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Avalanche Forecast

Archived

Feb 17th, 2020–Feb 19th, 2020

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

Regions

North Rockies.

60 - 120 cm of storm snow in the last week and surprisingly little avalanche activity, the sun may change that when it makes it's full appearance on Wednesday. Fat wind loaded features in wind exposed terrain remain suspect, and buried surface hoar at treeline is also a concern.

Confidence

Low - Uncertainty is due to the limited number of field observations. Uncertainty is due to the timing or intensity of solar radiation and its effect on the snowpack. Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather.

Weather Forecast

The forecast period is filled with plenty of sun and wind for the foreseeable future.

MONDAY NIGHT: Light to moderate northwest wind, freezing level at valley bottom, no significant precipitation expected.

TUESDAY: Scattered cloud cover at dawn clearing to just a few clouds in the afternoon, moderate northwest wind, freezing level at valley bottom, no significant precipitation expected.

WEDNESDAY: Clear skies, moderate southwest wind, freezing level rising to about 1300 m, no significant precipitation expected.

THURSDAY: Clear skies at dawn with some cloud building in the afternoon, strong southwest wind, freezing level rising to about 1300 m, no significant precipitation expected.

Avalanche Summary

Surprisingly little avalanche activity has been reported. On Sunday explosive control work produced small avalanches running in the upper 20 cm of storm snow.

Snowpack Summary

The weekend storm added 10 to 25 cm of snow to the phenomenal snowfall last week where the wed/thur/fri storm produced 50 (Kakwa) to 100 (Torpy) cm of storm snow under mild temperatures and moderate wind from the south, southwest and southeast. Most of the observations we're seeing suggest that the copious amount of storm snow arrived warm and is settling well in the cooler recent temperatures. The storm snow reportedly has very little slab property, but we're short on observations from more wind effected terrain. This MIN from the Renshaw on Saturday really tells the story.

Last week easy sudden planar shears and widespread storm slabs were observed. The presence of buried surface hoar about 40 to 60 cm below the surface has been confirmed in the Pine Pass, Torpy & Renshaw zones, it may be more widespread throughout the region too. 

Below all the recent storm snow there is a thin rain crust from the warm, wet storm on Feb 01. This rain crust has been reported to exist up to treeline elevations around 1700 m. Initially this layer was described as reactive and was the focus of a number of avalanches but no recent activity involving this crust has been reported.

Deep in the the mid-pack there may still be a layer of surface hoar buried in late December. It's gaining strength, but should remain a concern since snowpack tests still demonstrate this layer has the potential to slide, albeit in isolated areas and/or with very large triggers.

Terrain and Travel

  • Avoid exposure to steep, sun exposed slopes, especially when the solar radiation is strong.
  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Cornice failure may trigger large avalanches.
  • Be aware of the potential for remote triggering and large, deep avalanches due to the presence of buried surface hoar.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.