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Avalanche Forecast

Archived

Mar 29th, 2021–Mar 30th, 2021

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

Regions

Purcells.

Storm slabs may remain reactive to human triggers as natural avalanche activity tapers off. Start with small terrain features, look for signs of instability and gather information before considering bigger terrain.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

Monday night: Partly cloudy. Light to moderate northwest wind. Freezing level valley bottom. Alpine temperature -13.

Tuesday: Mix of sun and cloud. Light northwest wind. Freezing level 1500 m. Alpine temperature -9.

Wednesday: Sunny. Light southwest wind. Freezing level 2100 m. Alpine temperature -1.

Thursday: Sunny. Light southwest wind. Freezing level 2100 m. Alpine temperature -1.

Avalanche Summary

Preliminary reports of avalanche activity during the storm on Sunday include a numerous explosive triggered size 2 loose wet avalanche on a northeast aspect around 1700 m. It is suspected to have scrubbed down to a crust near the ground, as rain and warm temperatures destabilized the full depth of the snowpack.

Snowpack Summary

15-40 cm of recent snow came with west to northwest wind, likely loading the fresh snow into lee features at upper elevations. Crusty and/or moist surfaces exist below treeline depending on elevation and time of day.

A widespread crust layer from the mid-March warm spell can be found 30-60 cm deep, and small surface hoar has been observed at this depth on some isolated north-facing slopes. Reports suggest the snow is generally well bonded to these layers. Deeper layers are strong and have been unreactive over the past few weeks. 

Terrain and Travel

  • Keep in mind that human triggering potential persists as natural avalanching tapers off.
  • Start with conservative lines and watch for clues of instability.
  • Seek out sheltered terrain where new snow hasn't been wind-affected.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.