Regions
Northwest Coastal.
Fresh storm slabs will form above a hard crust on Tuesday, plus there's a lingering possibility of large persistent slab avalanches. Conservative terrain choices are recommended.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Tuesday
Weather Forecast
TUESDAY: Snow starting Monday afternoon will continue overnight and then ease off on Tuesday afternoon, accumulations of 15 to 25 cm, moderate southwest wind, alpine high temperatures near -8 C, freezing level up to 500 m.WEDNESDAY: Mostly cloudy with light snowfall, accumulations of 1 to 3 cm, light south wind, alpine high temperatures near -6 C, freezing level up to 700 m.THURSDAY: Snow starting in the morning with accumulations of 5-10 cm by the afternoon, light south wind, alpine high temperatures near -8 C, freezing level up to 500 m.
Avalanche Summary
Avalanche activity on Sunday was limited to a few small loose avalanches on steep sun-exposed slopes.On Saturday, a large persistent slab avalanche was reported in the north of the region, failing on the early-March weak layer. It failed on an easterly aspect around treeline. Otherwise, there was further evidence of Friday's natural cycle of storm slab and loose dry avalanches. They were small to large (size 1 to 2) and observed on all aspects and at all elevations.Last week, large persistent slab avalanches were reported on east to northeast aspects at all elevations. These were failing on the early-March and mid-March layers.
Snowpack Summary
Moderate amounts of new snow are expected in the region, but accumulations could reach 30 cm in some localized areas. The new snow will fall on a hard crust layer on south-facing slopes and at elevations below 800-1200 m.In the south of the region, there is about 70 to 90 cm of snow from last week that overlies two layers of surface hoar, which are most prominent on north to east aspects and were buried early-March and mid-March. In the north of the region, these layers are around 40 cm deep.Shallower parts of the region, such as the far north, have weak sugary facets near the bottom of the snowpack.
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.