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Avalanche Forecast

Archived

Apr 9th, 2021–Apr 10th, 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.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

Purcells.

New snow and wind may form pockets of reactive wind slab in exposed terrain. Loose snow avalanches can be expected in the new snow on steep terrain features. Where new snow sits on a crust, avalanches can trigger easily and run surprisingly far & fast.

Confidence

High -

Weather Forecast

The unsettled weather begins to clear up over the weekend as a ridge of high pressure sets up over the province.

Friday Night: Scattered flurries up to 5 cm. Moderate to strong southwest wind. Alpine temperature around -8. Freezing level 1000 m.

Saturday: Scattered flurries up to 10 cm. Light to moderate northwest wind. Alpine temperatures near -9. Freezing level 1200 m. 

Sunday: Sunny. Light west wind. Alpine temperature around -7. Freezing level 1600 m.

Monday: Mix of sun and cloud. Light east wind. Alpine temperature around -7. Freezing level 1600 m.

Avalanche Summary

Most activity in the recent snow has been limited to loose dry sluffing over a crust.

On Thursday, a significant natural cornice failure triggered a persistent slab from the slope below size 3. This occurred on a high alpine steep east-facing slope. This is an important reminder to stay well back from ridgetops that may be corniced and avoid slopes with overhead cornice exposure especially under the current conditions where recent rapid growth and daytime warming has made them large and fragile. 

Snowpack Summary

Incremental snowfall continues to accumulate 5-20 cm throughout the region. Shifting wind directions from the southwest to northwest may have formed isolated pockets of wind slab on leeward slopes and behind terrain features. The new snow sits on a series of melt-freeze crusts exist on all aspects below 1900 m and solar aspects to mountain top. North aspects in the alpine hold dry wintery and wind-affected snow. The mid-pack is generally strong and consolidated. 

Recent warm weather is expected to have promoted bonding around widespread crust layers formed over the last month in the mid to upper snowpack. One widespread crust from mid-March found 30-60 cm deep saw isolated instances of large solar-triggered slab avalanches during the last sunny period. The early November crust is buried down 180-200 cm with faceted crystals above and below it. These persistent weaknesses are definitely worth keeping in mind heading into the next big warmup.

Terrain and Travel

  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
  • Be aware of the potential for loose avalanches in steep terrain where snow hasn't formed a slab.
  • Brief periods of sun could quickly initiate natural avalanche activity.

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.