Avalanche Forecast

Issued: Feb 1st, 2021 4:00PM

The alpine rating is high, the treeline rating is high, and the below treeline rating is considerable. Known problems include Storm Slabs and Persistent Slabs.

swerner, Avalanche Canada

Lizard-Flathead.

Avoid avalanche terrain and overhead hazards on Tuesday as the new snow accompanied by a strong southwest wind will build touchy and reactive slab avalanches. They will especially be reactive in sheltered terrain where they sit above a surface hoar/ facet/ crust interface.

Summary

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Enhanced snowfall amounts may occur with the strong southwest flow followed by the cold front.

Monday Night: Snow amounts 5-10 cm with 20-60 km/hr ridgetop wind from the South. Alpine temperatures near -1 and freezing levels 1300 m.

Tuesday: Snow 10-25 cm. Strong and gusty southwest wind. Alpine temperatures near -1 and freezing levels 1400 m.

Wednesday: A mix of sun and cloud with isolated flurries. Alpine temperatures -6 and freezing levels at the valley bottom. Ridgetop wind generally light from the southwest.

Avalanche Summary

On Monday, several storm slab and loose dry avalanches up to size 2 were triggered with the use of explosives. Reports of natural cornice failures were seen up to size 2. On Sunday, numerous storm slab and persistent slab avalanches were reported up to size 1.5 primarily on NE aspects above 1700 m. These avalanches were easily triggered by the weight of a human either directly or remotely (from a distance away). Check out these reports that show how reactive the slab is: MIN Report, MIN report, or this MIN report.

With additional forecast snow and strong wind, the trend will likely continue on Tuesday resulting in natural avalanche activity and slabs primed for human triggers.

Snowpack Summary

Up to 45 cm of recent snow has formed a cohesive and reactive slab that sits above a weak interface of surface hoar and surface facets (weak sugar or feather-like snow crystals). In the alpine the recent snow sits on top of layers of hard wind slab, scoured areas, sastrugi and isolated pockets of soft snow. A hard melt-freeze crust underneath the new snow is found up to 1800 m.

A solid mid-pack sits above a deeply buried crust and facet layers near the bottom of the snowpack (150-200 cm deep), which are currently unreactive.

Terrain and Travel

Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
Avoid terrain traps where the consequence of any avalanche could be serious.
Pay attention to cornices and give them a wide berth when traveling on or below ridges.

Problems

Storm Slabs

Strong southwest wind and new snow forecast for Tuesday will build new and reactive storm slabs at all elevations.

Cornices are large and fragile. Cornice failures were reported on Monday, likely due to warming. They deserve respect and a wide berth from above and below.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1.5 - 2.5

Persistent Slabs

A cohesive slab 30-45 cm thick now sits above a buried weak layer of surface hoar and facets. These slabs have been very reactive to human triggers showing wide propagations in moderate to low angle terrain. They have been catching people by surprise, so pay attention to signs of instability and stay clear of terrain traps below.