Confidence
Fair - Intensity of incoming weather is uncertain for the entire period
Weather Forecast
Friday: Moderate to heavy snow and strong southwesterly winds with 20-30cm possible by the end of the day. Freezing levels are expected to drop to 1000m. Saturday and Sunday: Continued light to moderate snowfall and strong south to southwest winds with freezing levels remaining in the 500-1000m 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 last weekend's widespread large natural avalanche cycle are coming in with observations of Size 4.5 avalanches. Some of the larger slabs propagated 2-3Km along ridgelines and stepped down to the mid-February persistent weakness, and there's no reason why similar avalanches can't happen with this next round of loading.
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 60-80cm at treeline and over a metre at higher elevations, 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.