow, you could see wind slabs become larger and easier to trigger later in the day.
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
Expected Size
Loose Dry
Previous description only applies to winds sheltered areas: Chilly temperatures, light winds, and minimal additional snow should allow the surface to remain light, dry, and unconsolidated. As a result, you could trigger loose dry avalanches on slopes steeper than 35 degrees. Some of these slides could run far and fast. Don’t get surprised by these sluffs, knocked off your feet, and pushed into a terrain trap.
Even though we’re expecting mostly cloudy skies, you could see a few sun breaks or periods of filtered sunshine. If you start to notice the sun, odds are, so does the snowpack. This could cause the snow to change and behave differently. You may see the new snow settle into a slab or, if the sun stays out long enough, produce natural loose wet avalanches. If you find the surface snow becoming firmer, see new rollerballs, or find other signs of unstable snow, dial back your terrain use and steer around steeper convex rollovers and higher consequence terrain.
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Mar 15th, 2020 11:36PM
