The storm is expected to bring strong winds and rapidly rising freezing levels. Expect avalanche danger to increase as the storm develops. If you see more than 25cm of new snow on Tuesday, expect your local avalanche danger to be HIGH.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
The storm system is expected to bring 15-20mm of precipitation to the South Columbia region on Tuesday. Winds are expected to increase to strong from the SW and freezing levels will climb as high as 2100m by Tuesday afternoon. On Wednesday and Thursday, freezing levels should remain around 2000m and winds will remain moderate or strong in the alpine. There is currently some uncertainty regarding precipitation amounts for Wednesday and Thursday with models showing another 10-40mm possible.
Avalanche Summary
Reports of natural storm slab activity up to size 2.5 in the northern Monashees on Saturday night during the last storm system. These were typically releasing down about 30cm and occurring above 2100m on N-SE slopes. I expect these slabs were mainly releasing on a layer of surface hoar and wind loading may have contributed. In other parts of the region, operators are reporting more isolated problems including ski cutting size 1 slabs in lee features and sluffing from steep features.
Snowpack Summary
15-20cm of new snow sits on a weak layer of surface hoar and/or a sun crust. In windier places, this new snow has likely formed into wind slabs in leeward features. Below around 1800m elevation and down around 30-40cm is a rain crust but it appears that the snow above it is generally well bonded. About 1m down, a weak layer of surface hoar and/or facets can be found in some locations. A thick rain crust with facets from early November is buried over 1 m down. Snowpack tests on these deep weak layers are showing slowly improving results, but in some locations these layers are still reactive and have the potential to release large slab avalanches. We may see these deep weak layers become a problem again with the upcoming storm system.
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.
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.