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Avalanche Forecast

Archived

Nov 27th, 2020–Nov 28th, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Northwest Coastal.

Avalanches are likely at higher elevations where it has remained cold and windy. Hazard will increase when heavy snowfall and strong wind starts on Saturday afternoon.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength. Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

The next storm arrives on Saturday afternoon and continues until Monday morning.

FRIDAY NIGHT: Partly cloud, light wind from the west, alpine temperatures drop to -10 C.

SATURDAY: Storm arrives in the afternoon with up to 5 cm of snow near the coast by 4 pm, wind increases to 60-90 km/h from the south with arrival of the front, freezing level climbs to 1000 m.

SUNDAY: Steady precipitation throughout the day with 15-40 cm above 1000 m (and rain below), 60-90 km/h wind from the south, freezing level around 1000 m.

MONDAY: Intense snow overnight before the front leaves midday leaving another 30-50 cm of snow, 40-60 km/h wind from the west, freezing level drops to 500 m, alpine temperatures around -4 C.

Avalanche Summary

The main concern over the weekend is a developing storm slab problem when new snow accumulates. However, there is still potential for old storm snow to be a concern on Saturday, especially at higher elevations where it has stayed cold and windy. There have been a few reports of size 1-2 naturally triggered storm slab avalanches during the stormy weather over the past few days (including ones like this from Shames).

On Tuesday, a large (size 2.5) avalanche was triggered with explosives in the northern part of the region. This avalanche failed on weak snow around the crust at the bottom of the snowpack, which is a layer worth monitoring in parts of the region as the season progresses.

Snowpack Summary

Heavy precipitation and strong wind over the weekend will create fresh storm and wind slabs at higher elevations.

30 to 50 cm of snow from the past week is settling and strengthening, while a rain crust on the surface extends up to roughly 1000 m. As new snow starts to accumulate on Saturday keep in mind the snow from the past few days could still be unstable, especially on freshly wind loaded slopes. The total snowpack depth is roughly 100 to 150 cm at upper treeline and alpine elevations, and should grow much deeper by Monday. 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 areas where the snow feels stiff and/or slabby.
  • Pay attention to the wind, once it starts to blow fresh sensitive wind slabs are likely to form.
  • Be alert to conditions that change with elevation.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.