Regions
Kootenay Boundary.
Storm conditions will drive the up the danger on Saturday. Heavy precipitation, strong winds, rising freezing levels, and a well preserved surface hoar layer will create very dangerous conditions at all elevations.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
An atmospheric river is bringing warm air and heavy precipitation to the interior regions Friday night and Saturday (this animation shows the event well: https://goo.gl/00P1SB ). There is currently some uncertainty regarding exact timing but the heaviest amounts are currently forecast to fall Friday overnight or Saturday morning. A ridge of high pressure sitting over Idaho will determine how much precipitation will fall in the Kootenay Boundary region. Models are currently showing anywhere from 10-40mm and it looks like the north of the region will see the greatest amounts. The high danger rating is reflecting the larger amounts of 20-40mm. Alpine winds are forecast to be strong from the SW. Freezing levels look to start at around 1000m at the beginning of the storm but will climb to over 2000m by Saturday afternoon. Light precipitation may continue for Saturday night but things should dry out on Sunday morning. While the precipitation event is quite short, we will see the warm air stick around for several days. Freezing levels are forecast to be around 3000m on Sunday and Monday. Both days are showing a mix of sun and cloud with moderate-to-strong SW-W alpine winds.
Avalanche Summary
With a forecast of ~30mm of precipitation and rapidly rising freezing levels, we expect upside-down storm slabs to form very quickly. As the rain level climbs we are expecting to see widespread storm activity at all elevations. This is enhanced by the mid-Jan surface hoar which is preserved down 20-30cm and has not seen its critical loading in many parts of the region. Also in the last few days we have seen several large avalanches release on the mid-Dec layer which is still proving quite reactive. This layer is expected to continue to produce very large avalanches as new load is added to the snowpack.
Snowpack Summary
The new snow is sitting on a 20-30cm persistent slab that sits over the touchy mid-Jan surface hoar layer. In many places the surface hoar sits above a thin crust that can be found up to about 1900m on north aspects and all the way to ridgeline on south aspects. Strong SW winds during the storm loaded leeward features and the persistent slab may be 50-60cm thick in wind loaded areas (typically north through east aspects). A deeper crust/surface hoar combo buried in mid-December continues to be a concern. This layer was typically down 70-100cm before the storm. This layer has produced some large avalanches recently and is expected to become quite reactive with the additional loading.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.