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Archived

Avalanche Forecast

Dec 30th, 2019–Dec 31st, 2019
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
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
3: Considerable
The avalanche danger rating below treeline will be considerable
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

Regions: Sea To Sky.

The incoming storm will create dangerous avalanche conditions. New snow will form touchy storm slabs while testing the strength of deeply buried weak layers. Step-down avalanches are a distinct possibility. Avoid avalanche terrain and minimize overhead exposure on Tuesday.

Confidence

High - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather.

Weather Forecast

Monday night: Cloudy, 10-20 cm of snow, moderate southwest winds, alpine temperature near -2 C with freezing levels near 1200 m.

Tuesday: Cloudy, 30-40 cm of snow, moderate southwest winds gusting strong at ridge top, alpine high temperatures near-2 C with freezing level rising to 1600 m overnight.

Wednesday: Cloudy, 10-15 cm of snow, light southwest wind, alpine high temperatures near -1 C with freezing level near 1300 m.

Thursday: Mostly cloudy, 2-4 cm of snow, light southwest winds, alpine high temperature near -6 C with freezing level dropping below 500 m.

Avalanche Summary

With the incoming storm, a natural avalanche cycle is expected on Tuesday.

Avalanche activity has diminished over the past few days. A few small avalanches, both human and explosive-triggered, were reported over the weekend in the recent storm snow. These avalanches released on leeward aspects (north to northwest facing slopes) in the alpine, and one stepped down to the mid-November weak layer.

Reports from last week captured widespread large to very large (size 2-3) natural, human, and explosive-triggered persistent slab avalanches. Many of these avalanches either failed on the mid-November weak layer or stepped down to it, even scouring the lower snowpack away to reveal ground. A few of these avalanches were remotely triggered. See here for some photos of one of them.

Snowpack Summary

The incoming storm is expected to rapidly add a new layer to our snowpack. The new snow will fall on 15-25 cm of snow from over the weekend overlying a weak interface. At higher elevations, winds from the southwest drifted this snow into slabs on leeward features. Elsewhere this recent snow remains unconsolidated. The interface below it may present as surface hoar in many sheltered locations.

The upper snowpack consists of around 70 to 120 cm of snow that overlies a variable weak layer of surface hoar and a crust from mid-December (down 70-90 cm), as well as a deeper weak layer of sugary faceted grains and a crust buried in late-November (down 100-200 cm). Both of these persistent weak layers produced many large and destructive avalanches during and in the days after last week's storm. Snowpack tests continue to produce sudden and propagating results on these layers (like this MIN report from Disease Ridge on Sunday). This fundamentally unstable snowpack structure remains a serious concern with the incoming storm.

Terrain and Travel

  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Be aware of the potential for large avalanches due to the presence of buried weak layers.

Avalanche Problems

Storm Slabs

Forecast snowfall will add a new storm slab problem and will cover another recent surface hoar layer buried late last week. New snow is expected to become increasingly easy to trigger as its depth increases. Slab problems will become widespread, with wind-loaded areas seeing more rapid accumulation and slab formation and sheltered areas holding more pronounced weak layers.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Very Likely

Expected Size: 1 - 2

Persistent Slabs

Persistent weak layers that have been slowly healing will be tested by rapid loading from forecast snowfall and wind. Shallower but more touchy storm slab releases carry the risk of triggering one of these deeper weak layers to create larger, more destructive avalanches.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Possible - Likely

Expected Size: 2 - 3.5