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Avalanche Forecast

Archived

Mar 28th, 2021–Mar 29th, 2021

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

Regions

Purcells.

Overnight snow and moderate winds shifting northwest will continue to form wind slabs and elevate avalanche hazard at upper elevations. Crusty surfaces below treeline will make for low hazard but difficult travel conditions.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Convective instability behind the storm will bring additional snowfall to some areas overnight Sunday and into Monday. Specific locations and amounts are notoriously difficult to forecast for this type of weather pattern.

Sunday night: 5-10 cm new snow. Moderate to strong northwest wind. Freezing level valley bottom. Alpine temperatures around -12.

Monday: Mix of sun and cloud with isolated flurries. Moderate northwest wind. Freezing level 500 to 1000 m. Alpine temperatures around-12.

Tuesday: Mix of sun and cloud. Light westerly wind. Freezing level 1500 m. Alpine temperature -7.

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

Avalanche Summary

We are awaiting reports of avalanche activity from the weekend's storm. Prior to the storm, recent avalanche activity has been limited to small (size 1-1.5) wind slab avalanches, dry loose sluffs, and cornice falls.

Snowpack Summary

10-20 cm of new snow has likely been redistributed by wind at upper elevations. Crusty surfaces exist below treeline. 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

  • Storm snow and wind is forming touchy slabs. Use caution in lee areas in the alpine and treeline.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.