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Avalanche Forecast

Archived

Mar 7th, 2021–Mar 8th, 2021

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 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.

Regions

South Rockies.

Buried weak layers remain possible to be triggered by riders, where they exist. Cornices and sun-exposed slopes will deteriorate during the heat of the day, potentially increasing the likelihood of avalanche activity.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

SUNDAY NIGHT: Cloudy with light snowfall then clearing, accumulation 1 to 3 cm, 20 km/h southwest wind, alpine temperature -8 C.

MONDAY: Clear skies, 10 km/h northwest wind, alpine temperature -9 C, freezing level rising to 1500 m.

TUESDAY: Mix of sun and cloud, 20 km/h southwest wind, alpine temperature -6 C.

WEDNESDAY: Mix of sun and cloud with afternoon snowfall, accumulation 2 to 5 cm, 20 km/h west wind, alpine temperature -9 C.

Avalanche Summary

We received a few images of a rider that triggered a persistent slab avalanche near Crowsnest Pass on Saturday. The avalanche appeared to be large (size 2 or greater) and at below treeline or treeline elevation in a forest burn. It is suspected that the avalanche released on one or potentially both of the weak layers described in the Snowpack Summary, as the maximum slab depth was around 90 cm.

Otherwise, a few persistent slab avalanches were triggered naturally in adjacent Waterton National Park on Saturday. The avalanches were on northeast aspects between 1600 and 2000 m and released on the mid-February layer described in the snowpack summary. 

Snowpack Summary

Previous warm air temperature followed by a re-freeze have created a melt-freeze crust on the snow surface on sun-exposed slopes and below treeline. Dry snow may still be found on north aspects at treeline and alpine elevations. Wind slabs are likely still present on northerly aspects in the alpine and at treeline. Windward slopes at upper elevations have been scoured down to rocks or crusts in many areas due to recent strong southwest winds. 

Around 40 to 60 cm of snow sits above a persistent weak layer of sugary faceted grains that was buried in mid-February. There have been a handful of avalanches that released on this layer in the past few weeks. There is another persistent weak layer that was buried at the end of January, found around 50 to 80 cm deep. This layer consists of feathery surface hoar, facets, and/or a hard melt-freeze crust. Periodic avalanches continue to be triggered on these layers. 

Weak faceted snow and a decomposing melt-freeze crust can be found near the base of the snowpack. 

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of a persistent slab.
  • Avoid exposure to steep, sun exposed slopes, especially when the solar radiation is strong.
  • When a thick, melt-freeze surface crust is present, avalanche activity is unlikely.

Problems

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.

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.