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Avalanche Forecast

Archived

Jan 17th, 2020–Jan 18th, 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 unlikely, human triggered possible.
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.

Regions

South Coast Inland.

Forecast snowfall for Saturday's storm varies across the region, and it's not a clear cut split between north and south. Expect avalanche danger to increase over the day as new snow accumulates and is affected by wind.

Confidence

Low - Forecast snowfall amounts are uncertain.

Weather Forecast

Friday night: Cloudy with increasing flurries bringing about 5 cm of new snow by morning. Light to moderate south or southwest winds, increasing.

Saturday: Cloudy with flurries or snow showers bringing a variable 10-25 cm of new snow, easing overnight in most areas. Moderate to strong south winds. Alpine high temperatures around -5, closer to -3 in the south of the region.

Sunday: A mix of sun and cloud with isolated flurries and a trace of new snow, increasing overnight. Light to moderate south winds. Alpine high temperatures around -3, closer to 0 in the south of the region.

Monday: Cloudy with scattered flurries bringing a trace to 5 cm of new snow. Light south winds. Alpine temperatures around -3, closer to 0 in the south of the region.

Avalanche Summary

A report from Thursday in the McGillivray pass area in the north of the region included observations of several storm slabs reaching size 2.5 (large) releasing naturally from steep north aspects in the alpine during a period of intense snowfall.

Reports from the region have been limited during the recent cold snap, however a MIN from Tuesday describes winds actively forming new slabs in the Duffey area. Similar conditions were observed in the Coquihalla area:

Last Saturday, a few explosives-triggered avalanches in the north of the region stepped down to the Christmas surface hoar layer mentioned in our snowpack summary, buried 50-60 cm deep at the time.

Two Thursdays ago, explosive control work produced a size 2 avalanche off the Duffey Lake road that released on the mid November facet/crust layer.

Snowpack Summary

A variable 15-30 cm of new snow is expected to accumulate over the region by the end of the day on Saturday. This will bury recent wind slabs and wind-affected surfaces in exposed areas at all elevations and soft, low density snow in sheltered areas.

90-150 cm of snow currently sits above a weak layer of surface hoar from Christmas that professionals are tracking in the north of the region. It may be found at treeline and below on shaded aspects in sheltered areas. 

A deep persistent weak layer remains a serious concern at the bottom of the snowpack in the northern half of the region (e.g., Duffey, Hurley). It's made up of sugary faceted grains and a hard melt-freeze crust. This layer is most likely to be triggered in thin, rocky features where it exists closer to triggering forces on the surface.

Concern for very large avalanches releasing over this weak basal layer will increase in areas of the north where loading from new snow, wind, and rising temperatures strain the snowpack and cause avalanche activity in surface layers.

There are currently no concerns about deep weaknesses in the snowpack near Coquihalla summit and elsewhere in the south of the region.

Terrain and Travel

  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
  • Be alert to conditions that change with elevation and wind exposure.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.