Avalanche Forecast

Archived

Mar 16th, 2013–Mar 17th, 2013

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 unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

South Rockies.

Confidence

Fair - Forecast snowfall amounts are uncertain on Sunday

Weather Forecast

Saturday night: Light snow. Moderate to strong W winds. Freezing level at valley floor.Sunday: Moderate snow (amounts uncertain). Moderate to strong NW winds. Alpine temperature near -7.Monday: Light snow. Light SW winds. Alpine temperature near -8.Tuesday: Light snow. Light SW winds. Alpine temperature near -7.

Avalanche Summary

A widespread natural avalanche cycle occurred during the last storm in response to heavy rain and/or snow and wind loading. Many were loose wet avalanches in the size 1-2.5 range, which entrained loose moist snow as they traveled. Some size 3 avalanches were reported to have failed at ground in the SE corner of the region. Several wind slabs and cornice-triggered storm/wind slabs were also observed (photos here). By Friday, natural activity started to slow down.

Snowpack Summary

The recent warm storm dropped 10-30 cm of dense snow above about 2000 m and soaked the snowpack with rain at lower elevations. At high elevations, snow has been transported by strong SW and W winds, creating wind slabs. Cornices are large and fragile in some areas. The new snow was not bonding well to underlying surfaces during the storm. With cooling temperatures, the snowpack should start to stabilize and form a hard surface crust, which new snow may not bond well to. The midpack is generally well-settled.

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.