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Avalanche Forecast

Archived

Apr 17th, 2021–Apr 19th, 2021

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

Regions

North Rockies.

Cloudy skies may prevent the snow surface from freezing overnight. Expect the danger to be CONSIDERABLE in areas where the snow surface doesn't freeze into a strong crust overnight.

Check out the latest Forecaster's Blog for more information on managing current conditions.

Confidence

Moderate - Uncertainty is due to how the snowpack will react to the forecast weather.

Weather Forecast

  

SATURDAY NIGHT: Cloudy with isolated flurries; 3-10 cm. / Strong, northeast ridgetop wind / alpine low temperature -2 / Freezing level dropping to 1500 m.

SUNDAY: Mix of sun and cloud / Strong, northeast ridgetop wind / alpine high temperature 1 / Freezing level 1500 m.

MONDAY: Sunny / Light, northeast ridgetop wind / alpine high temperature 3 / Freezing level 1800 m.

TUESDAY: Sunny / Light, northwest ridgetop wind / alpine high temperature 7 / Freezing level 2300 m.

Avalanche Summary

Numerous explosive triggered wet slab avalanches size 3-3.5 and one size 3.5 deep persistent slab avalanche were reported on solar aspects on Friday.

There have been some large deep persistent slab avalanches triggered by warming to the south around Jasper and Banff, and we suspect this problem may extend into the North Rockies.

Sustained warming can increase the likelihood of large avalanches failing on deeply buried weak layers.This is most likely to occur on steep, rocky alpine slopes with a shallow or thin to thick snowpack. 

Snowpack Summary

Forecast flurries and strong northeasterly winds Saturday night may form small wind slabs on lee features below ridgetops.

Warm temperatures are melting the surface on all but the highest north-facing slopes, creating weak surface snow during the heat of the day. Crusts may form overnight. 

 Strong solar radiation and warming will likely trigger the large cornices that hang over ridgelines. Cornice falls could trigger large avalanches on deeply buried weak layers. 

As heat penetrates deeper into the snowpack it has the potential to reactivate deeper weak layers, including a layer from mid-February and weak facets at the bottom of the snowpack. The most likely areas to be concerned about deeper weak layers are shallow parts of the region along the eastern slopes of the Rockies like Core Lodge, Wolverine, Bullmoose, Upper Burnt and perhaps around Mt. Robson. In general, steep, rocky slopes with a shallow or thin to thick snowpack are most suspect.

Terrain and Travel

  • Avoid travelling on slopes below cornices.
  • Avoid exposure to sunny slopes, especially in the afternoon.
  • The more the snowpack warms-up and weakens, the more conservative you`ll want to be with your terrain selection.
  • If triggered loose wet avalanches may step down to deeper layers resulting in larger avalanches.
  • 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.