Avalanche Forecast

Issued: Mar 8th, 2020 5:00PM

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

Avalanche Canada astclair, Avalanche Canada

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Intense sun may make recent snow and cornices more sensitive to failure. Adjust travel to give steep, sun-exposed slopes and cornices a wide berth. A buried weak layer warrants a conservative mindset and terrain use strategy.

Summary

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Sunday night: Mostly clear, light west winds, alpine temperature -14 C.

Monday: Mostly clear, light west wind, alpine high temperature -5 C, freezing level 1000 m. 

Tuesday: Increasing cloud, isolated flurries with trace accumulations, moderate southwest winds with strong gusts, alpine high temperature -4 C, freezing level 1400 m.

Wednesday: Mostly cloudy, scattered flurries with up to 5 cm of snow, decreasing west winds, alpine high temperature -4 C, freezing level 1400 m. 

Avalanche Summary

On Saturday, several human-triggered and explosive-triggered storm slab avalanches were reported. These avalanches were small to large (size 1-2) and breaking 25-40 cm deep. Shallow storm slabs have the potential to step down to the buried surface hoar layer, creating very large avalanches. Use added caution on sun-exposed slopes during intense solar radiation, as rapidly warming snow and weakening cornices could trigger loose or slab avalanches.

Over the past week, human-triggered and remotely-triggered avalanches releasing on the February 22 surface hoar have been consistently reported. In many instances, slopes were ski cut with no results, only to have the third or fourth skier in the group trigger the slab. These avalanches have propagated widely across terrain features, warranting conservative terrain margins (see this MIN for a helpful example). Observations extend to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity. 

On Thursday, professionals reported large (size 2.5) explosive-triggered avalanches releasing on weak layers near the ground. These avalanches occurred on north and east facing slopes above 2300 m, and they confirm that the deep persistent slab problem remains a concern. 

Snowpack Summary

Snowfall amounts from the weekend storm ranged from 20-40 cm. Slab formation and human triggering will likely be specific to where the wind has transported the snow into deeper drifts. Intense solar radiation may rapidly destabilize recent snow on steep, sun-exposed slopes and weaken cornices.

A weak layer of surface hoar from February 22 may be found 35-70 cm deep. It may sit over a crust on solar aspects. With successive storms, more areas have reached critical loading and developed a slab over this layer. Riders will be capable of triggering this layer for some time until there is a substantial change in the snowpack. Read more about surface hoar on our forecaster blog.

Deep basal facets lurk near the bottom of the snowpack. This layer has been responsible for sporadic deep persistent slab avalanches, usually triggered from shallow, rocky start zones.

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Use conservative route selection and resist venturing out into complex terrain.
  • Minimize exposure to sun-exposed slopes when the solar radiation is strong.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

Problems

Persistent Slabs

An icon showing Persistent Slabs

A weak layer of surface hoar sits 35-70 cm deep, within the prime range for human-triggering. Over the past several days, large human-triggered and remotely-triggered avalanches have released on this layer. As more areas approach critical loading with settlement in the recent snow, human-triggered large avalanche remains likely. 

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 2.5

Wind Slabs

An icon showing Wind Slabs

Snowfall with wind has formed wind slabs in the alpine and open areas at treeline.This problem overlaps with where cornices may be reaching their breaking point. If triggered, wind slabs and cornices could step down to the buried surface hoar layer, forming large avalanches. Periods of intense sun may make these problems more susceptible to failure.

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

Weak snow at the bottom of the snowpack continues to pose a deep persistent slab avalanche problem. Treat wind-scoured, rocky, shallow terrain as suspect and avoid big alpine slopes that are threatened by cornices overhead. 

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 3.5

Valid until: Mar 9th, 2020 4:00PM