Avalanche Forecast

Issued: Mar 20th, 2021 4:00PM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs, Cornices and Persistent Slabs.

Avalanche Canada dsaly, Avalanche Canada

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Strong winds and flurries are forming fresh wind slabs around ridges and lee features. Be mindful that best riding conditions may overlap with the deepest deposits and touchiest slabs.

Summary

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

SATURDAY NIGHT: Cloudy with isolated flurries and trace accumulation. Light to moderate southwest wind, alpine temperature low -10C, and freezing level dropping to valley bottom.

SUNDAY: Flurries accumulating through Sunday, 5-15 cm by days end. Moderate to strong southwest wind, alpine high -5C, and freezing level beginning near valley bottom rising to about 1200 m.

MONDAY: Cloudy with sunny breaks and isolated flurries, no snow expected. Light northwest wind, alpine high -6C, and freezing level beginning near valley bottom rising to about 1300 m.

TUESDAY: Mostly cloudy with flurries starting through the day. Moderate southwest wind, alpine high -5C, and freezing level beginning near valley bottom rising to about 1200 m.

Avalanche Summary

No new activity to report from Thursday or Friday.

New snow and strong winds through the weekend could build fresh wind slabs on leeward slopes and behind terrain features. These would likely be reactive to human triggering.

On Wednesday our North Rockies Field team reported a size 2.5 natural cornice failure in the McBride area. This cornice triggered an East facing alpine slope with the suspected weak layer being the deeper weak facets.

Several reports of large size 2.5-3 natural slab avalanches occurred last weekend, likely during the first big warm-up. The suspect failing layer of these avalanches is the mid-February facet interface, see some photos of this activity in this MIN.

These reports indicate that the buried persistent weak interface remains active. However, it would likely take a large load to trigger it. That being said the weight of a human and/or machine may be enough to trigger something deeper. It is a low probability - high consequence scenario with large N-E facing alpine slopes being the most suspect.

Snowpack Summary

Strong ridge top winds accompanied by flurries are forming fresh wind slabs below ridgetops. Dry snow can be found on north aspects and crusty snow surfaces exist on solar aspects. Large cornices loom over alpine ridges. They are very unpredictable and if they fail they could trigger a slab on the slope below.

A persistent weak layer made up of surface hoar at treeline elevations and a crust with facets in the alpine can be found down 50-150 cm in some parts of the region. Recent reports indicate that some very large avalanches have occurred on this layer in the past week. It seems to need a large trigger like a cornice fall or a rapid flux in weather like a big warm-up. 

Terrain and Travel

  • Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
  • Wind slabs are most reactive during their formation.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.
  • Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.

Problems

Wind Slabs

An icon showing Wind Slabs

As southwesterly winds redistribute loose snow, the best riding conditions will overlap with the highest avalanche hazard. High elevation north, northeast and east facing slopes may have a thin layer of surface hoar in place and continued southwest winds are expected to build fresh slabs above this surface hoar. Carefully assess your line for touchy wind slabs before committing. This problem is expected to increase in sensitivity as new snow and wind continue to build the slab. Steep, convex slopes below alpine ridge tops are the most likely places to trigger these slabs.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

JavaScript chart by amCharts 3.20.8

Likelihood

Possible - Likely

Expected Size

1 - 2

Cornices

An icon showing Cornices

Where cornices exist they pose a threat both from the potential for them to collapse under your feet (or machine) and from the potential to send large chunks of snow far down a slope. If anything can trigger the more stubborn persistent weak layers it's a large falling cornice. They are most likely to fail during periods of solar radiation or loading from snow/wind.

Aspects: North, North East, East, South East.

Elevations: Alpine.

JavaScript chart by amCharts 3.20.8

Likelihood

Possible

Expected Size

2 - 3

Persistent Slabs

An icon showing Persistent Slabs

A persistent weak layer of sugary facets continues to lurk deep in the snowpack. Three large avalanches (size 2-3) have failed on this interface in the past week. The first two occurred last weekend during an intense warm-up and the third was initiated by a cornice failure Wednesday near McBride. These avalanches were all reported from northeasterly slopes in the alpine. The persistent slab will likely need a large trigger, however, the weight of a person plus a machine might just be enough. This is a low probabilty but high consequence scenario and very hard to predict. 

Aspects: North, North East, East.

Elevations: Alpine.

JavaScript chart by amCharts 3.20.8

Likelihood

Unlikely - Possible

Expected Size

2 - 3

Valid until: Mar 21st, 2021 4:00PM

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