Regions
Kootenay Boundary.
30 to 60 cm of snow is slowly coalescing into a slab above the mid-February persistent weak layer that has recently been sensitive to human triggering. Watch for the formation of fresh wind slabs at upper elevations too.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain on Wednesday
Weather Forecast
A weak upper trough Tuesday should deliver a small shot of precipitation and wind before a vigorous low makes landfall on Wednesday. This system may push into the Kootenay Boundary Wednesday evening. Unfortunately the weather models cannot seem to agree on a solution with regard to track and intensity but there will likely be less uncertainty as we get closer to Wednesday. Stay tuned for more details. TUESDAY: Broken cloud cover, freezing level beginning at valley bottom rising to around 700 m in the afternoon, moderate to strong west/southwest wind, 1 to 5 cm of snow.WEDNESDAY: Overcast, freezing level rising to around 1100 m in the afternoon, potentially strong to extreme south/southwest wind, latest model run shows about 10 cm of snow. Stay tuned for more details. THURSDAY: Overcast, freezing level around 1500 m in the afternoon, light to moderate south/southeast wind, 1 to 5 of snow possible.
Avalanche Summary
On Sunday storm slab avalanches were susceptible to both skier triggering and explosive control work to size 2. Storm slabs were reported from an east facing slope at 2025 m and northeast through northwest facing slopes between 1900 and 2100 m. In the southern portion of the neighboring South Columbia region a skier triggered size 2 avalanche was reported from a southwest facing slope at 1550 m. The avalanche had a crown 40 to 60 cm in depth and ran on the mid-February weak layer. On Friday and Saturday storm slabs were triggered by skiers and explosives at treeline and alpine elevations, 10 to 25 cm deep.
Snowpack Summary
Two successive storms over the weekend produced 10 to 40 cm of new snow accompanied by wind out of the southwest, west and northwest. 30 to 60 cm of snow now rests on the mid-February weak layer that is composed of facets, surface hoar and a sun crust on solar aspects.Deeper in the snowpack weak layers that formed in January and December are gradually gaining strength. Several surface hoar and facet layers are buried 1 to 2 m below the surface and there is a crust/ facet interface near the base of the snowpack. No avalanches have been reported on these layers for the past two weeks and snowpack tests are showing improved bonding. Despite these signs, avalanche professionals are still treating these layers with respect and being cautious around shallow start zones and big avalanche paths.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.