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
A cool arctic air mass will dominate the weather pattern over the next few days bringing cooler and much dryer conditions. Coastal areas may see some flurries and cloudy skies tonight and early tomorrow before the arctic front locks in on Tuesday. Skies will mostly be a mix of sun and cloud. Alpine temperatures will drop to -15 accompanied by light NE winds. Expect strong, cold outflow winds.
Avalanche Summary
On Saturday, ski tourers reported numerous persistent slab avalanches 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. Rider triggers are likely on Monday where this instability exists.On Friday, explosive triggered storm slabs were reported from northerly aspects at 1400 m and above up to size 2.5. Some of these ran on the buried surface hoar layer. At treeline and lower elevations numerous loose wet natural avalanches were reported from steep, rocky terrain features. Natural avalanches are still possible and rider triggered avalanches remain likely through Monday. Light winds from the northeast may reverse load slopes and catch you by surprise.
Snowpack Summary
Storm snow totals reached 30-75 cm by Sunday, with the potential for much 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 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 120-140 cm in the Terrace and Stewart areas, but substantially less further north.
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.