The storm is over and the new storm slab may take several days to settle and bond to the old surface. The mid-December buried surface hoar crust combination is expected to continue to be a tricky problem to solve.
Weather Forecast
Warm air that has been trapped in the alpine should move out and let the alpine cool down to about -3 by Thursday morning. The freezing level should also drop down to valley bottoms overnight, and stay there throughout the forecast period. Moderate Northwest winds overnight should become light Northerly by Thursday morning. Light winds, cooling temperatures, and clearing skies are expected during the day Thursday. Mostly overcast with light Southerly winds on Friday. Flurries beginning Friday evening and becoming light snow by Saturday morning. Saturday should be overcast with flurries or light snow combined with moderate Westerly winds.
Avalanche Summary
Widespread natural avalanches reported up to size 3.0 on Tuesday. Explosives controlled avalanches up to size 3.0 on Wednesday. I suspect that natural avalanche activity ended on Wednesday and that human triggered avalanches continue to be likely-very likely. New storm slab avalanche activity is expected to become less likely, but persistent slab activity may continue for some time.
Snowpack Summary
There has been about 60 cm of new snow in the Monashees, and about 50 cm in the Selkirks in the past 48 hours. On Tuesday warm (above freezing) air moved into the alpine and freezing levels climbed up to about 1800 metres. I suspect that the warm temperatures caused rapid settlement in the dry storm slab resulting in a higher likelihood of triggering. Rain or freezing rain have created a moist slab at treeline in the Monashees. The Selkirks may be a bit cooler and drier; alpine temperatures around Revelstoke were -2.0 on Tuesday afternoon. The new storm slab is 60-80 cm thick and is sitting on a mix of old surfaces including patchy surface hoar and old windslabs on most aspects. Deeper down (around 100 cm) the mid-December surface hoar/crust layer continues to allow for remote triggering and long fracture propagations.
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.