Avalanche Forecast

Issued: Nov 30th, 2020 4:00PM

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 ghelgeson, Avalanche Canada

30 to 70 cm of snow fell across the region over the last few days with strong to extreme wind. Storm slabs may rest on buried weak layers. Storm slabs, especially those at upper elevations, likely remain sensitive to human triggering. 

Summary

Confidence

Low - Uncertainty is due to the limited number of field observations. Uncertainty is due to the extreme variability of wind effect on the snowpack. Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

The freezing level is expected to rise dramatically Tuesday evening.

MONDAY NIGHT: Freezing level around 500 m, moderate to strong west/southwest wind, 2 to 8 cm of snow expected. 

TUESDAY: Overcast, freezing level climbing, potentially as high as 1400 m in the afternoon, strong to extreme southwest wind, 4 to 8 cm of snow possible. The freezing level is expected to climb to about 2500 m Tuesday night.

WEDNESDAY: A few clouds in the AM with cloud cover increasing throughout the day, freezing level holding at 2500 m, strong to extreme south wind, no precipitation expected.

THURSDAY: Overcast, freezing level between 700 and 1200 m, moderate to strong southwest wind, 5 to 10 cm of snow.  

Avalanche Summary

Observations are very limited at this time. 30 to 70 cm of snow fell over the last few days with more than enough wind to form stiff storm slabs in wind exposed terrain. There was almost certainly a natural avalanche cycle during the storm, but we don't have any information about that cycle. In areas where there is enough snow to ride, there is enough snow to slide. Observations like this from Shames show us that avalanches in the trees are possible at this time.

There have been repots of very large size 2.5 to size 3.5 explosive triggered avalanches on the west central portion of the coast, those avalanches are running on the November 3rd crust. We will continue to monitor this crust as the season progresses.

Snowpack Summary

The storm over the last few days produced 30 to 70 cm of snow above 500 m. Areas to the north and west of the region got the bulk of the snowfall.  

The new snow may be resting on surface hoar and facets which could set up a scenario where storm slabs are surprisingly touchy.

Strong to extreme winds generally out of the south/southwest have formed stiffer storm slabs in open features at upper elevations.

We're learning that the early November crust is widespread, even in the alpine and treeline elevations. You should be able to find it 25 to 50 cm above the ground. 

The total snowpack depth is roughly 150 to 200 cm at upper treeline and alpine elevations. The bottom of the snowpack consists of several thick crusts. There could be weak snow developing around these crusts in colder inland parts of the region.

Terrain and Travel

  • Avoid freshly wind loaded terrain features.
  • Be alert to conditions that change with elevation and wind exposure.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.
  • Caution around slopes that are exposed to cornices overhead.

Problems

Storm Slabs

An icon showing Storm Slabs

30 to 70 cm of storm snow fell across the region over the last few days. Careful with the new storm snow, especially at upper elevations. Large human triggered storm slabs remain likely. The new snow may be resting on surface hoar and facets which could set up a scenario where storm slabs remain surprisingly touchy.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1.5 - 2.5

Persistent Slabs

An icon showing Persistent Slabs

A buried rain crust is widespread in the region. The snowfall from this storm could be enough to wake up this deeper instability, especially in the central and northern portions of the region.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2 - 3.5

Valid until: Dec 1st, 2020 4:00PM