Avalanche Forecast
Issued: Mar 19th, 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.
ghelgeson,
Avalanche Canada
Potentially touchy wind slabs may form throughout the weekend on upper elevation north, northeast & east facing slopes. Clearing Saturday afternoon could heat things up fast. Large loads like cornice fall may trigger a persistent slab from the slope below.
Summary
Confidence
Weather Forecast
Looks like it’s back to winter for the next few days.
FRIDAY NIGHT: Freezing level lowering to about 600 m, moderate southwest wind with potential for strong gusts, 1 to 5 cm of snow possible.
SATURDAY: Broken cloud cover with potential for some clearing in the late afternoon, freezing level rising to about 1400 m, moderate southwest wind, 1 to 5 cm of snow possible.
SUNDAY: Broken cloud cover, freezing level beginning near valley bottom rising to about 1400 m, moderate southwest wind, 2 to 4 cm of snow possible during the day with potential for 3 to 6 cm Sunday night.
MONDAY: A few clouds, freezing level beginning near valley bottom rising to about 1400 m, light variable wind, no snow expected.
Avalanche Summary
No new activity to report from Thursday.
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 wind accompanied by dribs and drabs of snowfall through the weekend may form fresh wind slabs below alpine 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.
- 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
The best riding conditions and the highest hazard will overlap this weekend. 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 over the next few days 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.
If the sun pokes out it may have enough punch to initiate the recent snow as loose wet or dry avalanches from steep terrain features.
Aspects: North, North East, East.
Elevations: Alpine.
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.
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 20th, 2021 4:00PM
