Avalanche Forecast
Issued: Dec 28th, 2020 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
jfloyer,
Avalanche Canada
A close call occurred on Rainbow Mountain on Monday when an avalanche on a persistent weak layer resulted in a full burial. Remain conservative in your terrain choices to account for significant variability from one slope to another.
Summary
Confidence
Weather Forecast
Monday night: Dry with clear skies. Cooling temperatures.
Tuesday: 5-10 cm new snow. Freezing level around 600 m. Winds light to moderate southwesterly.
Wednesday: 15-20 cm new snow. Freezing level around 800 m. Winds moderate southerly.
Thursday: Flurries. Freezing level around 900 m. Winds light southwesterly.
Avalanche Summary
A large (size 2.5) avalanche was triggered on Monday by a snowmobiler on Rainbow mountain in alpine terrain, which resulted in a full burial and, thankfully, a successful rescue. The avalanche failed on a layer 60 cm deep. At this time the nature of the failure plane is unknown, but it is reasonable to assume it failed on one of the two weak layers discussed in the snowpack section. This avalanche points to the ongoing potential to trigger persistent weak layers in the snowpack.
A small (size 1) avalanche was also triggered on Monday by skiers on a north aspect in the Whistler backcountry, most likely in recent storm snow.
Snowpack Summary
15-30 cm of recent snow has been redistributed by strong south to east winds, depositing deep pockets of storm slab into lee terrain features.
Two potential concerning weak layers may be present in the snowpack:
- The shallower layer, around 60 to 100 cm deep, includes feathery surface hoar crystals. This layer may be found in sheltered terrain features at treeline and lower alpine elevations but has been reported as spotty across the region.
- The more widespread weak layer includes sugary faceted grains that overlies a hard melt-freeze crust. This layer ranges in depth from 10 cm to 120 cm due to the wind scouring and loading the snow in different terrain features over the past two weeks. The layer has been reported as being widespread up to around 2000 to 2200 m.
To date we have mostly seen sporadic avalanche activity on these layers, but they remain possible to trigger where they exist in the mountains.
The remainder of the snowpack is well-settled.
Terrain and Travel
- Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
- Conditions may have improved, but be mindful that deep instabilities are still present.
- Surface hoar distribution is highly variable. Avoid generalizing your observations.
Problems
Persistent Slabs
A couple problematic layers may be found around 50 to 120 cm deep, including weak layers of surface hoar and faceted grains above a hard melt-freeze crust. There remains potential for storm slab avalanches to step down to these layers, resulting in large and destructive avalanches.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Wind Slabs
Winds have shifted from southwesterly to northerly. Watch for reverse-loading on south-facing slopes in areas where winds are sufficiently strong to move snow around.
Aspects: North, North East, East, South East, South, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 29th, 2020 4:00PM
