Regions
Northwest Coastal.
Change is coming! Expect stormy weather brewing in the Pacific to move onto the North Coast. Persistent slab avalanches may be tricky to predict with new loading.
Weather Forecast
Cloud developing overnight as the next system moves down the northwest coast. Expect moderate to strong southwest winds combined with 5-10 mm of precipitation. Heavier precipitation amounts are possible close to the coast. Freezing levels should drop to valley bottoms overnight and rise up to about 400 metres on Sunday. A second wave of moisture is forecast for Sunday night or early Monday morning that should bring 10-15 mm of precipitation and strong westerly winds with freezing levels around 700 metres. There is a big system approaching the coast on Tuesday that may bring 20-30 mm of precipitation combined with strong to extreme winds and freezing levels rising up to at least 1200 metres.
Avalanche Summary
Skiers in the Shames area reported natural and skier accidental avalanche activity up to size 1.5, and skier controlled up to size 2.0 on Friday. These avalanches are suspected to have released on a buried layer of decomposing surface hoar that is down about 50 cm in the alpine. There is only 10-15 cm of snow at 700 metres elevation, that quickly becomes a metre on the ground by 1000 metres elevation. This persistent weak layer is expected to continue to be reactive for the duration of this forecast. Other areas like Sterling, Wesach/ South Douglas are expected to have the same problem with buried surface hoar. Please submit your observations to the Mountain Information Network.
Snowpack Summary
Recent snowpack information has been limited to reports from the Shames backcountry. Some earlier information came from Wesach that suggested the buried surface hoar problem exists there as well. The surface hoar in the Shames area was buried in mid-November and is now down about 30-60 cm. This weak layer has been reactive to light loads like individual skiers and riders. Shaded slopes in the alpine are the most likely spots to encounter this problem. Steep roll-overs and convex slopes tend to be more reactive to persistent weak layers like buried surface hoar. Areas that have released naturally or from skier traffic may be the safest spots after the next storm moves in early next week. Terrain that has not released may continue to be a problem for the duration of the forecast period. There is very little snow in the north of the region at Ningunsaw and Cassiar. Snow that is on the ground in these areas may be facetting and developing a weak basal layer. Due to a lack of observations, there is a lot of uncertainty surrounding the distribution and persistence of these layers and it is best to dig down and test these layers before committing to big slopes.
Problems
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.