Regions
Northwest Coastal.
Rider triggered storm slab avalanches are likely, especially where there is buried surface hoar. Use a very conservative approach to terrain and gather info while you travel.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
The arctic air has dominated the North bringing some of the lowest temperatures of the season. Cold, dry arctic outflows will blow strong and start pouring from the coastal inlets. The next change in the weather pattern looks to come Thursday as the moist Pacific pushing onto the coast battles the arctic air. Track and timing will play a huge role and mostly affect the south, however; the north coast may see a slight rise in temperatures and more cloud cover.Tuesday/ Wednesday: Sunny with some cloudy periods. Alpine inversions will exist with temperatures near -12 up high and -20 in the valley. Ridgetop winds will blow moderate strong from the east.Thursday: Mostly cloudy with nil precipitation. Alpine temperatures -12 and moderate ridgetop winds from the east.
Avalanche Summary
On Sunday, explosives control triggered numerous large size 2-3.5 slab avalanches running full path and suspect the failure plane to be the November 22nd surface hoar interface. There was also a report of a remote (from 15m away) machine triggered size 2 slab avalanche that propagated 60 m up slope into flat terrain partially burying a person. No injury's were related. On Saturday, a group od ski tourers reported numerous slab avalanches which were triggered remotely and running on the buried surface hoar interface. The avalanche crowns were 50-100 cm deep, 200 m wide and 100 m long from northeast aspects at upper elevations. They also noted another avalanche from a distance away running 500 m in length and width.
Snowpack Summary
Storm snow totals reached 30-80 cm over the weekend with the potential for more on lee features due to strong southwest winds. The new snow buried a surface hoar layer reported 60-100 cm below the surface in many parts of the region. This layer was reportedly reactive during and after the storm. A thick rain crust exists 20-30 cm below the surface hoar layer, with isolated reports of weak facets (sugary snow) forming above the crust. Treeline snow depths are around 140-200 cm in the Terrace and Stewart areas, but substantially less further north. Concerns in the north are basal weaknesses that may exist in the shallower snowpack areas especially on smooth alpine features like glaciers.
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.