Avalanche Forecast

Issued: Mar 9th, 2020 5:00PM

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

Avalanche Canada astclair, Avalanche Canada

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Fresh snow and wind may build touchy slabs on Tuesday. Anticipate changing conditions and dial back terrain where more than 20 cm accumulates. A buried weak layer warrants a conservative mindset and terrain use strategy.

Summary

Confidence

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

Weather Forecast

Monday night: Mostly clear, moderate southwest winds, alpine temperature -12 C.

Tuesday: Cloudy, 10-20 cm of snow, moderate southwest winds gusting strong at ridge-tops, alpine high temperature -4 C, freezing level 1100 m.

Wednesday: Mostly cloudy, 5-15 cm of snow overnight, light northwest winds, alpine high temperature -5 C, freezing level 1100 m.

Thursday: Mostly cloudy, isolated flurries with trace accumulations, alpine high temperature -6 C, freezing level 1100 m.

Avalanche Summary

Reports of large (size 2-2.5) human-triggered and explosive-triggered avalanches releasing on the February 22 surface hoar layer continued throughout the weekend from operators region-wide. This layer continues to surprise people with remote-triggered activity, wide propagation, and a false sense of stability from unreactive ski cuts or from multiple people moving through the terrain before an avalanche releases. Observations have extended to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity. Avalanches have also occurred at unusually high elevations for surface hoar, with several cases of wind slabs stepping down in the alpine. While the number of reported avalanches has decreased, the size has increased, with more of the activity releasing size 2+.

New snow and wind on Tuesday may build reactive storm slabs and add a new load to cornices. Shallow storm slab avalanches or cornice falls have the potential to step-down to deeper layers and produce very large avalanches. 

Snowpack Summary

A storm moving into the region is expected to bring 10-20 cm of snow with strong southwest winds by Tuesday afternoon. The new snow is not expected to bond well with previous surfaces and may become increasingly touchy as snow accumulates. The new snow will likely develop into a slab sooner on lee features at upper elevations, where southwest winds are transporting the snow into deeper, more cohesive drifts.

A widespread weak layer of surface hoar is buried 50-90 cm deep. On solar aspects, this layer may sit over a crust. This persistent slab problem is transitioning into a low probability/high consequence scenario. The snow above the weak layer has increased in depth and slab properties, making avalanches more difficult to trigger and masking obvious clues that the problem is present (i.e. cracking, whumpfing). However, if triggered, avalanches will be large and getting caught could have serious consequences. Managing this problem requires a patient and diligent mindset, implemented by avoiding suspect slopes and maintaining conservative terrain margins. This persistent weak layer will likely pose the threat of a low probability/high consequence avalanche until there is a substantial change in the snowpack. Read more about surface hoar on our forecaster blog.

Terrain and Travel

  • Watch for changing conditions today, storm slabs may become increasingly reactive.
  • Dial back your terrain choices if you are seeing more than 20 cm of new snow.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.

Problems

Storm Slabs

An icon showing Storm Slabs

10-20 cm of snow is forecast with strong southwest winds. The new snow is not expected to bond well with previous surfaces, and storm slabs may become increasingly reactive to human triggering as snow accumulates. Slabs are likely to build sooner on lee features at upper elevations, where winds are transporting the snow into deeper, more cohesive drifts.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible - Likely

Expected Size

1 - 2

Persistent Slabs

An icon showing Persistent Slabs

A weak layer of surface hoar is buried 50-90 cm deep and is still possible to human-trigger. Over the past week, large avalanches have been observed extensively across the region. The distribution of this problem extends to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 3.5

Valid until: Mar 10th, 2020 4:00PM