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Archived

Avalanche Forecast

Dec 19th, 2020–Dec 20th, 2020
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
4: High
The avalanche danger rating at treeline will be high
Below Treeline
4: High
The avalanche danger rating below treeline will be high
Alpine
5: Extreme
The avalanche danger rating in the alpine will be extreme
Treeline
5: Extreme
The avalanche danger rating at treeline will be extreme
Below Treeline
5: Extreme
The avalanche danger rating below treeline will be extreme

Regions: Glacier.

Natural triggering has slowed, but with strong winds forecast for this afternoon (and heavy snowfall tonight) another natural cycle will begin later today. Avalanche terrain that has not had recent activity should be avoided.

Weather Forecast

A brief respite from a weak ridge today, then a couple more frontal systems give moderate-heavy snowfall tonight and early next week.

Today: Sunny periods. Alpine High -7 °C. Ridge wind moderate (gusting strong) SW.

Tonight: Snow (20 cm). Low -8 °C. Strong (gusting Extreme) SW wind.

Sunday: Flurries (9 cm). High -6 °C. Mod-Extreme SW wind.

Snowpack Summary

20+ cm of new snow, strong SW winds and warm temps continue to build a sensitive slab at all elevations. Weak layers of concern are Dec 13 surface hoar/facets down 60cm+ and Dec 7 crust/surface hoar layer down 75cm+. The Nov 5 crust lingers near the base of the snowpack but has been unreactive recently.

Avalanche Summary

A widespread cycle of natural and artillery controlled avalanches up to size 3.5 occurred in the highway corridor (and likely in the backcountry) overnight.

There have been several reports of near misses from both skier triggered and natural avalanches in the park over the past few days (see MIN).

Confidence

Timing of incoming weather systems is uncertain

Avalanche Problems

Persistent Slabs

The slab continues to build over the Dec 13 surface hoar/facet layer, now down 60+ cm. This layer has been sensitive to human and natural triggering and is reactive at all three elevation bands.

  • Use conservative route selection, choose moderate angled and supported terrain with low consequence.
  • Avoid overhead exposure during periods of heavy loading from new snow and/or wind.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Likely - Very Likely

Expected Size: 1.5 - 3

Storm Slabs

Last nights heavy snowfall, strong SW winds and warm temps were the perfect recipe for building fresh reactive storm slabs.

  • If triggered the storm slabs may step down to deeper layers resulting in large avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Very Likely

Expected Size: 1 - 2.5

Persistent Slabs

70cm+ sits on the Dec 7th weak interface of rain crust/ surface hoar. If triggered the storm slab has potential to step down to this interface triggering large avalanches.

  • Avoid exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Possible - Likely

Expected Size: 2 - 4