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Avalanche Forecast

Archived

Mar 21st, 2021–Mar 22nd, 2021

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Cariboos.

Carefully assess your line for reactive wind slabs before committing. Steep, convex slopes below ridgetop are the most likely places to trigger these slabs.

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. 

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

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.