Avalanche Forecast

Issued: Feb 26th, 2020 4:00PM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs and Deep Persistent Slabs.

Avalanche Canada ghelgeson, Avalanche Canada

Cornice failure, loose wet avalanches and even slab avalanches are possible as the upper snowpack feels the strong sun and warming temps for the first time today. Really watch your exposure to overhead hazard, it would be easy to be surprised by weird avalanches today.

Summary

Confidence

Low - Uncertainty is due to the timing or intensity of solar radiation and its effect on the snowpack. Uncertainty is due to the fact that deep persistent slabs are particularly difficult to forecast.

Weather Forecast

Clear skies and warm sun for the next couple of days...

WEDNESDAY NIGHT: Freezing level at valley bottom, light southwest wind at lower elevations, moderate to strong northwest wind at ridge top, no precipitation expected.

THURSDAY: A few clouds, freezing level starting at valley bottom rising to about 1900 m, light southwest wind at lower elevations, strong northwest wind at ridge top, no precipitation expected.

FRIDAY: Clear skies, freezing level starting near valley bottom rising to about 2300 m, light to moderate southwest wind at lower elevations, strong southwest wind at ridge top, no precipitation expected. Freezing level holding around 1900 m Friday night.

SATURDAY: Broken cloud cover, freezing level lowering from 1900 m to 1500 m, light to moderate southwest wind, 4 to 8 cm of snow expected.

Avalanche Summary

We're short on post storm observations, but control work produced storm slabs to size 1.5 Monday and Tuesday that were most problematic near ridge crest. On Wednesday a small storm slab was reported that may have failed on surface hoar.

The most recent deep persistent slab activity we know about is described in this MIN report from February 11 and this MIN report on February 9th. This layer may become reactive again with warming temperatures and the appearance of the strong late winter sun.

Snowpack Summary

The 10 to 35 cm of snow from Sunday/Monday has been absolutely hammered by wind Wednesday as evidenced in this MIN submission. Wind was strong enough to get into below treeline features even. Wind stiffened slabs now rest on a variety of old surfaces including a firm crust on solar aspects, soft faceted snow, possibly surface hoar in sheltered areas, and old wind slabs near ridge crest. 

A thick rain crust sits 30-60 cm below the surface and can be found up to 2100 m. Avalanche activity was last observed at the faceting interface between this crust and overlying snow on February 17. 

The mid-pack is well settled and strong, but the base of the snowpack contains basal facets that are most prominent in shallow rocky start zones.

Terrain and Travel

  • Avalanche hazard is expected to to increase througout the day, think carefully about your egress.
  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
  • Recent wind has varied in direction so watch for wind slabs on all aspects.
  • The likelihood of deep persistent slab avalanches will increase with each day of warm weather.

Problems

Wind Slabs

An icon showing Wind Slabs

10 to 35 cm of storm snow was heavily affected by shifting wind Wednesday that even got into tree'd terrain. As temperatures warm and the sun comes out over the next few days human or potentially even natural avalanches become more likely. 

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1.5 - 2.5

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

Weak snow at the bottom of the snowpack hasn't gone away unfortunately. As temperatures warm and the strong sun comes out over the next few days we need to be thinking about the potential for large destructive avalanches. Cornice failures become more likely during these kinds of conditions and a failing cornice could be the perfect trigger for the deep persistent slab. Human triggering would be most likley around steep rocky terrain features, or anywhere the snowpack is thin and weak.  

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2.5 - 3.5

Valid until: Feb 27th, 2020 5:00PM