Regions
Kootenay Boundary.
It's getting trickier as the recent storm snow settles into a cohesive slab and the mid-December surface hoar becomes active. With all this uncertainty conservative terrain selection remains prudent.
Weather Forecast
After a succession of snowstorms over southern BC, the weather over the province will settle into a dry cold period as 2017 comes to an end and 2018 begins. The culprit for the cold is our old friend the Arctic high that has been waiting patiently over northern BC for an opportunity to push south and envelope the entire province in cold, dry Arctic air. All indications point to the cold dry spell lasting until mid-week.SUNDAY: A few clouds, freezing level at valley bottom, moderate to strong northwest wind, no snow expected.MONDAY: A few clouds, freezing level at valley bottom, light to moderate west/northwest wind, no snow expected.TUESDAY: Clear skies at dawn, cloud cover increasing in the afternoon, freezing level at valley bottom, with an Above Freezing Layer (AFL) between 2000 m and 3000 m, light variable wind, no snow expected.
Avalanche Summary
On Saturday explosive control work produced what might be the first large avalanche failing on the mid-December interface. The size 2.5 avalanche ran on a 35 to 40 degree slope that was southeast through southwest facing at treeline. Control work produced numerous other storm slabs to size 2 on northwest, north, northeast, southeast, south and southwest aspects. Natural avalanches to size 2 were reported on north facing features at 2100 m. Last Tuesday a skier triggered a size 1.5 wind slab on a north-facing alpine slope in Kokanee Glacier Provincial Park. The skier went for a short ride, but was uninjured in the event.
Snowpack Summary
Two successive storms produced 25 to 50 cm of low density storm snow that was accompanied by moderate to strong winds out of the east, southeast, south and southwest. The new snow overlies 3 to 5 mm surface hoar, but we don't know much about the distribution of this weak layer yet.Between 60 and 100 cm below the surface you'll find the December 15th interface which consists of a melt-freeze crust on steep, solar, higher elevation slopes and well-developed surface hoar which seems most pronounced in sheltered terrain at and below treeline. The bond at this variable interface is of critical importance, especially in areas where the overlying slab is deep and well-consolidated.The lower snowpack is thought to be strong and well-settled.
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.