Avalanche Forecast

Archived

Mar 29th, 2021–Mar 30th, 2021

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 possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

South Rockies.

Storm slabs are expected to stay reactive throughout the day, especially in wind loaded terrain and/or when sun is on the slopes. Continue to make conservative choices as the new snow stabilizes.

Confidence

Moderate - Uncertainty is due to how the snowpack will react to the forecast weather. Uncertainty is due to the limited number of field observations.

Weather Forecast

There is some uncertainty between weather models about the extent of cloud cover during the day on Tuesday. Sunnier skies may increase the likelihood of both natural and human triggered avalanches. Continued moderate to strong winds will also be driving avalanche concerns.

MONDAY NIGHT - Clearing overnight / moderate northwest wind / alpine low temperature near -11 / freezing level at valley bottom

TUESDAY - Mostly sunny / moderate to strong north west wind / alpine high temperature near -4 / freezing level 1400 m

WEDNESDAY - Mostly clear / moderate south west wind / alpine high +3 /freezing level 2100 m

THURSDAY- Potential for no overnight freeze / partly cloudy / alpine high +5 / freezing level 2300 m

Avalanche Summary

Natural avalanches up to size 2 were reported by our field team on Monday. On the east slope there were reports of storm slabs forming during the day that were reactive to ski cutting in the size 1.5 range.

Snowpack Summary

By Monday we had 20-40cm of recent storm snow in sheltered areas. On Monday northwesterly winds were transporting fresh snow, potentially building new wind slabs and growing cornices. This is expected to continue into Tuesday.

This new snow sits over a variety of old surfaces such as crusts on all but north aspects in the alpine and treeline and crusts at all elevations below 1700 m.

The mid-pack is firm and well settled. Some faceted snow and a decomposing melt-freeze crust can be found near the base of the snowpack.

Terrain and Travel

Continue to make conservative terrain choices while the storm snow settles and stabilizes.
Be especially cautious as you transition into wind affected terrain.
Even brief periods of direct sun could produce natural avalanches.
Closely monitor how the new snow is bonding to the old surface.
Pay attention to cornices and give them a wide berth when traveling on or below ridges.

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.