Regions
Northwest Coastal.
The storm is coming to an end, but the avalanche danger will continue to be HIGH in the alpine for tomorrow.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
Winds becoming moderate from the south overnight with 5-8 cm of new snow above a freezing level of about 600 metres. Another 3-5 cm of new snow combined with light southwest winds during the day on Thursday and a freezing level of 500 metres. A few more cm of new snow by Friday morning when skies begin to clear and freezing levels drop to near valley bottoms. Another small pulse of wind and snow is expected on Saturday.
Avalanche Summary
Natural storm slab avalanches were reported on Tuesday up to size 2.5 near Terrace, as well as loose wet avalanches to size 1.5 below treeline a little northwest of Terrace. I suspect that widespread natural activity will be reported once travel and visibility conditions improve.
Snowpack Summary
There is 40-60 cm of storm snow in the south of the region that has been transported by strong southwest winds in the alpine and at treeline over the past few days. The freezing level has been around 1200 metres, and below this level the snowpack is moist or wet. It is cooler in the north, and the storm totals were between 60-80 cm with significant wind transport. As the storm snow settles into a slab, we are concerned that several of the weak surface hoar or facet interfaces that developed during cold and clear weather may become reactive. The January 5th layer is down 50-80 cm , and the December 25th is now down close to 100 cm in deeper snowpack areas. These are not typical conditions for this coastal region, and it may take some time to gain confidence regarding the distribution and likelihood of triggering these deeply buried weak layers. Areas exposed to cornice fall should be avoided due to the possibility of a cornice triggering a deep weak layer. Below treeline the snow is moist or wet and may be easily triggered as a loose wet avalanche in steep terrain. I suggest a conservative approach to terrain while gathering info after the storm.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.