Avalanche Forecast
Issued: Mar 22nd, 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 wind slabs before committing. Steep, convex slopes below ridge tops are the most likely places to trigger these slabs.
Summary
Confidence
Weather Forecast
MONDAY NIGHT: Mostly cloudy with isolated flurries, little accumulation expected. Light to moderate northwest wind, alpine low -12C, and freezing level valley bottom.
TUESDAY: Mostly cloudy with scattered flurries, trace to 5 cm. Moderate southwest wind, alpine high -4C, and freezing level rising around 1000 m.
WEDNESDAY: Snow, 10-20 cm. Moderate to strong southwest wind, alpine high -1C, and poor overnight recovery with the freezing level hovering around 1000m overnight Tuesday and rising above 1500m during the day.
THURSDAY: Mostly cloudy and unsettled. Light west wind, alpine high -1C, and freezing level rising to about 1300 m.
Avalanche Summary
There have been no recent avalanche reports.
Last 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. Just over a week ago, and likely during the first big warm-up, several large size 2.5-3 natural slab avalanches were reported - the suspect failing layer the mid-February facet interface, see some photos of this activity in this MIN. These reports indicate that the buried persistent weak interface remains somewhat active, but 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
Variable winds have impacted loose snow, wind slabs may be found on a variety of aspects, likely in lee features. 15-30 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.
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 fresh snow formed wind slabs in lee terrain features. Now in a west-northwesterly flow, winds may may "reverse load" slopes, building fresh wind slabs on more south and easterly aspects. 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 23rd, 2021 4:00PM
