Storm slabs continue to develop at higher elevations and will be reactive to human-triggering on Thursday. Conservative terrain choices remain critical and it is important to continually assess conditions as they change throughout the day.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Active weather continues on the Thursday with the next storm front arriving Wednesday overnight. Another 15-30cm is forecast between Wednesday night and Thursday afternoon. Amounts are forecast to be greatest directly along the coast and taper off as you move inland. Alpine winds are expected to be moderate to strong from the southwest and freezing levels are forecast to fall to around 1200m on Thursday. Snowfall is expected to continue of Friday with another 15-30cm forecast between Thursday night and Friday afternoon. Alpine winds are expected to stay moderate to strong from the southwest and freezing levels are expected to fluctuate between 1000 and 1500m. Light snowfall is currently forecast to continue on Saturday but conditions may start to clear off as a ridge of high pressure approaches.
Avalanche Summary
On Tuesday, several ski cut and skier accidental avalanches were reported throughout the region. These were mostly storm slabs and wind slabs size 1-2. Most of the reported activity was on north through east aspects between 1800 and 2000m elevation. These slabs were typically 20-40cm thick and were failing within the recent storm snow. However, a couple storm slabs were also reported at 1600m and were failing on a rain crust. One wind slab was remotely triggered from 10m away on a bench feature. One natural size 2 storm slab was reported from an east aspect at 1900m and had a slab thickness of 20cm. Explosive use on Tuesday also produced several size 2 storm slabs and several size 2 cornice releases. Lots of avalanches were also reported through social media and there are some very impressive photos floating around due to the clear weather. Storm slab avalanches are expected to be remain very reactive to human-triggering at higher elevations with ongoing snowfall and wind continuing Thursday. Cornices are reported to large and fragile, and may fail under the weight of a person.
Snowpack Summary
50-70cm of new snow has accumulated in the last week and overlies a thick melt-freeze crust which extends into the alpine. The snow is reported to be bonding well to the crust and all the recent avalanches have been failing within the recent storm snow, not on the crust interface. At treeline and below, other crusts may exist in the upper snowpack due to the recently fluctuating freezing levels. Freezing levels on Wednesday climbed to around 2000m and are expected to fall to around 1200m on Thursday so a new crust can be expected in the upper snowpack as rain changes to snowfall. Ongoing southeast through southwest winds have been loading leeward features in the alpine and large cornice development has been reported over the last few days. The weak surface hoar layer from early January can be found down over a meter and is still reactive in isolated snowpack tests but triggering an avalanche on this layer has become unlikely.
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.