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Avalanche Forecast

Archived

Mar 22nd, 2020–Mar 23rd, 2020

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

New snow drifted by wind may form reactive slabs at upper elevations. Anticipate changing conditions and monitor the depth, cohesiveness, and bond of the new snow to the old snow surface. A deep weak layer remains a concern.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Sunday night: Mostly cloudy, up to 5 cm of snow, light southwest wind with moderate gusts at ridge-tops, alpine temperature -7 C, freezing level 1000 m.

Monday: Mostly cloudy, scattered flurries with up to 5 cm of snow, light southwest wind, alpine temperature -6 C, freezing level 1500 m. 

Tuesday: Mostly cloudy, 5-10 cm of snow, light southwest wind, alpine temperature -8 C, freezing level 1300 m. 

Wednesday: Mix of sun and cloud, light west wind, alpine temperature -11 C, freezing level 1200 m. 

Avalanche Summary

Over the past several days, avalanche activity was reported as natural loose wet avalanches (size 1-2.5) running in steep, sun-exposed terrain. One large (size 2.5) cornice failed as a result of explosive mitigation.

Last week, there was a report of a skier remotely triggering (from a distance) a large (size 2) deep persistent slab avalanche at 2600 m on a northwest aspect. The avalanche was triggered from a thin spot, released 40-80 cm deep, and ran on depth hoar near the base of the snowpack.

If you decide to travel in the backcountry, consider sharing your observations via the Mountain Information Network (MIN) to supplement our data stream as operators are shutting down. A conservative mindset and margins are recommended at this time.

Snowpack Summary

Up to 10 cm of new snow is forecast to accumulate by Monday afternoon in favored areas. Moderate southwest winds at ridge-tops may build wind slabs primed for human triggering on lee features.

The new snow is falling on a variety of snow surfaces, consisting of sun crusts, hard wind-affected snow, and soft faceted snow. There have also been reports of surface hoar forming on sheltered, shady slopes. See this MIN for a helpful illustration from nearby Glacier National Park. It will be important to track the depth, cohesiveness, and bond of the new snow to these various old snow surfaces across aspects and elevations where you are travelling.

Cornices are large and looming. Surface hoar buried February 22 may be found 50-100 cm deep. Avalanche activity on this layer was last reported March 11th. 

Deeply buried facets lurk near the bottom of the snowpack. This layer has been responsible for sporadic deep persistent slab avalanches, usually triggered from shallow, rocky start zones.

Terrain and Travel

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Make observations and assess conditions continually as you travel.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.