Avalanche Forecast
Issued: Mar 18th, 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.
swerner,
Avalanche Canada
New snow and strong wind may form isolated pockets of fresh wind slab.
The region will see mostly cloud cover on Friday, but if the sun shines it could heat up fast. Large loads like cornice fall may trigger a persistent slab from the slope below.
Summary
Confidence
Weather Forecast
A Pacific frontal system reaching the Coast today will bring cloudy skies and snowfall to the Interior regions through the weekend.
Thursday Night: Snow 5-10 cm. Ridgetop wind moderate from the southwest and alpine temperatures near -10. Freezing levels 1500 m.
Friday: Cloudy with sunny periods and possibly 5-10 cm. Ridgetop winds moderate from the South and freezing levels 1500 m.
Saturday: Mix of sun and cloud. Ridgetop wind strong from the West and alpine temperatures near -10. Freezing levels 1100 m.
Sunday: Snow 5-10 cm. Ridgetop wind light to moderate from the southwest. Freezing levels 1200 m.
Avalanche Summary
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. This morning 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 suspect interface being the deeper weak facets.
These recent reports indicate that the buried persistent weak interface still lurks, however; it's likely that it would 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 most suspect.
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 rider triggers.
Snowpack Summary
Strong ridgetop wind accompanied by 5-10 cm of new snow is expected for Friday with additional snow through the weekend. This 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
Increasing winds on Friday accompanied by new snow may form wind slabs in isolated alpine locations. Steep, convex slopes below alpine ridgetops 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 yesterday 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 19th, 2021 4:00PM
