Avalanche Forecast
Issued: Mar 21st, 2021 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Cornices and Persistent Slabs.
dsaly,
Avalanche Canada
Carefully assess your line for reactive wind slabs before committing. Steep, convex slopes below ridgetop are the most likely places to trigger these slabs.
Summary
Confidence
Moderate - Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.
Weather Forecast
SUNDAY NIGHT: Snow and flurries, most areas should see 5-10 cm however localized enhancement may produce up to 15 cm in isolated areas. Strong southwest wind, alpine low -8C, and freezing level dropping to valley bottom.
MONDAY: Cloudy with isolated flurries and sunny breaks, up to 5 cm. Light to moderate northwest wind, alpine high -6C, and freezing level rising to about 1200 m.
TUESDAY: Mostly cloudy with flurries starting through the day. Moderate southwest wind, alpine high -4C, and freezing level beginning near valley bottom rising to about 1500 m.
WEDNESDAY: Snow, 10-15 cm. Moderate southwest wind, alpine high -2C, and poor overnight recovery with the freezing level hovering around 1000m and rising above 1500m during the day.
Avalanche Summary
No avalanche reports in the last few days.
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
South-southwesterly winds have impacted loose snow developing slabs in immediate lees. 10-20 cm fresh snow covers dry settled snow on northerly aspects above 1800m, and crusts on solar aspects and lower elevations. Large cornices loom over alpine ridgetops. A thick rain crust extends up to 1600m.
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.Â
Terrain and Travel
- Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
- Dial back your terrain choices if you are seeing more than 25cm of new snow.
- 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
Recent southwest winds and new snow have formed wind slabs in lee terrain features. Winds are expected to transition to a west-northwesterly flow, which may encourage "reverse loading" building fresh wind slabs on more south and easterly slopes. Carefully assess your line for touchy wind slabs before committing. 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.
Likelihood
Expected Size
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.
Likelihood
Expected Size
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.
Likelihood
Expected Size
Valid until: Mar 22nd, 2021 4:00PM
