New storm slabs developing during the day on Friday. If the storm is earlier or more intense than forecast, avalanche danger may reach HIGH by Friday afternoon.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Light snow and strong southwest winds overnight with the freezing level close to 800 metres. Snow becoming heavy (20-30 cm in 12 hours) with strong to extreme southerly winds on Friday and freezing level around 1200 metres. Another 10-15 cm by Saturday morning as the storm moves to the east and cooler air brings the freezing level back down to 800 metres with moderate westerly winds. Some sunny breaks on Sunday as a ridge of high pressure moves over the south of the province, resulting in light southerly winds and freezing levels rising up into alpine elevations.
Avalanche Summary
No new avalanches reported on Wednesday. I suspect that strong winds and new snow are developing storm slabs in the alpine and at treeline. Storm slabs should continue to develop during the day on Friday and may start to release naturally sometime late Friday or Saturday morning depending on the timing of the new storm.
Snowpack Summary
There is a new storm slab developing; the following is the snowpack structure below the forecast new storm snow. Up to 50 cm of dry new snow sits on 60-80 cm of moist settled snow from last week's warm storm system, or a rain crust below around 1800 m. Expect to find pockets of fresh wind slab in exposed lee terrain and the potential for loose dry avalanches on steep open slopes. Cornices are also becoming large and potentially weak. Where it still exists, the mid-January surface hoar layer may be found between 100 and 180 cm below the surface. The combination of heavy storm loading and warm temperatures has likely flushed out this weak layer in most areas. However, there is still some uncertainty regarding the sensitivity and distribution of this persistent avalanche problem, especially at high elevations in the north of the region where it remains reactive in snowpack tests (hard to initiate but sudden "pops" results).
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.