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Avalanche Forecast

Archived

Apr 15th, 2021–Apr 16th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Purcells.

Solar radiation and warm temperatures will continue to weaken the snowpack and cornices. Avoid exposure to sun exposed slopes and cornices, especially in the afternoon. 

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 continues to bring sunny and dry weather with high freezing levels even at night. On Sunday the ridge is forecast to break down bringing light precipitation and lowering freezing levels.

Thursday night: Clear, moderate to strong northeast wind, alpine low 0 C, freezing level 2400 m.

Friday: Sunny, moderate northeast wind, alpine high +8 C, freezing level 2800 m.

Saturday: Sunny, moderate northwest wind, alpine high +9 C, freezing level 2900 m.

Sunday: Mix of sun and cloud with isolated flurries, up to 5 cm new snow, moderate northwest wind, alpine high +5 C, freezing level lowering to 2000 m.

Avalanche Summary

A large natural cornice fall of size 3 likely released a slab on the northeast facing slope below and was reported on Wednesday. Small wet loose avalanches to size 1 and a couple size 2 were observed on steep solar slopes. A few 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 last 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 up to treeline which transitions into moist/wet snow during the day. Dry snow can still be found on northerly aspects 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. 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

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.

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.

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.