Fresh wind slabs are expected to form throughout the day as northwesterly winds redistribute the surface snow into lee areas in the alpine and treeline.
Concern for buried weak layers continues to demand conservative terrain selection and good travel habits.
Northwesterly winds are expected to form fresh, reactive wind slabs in the alpine and treeline. Watch for pockets of wind slab on the lee side of ridges or ribs.
Wind slabs have the potential to step down to the mid-November weakness in the middle of the snowpack, resulting in large avalanches.
Last week's storm buried a weak layer of surface hoar, which has been the culprit for several skier-triggered avalanches in the past week. This layer is now buried 40-70 cm down and is most likely to exist in sheltered and shaded locations.
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 this week. 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 Squamish 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 Wednesday, we expect fresh, reactive wind slabs to form as northwest winds redistribute the surface snow into lee areas. Keep in mind that small avalanches have the potential to 'step down' to deeply buried weak layers resulting in larger than expected avalanches.
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 30 to 50 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 and warm temperatures.
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.
Tuesday night
Clear periods. Alpine temperatures drop to a low of -5 °C. Ridge wind light to 25 km/h from the northwest. Freezing level at valley bottom.
Wednesday
Sunny with cloudy periods. Alpine temperatures reach a high of -2 °C. Ridge wind 25km/h gusting to 40 km/h. Freezing level at valley bottom.
ThursdaySunny with cloudy periods. Alpine temperatures reach a high of 2 °C. Ridge wind 30 km/h gusting to 55 km/h. Freezing level rises to 1900 metres.
FridaySunny. Alpine temperatures reach a high of 3 °C. Mostly light ridge wind occasionally gusting to 45 km/h. Freezing level rises to 2100 metres.
More details can be found in the Mountain Weather Forecast.
