Avalanche Forecast

Issued: Feb 20th, 2021 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 Persistent Slabs.

Avalanche Canada ahanna, Avalanche Canada

As a warm front brings rising temperatures, bits of new snow and plenty of wind to the region, watch for fresh wind slabs forming Sunday. These new wind slabs will be sitting on old, faceted snow, which may make them surprisingly sensitive to triggering.

Summary

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

A warm front will bring rising temperatures and wind on Sunday, with the bulk of precipitation forecast to fall on Monday.

SATURDAY NIGHT: Mostly cloudy, up to 5 cm of new snow, westerly wind increasing to moderate, freezing level valley bottom.

SUNDAY: Flurries up to 5 cm becoming wetter as freezing levels rise to 1500 m. Moderate west wind at treeline with strong to extreme west wind in the alpine.

MONDAY: 5 to 10 cm of snow at upper elevations with rain in the valleys, freezing level holding at 1500 m. Moderate southwest wind at treeline with strong west to extreme west wind in the alpine.

TUESDAY: Mix of sun and cloud, wind easing and switching northwest, freezing level 1000 m.

Avalanche Summary

There have been a few reports of small natural wind slabs and loose dry avalanches running in the facets this week. Cornices remain large and fragile but there have been no reports of cornice falls since last weekend.

Earlier in February, there were a few reports of large avalanches (size 2.5-3) on southeast aspects in the alpine, likely releasing on a layer of facets on a crust (see a photo here), and a large human-triggered avalanche (size 2.5) on surface hoar near Quartz Creek.

Snowpack Summary

Winds have varied in strength and direction over the past week, loading cornices and forming shallow wind slabs in complicated cross-loading and reverse-loading patterns at upper elevations. These wind slabs may overlie weak, sugary, faceted snow, meaning that they could break wider than expected and will likely be slow to bond. 

In wind-sheltered areas, the upper snowpack is becoming soft and faceted with the cold temperatures and the riding looks pretty good. A weak layer that formed in late January is now 30-60 cm deep. In the northern Purcells this layer has been reported as a surface hoar layer at treeline and below treeline elevations, but a combination of facets on crusts could exist at all elevations throughout the region. 

The northern Purcells also have an older surface hoar layer that can still be found 60-100 cm deep at treeline. Additional weak layers may exist near the bottom of the snowpack and steep rocky areas with a shallow or thin to thick snowpack should be considered suspect trigger points.

Terrain and Travel

  • Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
  • Be aware of the potential for large avalanches due to the presence of buried persistent weak layers.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

Problems

Wind Slabs

An icon showing Wind Slabs

New snow, rising temperatures and strong westerly wind will likely form fresh, thin wind slabs ontop of weak facets and surface hoar. This combination could make them surprisingly sensitive to human triggering. Cornices may be reaching their breaking point and can act as triggers on slopes below.

Aspects: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood

Likely - Very Likely

Expected Size

1 - 1.5

Persistent Slabs

An icon showing Persistent Slabs

There is a lingering possibility of triggering persistent slab avalanches in isolated steep terrain features. The primary layer of concern is layer of surface hoar, facets, and/or crusts buried 30-60 cm deep throughout the region.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely - Possible

Expected Size

1.5 - 2

Valid until: Feb 21st, 2021 4:00PM