Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain for the entire period
Weather Forecast
Thursday: Dry with increasing high clouds throughout the day and moderate snowfall starting in the evening. Light winds increasing to strong southeasterlies with the onset of the storm. Freezing levels rising as high as 2000m, or higher in the south, but dropping in the evening. Friday: Heavy snow and strong southwesterly winds tapering off in the morning with 20-30cm total possible by midday. Freezing levels are expected to drop to 700m. Saturday: Continued moderate precipitation and strong winds with freezing levels remaining in the 500-700m range.
Avalanche Summary
Tuesday's reports from Duffey Lake and Chilcotin areas suggest that a persistent facet/crust weakness is very touchy in treeline gullies and below treeline cutblocks. Several Size 2.0-2.5 persistent slabs, including multiple sympathetic releases, were skier-triggered in treeline features (gullies) and below treeline cutblocks. More reports of Mondays widespread avalanche cycle are coming in with loose snow and storm slab avalanches p to Size 3 observed on all aspects and at all elevations. In the Coquihalla area recent heavy loading from snow and wind resulted in several Size 3-4 natural avalanches. Two Size 4 slab avalanches were observed on northwest through east aspects. One was a 5-10m thick slab and took out large swaths of mature forest in the runout, while the other was a relatively thin slab at 1-2m but propagated approximately 1.5 Km across a bowl.
Snowpack Summary
Strong and variable winds in exposed treeline and alpine areas has resulted in reverse loading, widespread surface crusts, and looming undermined cornices. In the Duffey Lake and Chilcotin regions south aspects are described as spooky with a very hollow feeling with a couple of different poorly-bonded crusts in the upper snowpack. The lower weakness, down 50-75cm at treeline is not only particularly touchy on south aspects, but on all aspects at lower treeline and below, where there is also an associated crust. Lingering pockets of this mid-February surface hoar and/or facets elsewhere (steep easterly aspects, sheltered concave slopes, and protected valleys, for instance) necessitates continued "heads-up" travel. Meanwhile in the Cascades, 70-100cm of recent storm snow combined with five straight days of sustained extreme winds has created a highly unstable wind and storm slab problem that will probably take a few days to settle and strengthen.
Problems
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.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
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.