Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain
Weather Forecast
Sunday: 10-15cm overnight and a further 5-10cm through the day. Freezing level around 600m. Strong south-westerly winds.Monday: 10cm snow, mainly in the afternoon/evening. Strong to gale southerly winds. Freezing level near 600m.Tuesday: Flurries with possible sunny breaks. Light south-westerly winds. Freezing level near 600m.
Avalanche Summary
On Friday, the newly-forming storm snow could be easily triggered by ski-cutting at alpine elevations. On Thursday, widespread activity to size 2.5 was reported from the Coquihalla in response to warm temperatures. A size 2 avalanche was reported from Mt Rohr on a southerly aspect at 2000 m. 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 winds and new snow are expected to create new wind slabs and storm slabs which could overload persistent weak layers. Previous fierce and variable winds in exposed treeline and alpine areas have 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, comprising surface hoar, facets and a crust, is down 60-80cm at treeline and over a metre at higher elevations. It is touchy on all south aspect slopes as well as on slopes of all aspects at treeline and below. In the Cascades, 70-100cm of recent storm snow combined with five straight days of sustained extreme winds created a highly unstable wind and storm slab problem that could easily re-awaken with additional loading by new snow.
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.