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Avalanche Forecast

Mar 10th, 2020–Mar 11th, 2020
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Purcells.

Between fresh wind slabs and a reactive persistent weak layer, human triggered avalanches are likely. Keep in mind it's not always the first rider who triggers the slab and keep terrain choices conservative on Wednesday.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Tuesday night: Mostly cloudy, 15-20 cm of snow, strong westerly wind, alpine high temperature -5 C, freezing level 1300 m.

Wednesday: Mostly cloudy, scattered flurries with up to 5 cm of snow accumulation, moderate northwest wind, alpine high temperature -10 C, freezing level 1000 m. 

Thursday: Mix of sun and cloud with isolated flurries, light westerly wind, alpine high temperature -10 C, freezing level 1000 m.  

Friday: Mostly cloudy, scattered flurries with up to 5 cm of snow accumulation, moderate easterly wind, alpine high temperature -10 C, freezing level at valley bottom.

Avalanche Summary

Over the weekend, a natural storm slab avalanche cycle was observed. These avalanches were small to large (size 1-2.5) and breaking 20-40 cm deep. Shallow slabs in the recent snow have the potential to step down to the buried surface hoar layer, creating very large avalanches. 

Over the past week, human-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 been remotely-triggered and have propagated widely across terrain features, warranting conservative terrain margins (see this MIN or 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. Human triggering remains possible where the wind has transported the snow into deeper drifts. Incremental snowfall and wind forecast for Tuesday night is expected to add to this wind slab problem.

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. 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 remain a problem 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.
  • 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.

Avalanche Problems

Wind Slabs

15-20 cm of snow by Wednesday morning combined with strong westerly wind is forming 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.

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

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 2

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 avalanches have released on this layer. As more areas approach critical loading from additional snow and wind, human-triggering remains likely. 

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Possible - Likely

Expected Size: 1.5 - 2.5

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