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 fresh wind slabs at upper elevations too.
Confidence
Low - Timing, track, or intensity of incoming weather system is uncertain on Wednesday
Weather Forecast
We are entering what looks to be a very snowy period. The Wednesday night storm has potential to be a good re-fresh, and then the gift should keep on giving as the associated upper low drops south over the next few days. Weather models are converging on a solution but this is an incredibly dynamic period and the following should be taken as loose suggestions. If you are craving detail, now is a great time to click on the ACMWF link below.WEDNESDAY: Broken cloud cover, freezing level around 1000 m, moderate southwest wind, trace of snow.WEDNESDAY NIGHT: Freezing level around 500 m, 2 to 10 cm of snow, strong south/southwest wind. THURSDAY: Overcast, freezing level rising to around 1500 m, light to moderate south/southeast wind, 1 to 5 cm of snow possible.FRIDAY: Overcast, freezing level rising to around 1500 m, moderate southeast wind, 2 to 10 cm of snow possible.
Avalanche Summary
On Monday control work produced wind slab avalanches to size 1 on south and east facing features around 2150 m. A skier triggered size 1.5 avalanche was reported on a southwest facing feature at 2000 m which likely ran on the February 23rd sun crust that was buried by the recent storm. Loose dry avalanches to size 1 were also reported from northwest through northeast facing terrain between 2000 and 2100 m. 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.
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. In some locations this snow rests on the February 23rd weak layer which is composed of small surface hoar on polar aspects and a thin crust on solar aspects. 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.