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Avalanche Forecast

Archived

Apr 14th, 2021–Apr 15th, 2021

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

Regions

Purcells.

Solar radiation and warm temperatures will weaken the snowpack and cornices. Avoid exposure to sun exposed slopes and cornices, especially in the PM. The danger rating is for the hottest time of the day.

Check out the latest Forecaster's Blog on warming and how to stay safe.

Confidence

High - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather.

Weather Forecast

A ridge of high pressure over the province continues to bring sunny, dry weather. Freezing levels are forecast to steadily rise through the week.

Wednesday night: Clear, moderate to strong east wind, alpine low -8 C, freezing level 1600 m.

Thursday: Sunny, moderate northeast wind, alpine high +6 C, freezing level 2500 m.

Friday: Sunny, moderate northeast wind, alpine high +9 C, freezing level 2800 m.

Saturday: Sunny, light to moderate westerly wind, alpine high +10 C, freezing level 3000 m.

Avalanche Summary

A large natural cornice fall of size 3 on a northeast aspect was reported on Wednesday. Small wet loose avalanches to size 1 and a couple size 2 were observed on steep solar slopes. A couple slab avalanches were reported that might have been triggered by cornices in the last few days. 

Natural and skier controlled wind slabs have been stubborn to trigger and limited to size 1 over the weekend. 

Last Thursday, a significant natural cornice failure triggered a persistent slab on the slope below and resulted in a size 3 avalanche. 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 warm temperatures have made them large and fragile. 

Snowpack Summary

The snow surface consists of a melt-freeze crust on solar aspects and shaded aspects below treeline which transitions into moist/wet snow during the day. Dry snow can still be found on northerly aspects at treeline and in the alpine. 10-30 cm of recent snow has formed isolated pockets of wind slab on leeward slopes and behind terrain features. The new snow sits on a series of melt-freeze crusts on all aspects below 1900 m and southerly aspects to mountain top. Reports indicate the snow is bonding well at these interfaces.

Cornices are large, fragile and primed to fail with warm temperatures. Many natural cornice falls were recently observed in neighboring regions. Some triggered slabs on the slopes below.

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. With each day of warm weather the likelihood of deep persistent slab avalanches will increase. 

Terrain and Travel

  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • Avoid slopes when the solar radiation is strong, especially if they have large cornices overhead.
  • Avoid sun exposed slopes, especially if snow surface is moist or wet.
  • The likelihood of deep persistent slab avalanches will increase with each day of warm weather.

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.