Regions
Northwest Coastal.
Heavy snowfall and strong winds are a recipe for large destructive avalanches.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain on Saturday
Weather Forecast
Strong winds plus an incoming storm with warmer temperatures are in stock. This will be an extended stormy period for the Northwest, with lots more to come on Monday. SATURDAY: An additional 10-20 cm of new snow, with higher amounts in the north of the region. Winds strong (60 Km/hr) from the southwest. Freezing level rising to 1000 metres with Alpine temperatures around -2 Celcius. SUNDAY: A brief lull in the action - some flurries and up to 5cm snow. Freezing levels around 1100m. Winds moderate to strong southwesterly. Alpine temperatures near -2 Celcius. MONDAY: The big wallop comes in the afternoon with 30-50 cm of new snow by the end of the day. Winds strong (60 Km/hr +) from the southwest. Freezing level rising to 1500m and alpine temperatures around -1 Celcius.
Avalanche Summary
On Thursday several Size 2 natural slab avalanches were seen just north of Terrace on east and west aspects near treeline, running mostly likely on the Jan 5/6 surface hoar. On Wednesday natural avalanches were observed on northern aspects around 1300m, running on the Jan 5/6 surface hoar layer.
Snowpack Summary
We've had 15cm new snow on Thursday into Friday near Stewart (and only 3-4 cm near Terrace). Expect this new snow (and all the snow that will come on Saturday) to bond poorly to all the windslabs that formed during the latest arctic outbreak winds. Those strong arctic winds (southeast through northeast) redistributed the 20-40 cm of snow from a week ago at all elevation bands... and it sits on a variable interface composed of hard wind slabs, weak surface hoar (Jan 5/6 layer) and faceted snow. The net result is touchy slabs on wind-loaded features that are giving easy to moderate sudden results in snowpack tests. Below the new snow, a well settled slab sits above the Christmas surface hoar layer which is well preserved in southern areas. This surface hoar is now buried 60-100 cm deep, and is still reactive in sheltered areas and steep open features at and below treeline. Deeper weak layers have only been reactive in areas with thin snowpacks. This includes a facet layer from early December that has been reactive in snowpack tests at lower elevations in the southern part of the region, and weak facets near the ground that have produced avalanches in the northern part of the region.
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.