Regions
Northwest Coastal.
Developing avalanche danger is expected to reach HIGH in the alpine on Friday. If the storm arrives early danger may reach HIGH at all elevations.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Stormy weather for the next few days. Overnight: Extreme southwest winds combined with 10-20 cm of new snow and freezing levels around 800 metres. Friday: Extreme south winds, 20-30 cm of new snow during the day and another 20-40 cm overnight, freezing level 1000 metres. Saturday: another 25-40 cm of new snow combined with extreme south winds and freezing levels rising to 1500 metres. Sunday: Winds becoming moderate to strong southwest combined with 10-20 cm of new snow and dropping freezing levels.
Avalanche Summary
No new avalanches reported. One report on Wednesday of a skier triggered wind slab size 1.0 on a convex roll at 1500 metres that was about 25cm deep. Expect avalanche size and frequency to increase as the forecast storm moves into the region.
Snowpack Summary
There is 10-20 cm of new snow above the latest buried surface hoar layer from January 23rd that has been found up to size 10 mm at treeline and below. This surface hoar layer may be associated with a crust below 1100 metres. There continues to be several other surface hoar layers of concern from early January and Christmas. Some reports show these persistent weak layers to be rounding and bonding, however we are still getting some reports of sudden planar releases in snowpack tests with hard forces applied. I suspect the forecast heavy precipitation over the next few days will be a good test for these older buried weak layers.
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.