Riders may be drawn into sheltered areas at treeline to avoid wind slabs at upper elevations. Large surface hoar has been reported below this weekend's storm snow. The potential for a temperature inversion and sun on Tuesday means 40-50 cm of recent storm snow could settle into a cohesive slab above this weak layer. A recipe for avalanches!
All this above a weak and facetted snowpack... keep your terrain choices conservative and be alert to changing conditions throughout the day.
Strong southerly winds during last week's storm have created deep deposits of wind slab near ridges and peaks.
Wind slabs have the potential to step down to the mid-November weakness in the middle of the snowpack, resulting in large avalanches.
It is possible that warm temperatures on Tuesday could settle the 40-50 cm of storm snow into slabs in areas where the storm snow was previously loose and powdery. Watch for changing conditions throughout the day and dial back your exposure to avalanche terrain if the sun or temperature starts to affect the upper snowpack.
Soft facets on a crust about halfway down into the snowpack may become more sensitive to triggering with the additional load from last week's storm and rising temperatures throughout the day on Tuesday. Small avalanches or large cornice falls could step down to this layer resulting in large, consequential avalanches.
During the storm on Friday, a natural avalanche cycle occurred, with storm slab avalanches up to size 2 observed in the alpine and treeline. Numerous skier-triggered storm slabs were reported in this time period as well. Notably, some explosive control produced persistent slab avalanches that failed on the mid-Nov crust down about 50cm and propagated around 50m wide.
Over the weekend, natural activity began to taper but human triggering remained likely! Many riders in the Sky Pilot and Whistler area got surprised by size 1-2 slab avalanches. These slabs mainly occurred on wind-loaded slopes on north and west aspects in the alpine and treeline. Check out these MIN reports for examples (MIN1) (MIN2).
Looking forward to Tuesday, we suspect human-triggered avalanches will remain possible. The recently formed wind slabs overlie a weak layer of surface hoar or facets in some areas, which means that reactivity may persist for longer than normal. Be particularly cautious if rising temperatures or strong solar radiation start to form a denser, more cohesive slab in the upper snowpack.
Thank you for sharing your observations over the weekend! Please keep posting to the MIN; it helps strengthen our information gathering.
Surface: Large surface hoar growth has been observed in sheltered areas. Around 20 to 30 cm of recent snow, but also at or near the surface you can find wind-scoured, wind-pressed, and of course wind slabs. In wind-sheltered deep snow areas, there could be as much as 70 cm of storm snow accumulations.Upper-pack: The upper snowpack consists of 20-50 cm of light but settling snow. 50 cm down large surface hoar (up to 10 mm) has been observed mainly in shaded or sheltered areas. Mid-pack: 40-150 cm down is a late November crust, with soft facets above. This is likely the critical snowpack weakness. Professionals are concerned about the possibility of avalanches releasing at this deeper interface with the new snow load. So is Sofia (thanks for the MIN); I especially like her snow profile photo!
Total snow depths remain low for early December with 90 to 140 cm at treeline and up to 200 cm in the alpine. Much of the below treeline elevation band is below the threshold for avalanches.
Monday night
Clear with cloudy periods. A strong alpine temperature inversion. Alpine temperatures drop to a low of -4 °C. Ridge wind light to 15 km/h from the north. Freezing level at valley bottom.
Tuesday
A mix of sun and cloud. Alpine temperature inversion. Alpine temperatures reach a high of 0 °C. Ridge wind light to 20 km/h from the northwest. Freezing level rises to 2400 metres.
WednesdaySunny with cloudy periods. Alpine temperatures reach a high of 0 °C. Mostly light ridge wind occasionally gusting to 45 km/h from the northwest. Freezing level rises to 1700 metres.
ThursdaySunny with cloudy periods. Alpine temperatures reach a high of 1 °C. Light to moderate ridge wind occasionally gusting to 50 km/h from the north. Freezing level rises to 1700 metres.
More details can be found in the Mountain Weather Forecast.
