Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain
Weather Forecast
Sunday: 10-15cm overnight and a further 10-15cm through the day. Freezing level around 600m. Strong south-westerly winds.Monday: 10-20cm snow, mainly in the afternoon/evening. Strong to gale southerly winds. Freezing level near 600m.Tuesday: Light snow. Light south-westerly winds. Freezing level near 600m.
Avalanche Summary
There have been at least two close calls involving snowmobile-triggered avalanches in the region over the last couple of days, one of which was triggered from 200m below the fracture line. A size 2 natural avalanche was reported from Garibaldi Park at 1800m on a north aspect on Friday. There was an avalanche fatality at Grizzly Lake on Powder Mountain on Tuesday. The avalanche occurred on an east aspect near 2000m. The slab was 80cm-150cm thick and 525m wide and failed on the mid-February persistent weakness. 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
New snow, accompanied with strong southerly winds, is setting up new storm slabs and wind slabs overlying a variable previous snow surface comprising crusts, moist snow and old wind slabs. Previous storm snow amounts were in the region of 50+ cm and this snow bonded poorly to the older surfaces below. The instabilities associated with both these storm slabs as well as deeper weak layers buried within the snowpack have given us a situation where avalanches could initiate in the upper storm snow and easily propagate to lower weaknesses in the snowpack. This dramatically increases the potential size of avalanche occurrences, which is pushing up into the size 3-4 category. There is also the possibility for avalanches to be remote-triggered, triggered mid-slope, and/or propagate into low-angled terrain.
Problems
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.
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.