Regions
Northwest Coastal.
A storm slab sitting on a touchy weak layer means conditions will remain tricky over the next few days. Continue to make conservative decisions and have a safe holiday.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Conditions on Wednesday will be unsettled as the storm is replaced by a ridge of high pressure that will establish itself for Thursday and Friday. By Wednesday afternoon the precipitation should have ended but scattered flurries are possible. Freezing levels should drop to around 500m and alpine winds should taper off to moderate or light from the NW. A mix of sun and cloud is expected for Thursday with freezing levels around 500m and light alpine winds. By Thursday afternoon, a weak disturbance may bring light precipitation to the coast. Friday should be mostly dry and cloudy with light alpine winds.
Avalanche Summary
Reports from Monday of explosive control producing storm slabs up to size 2.5 on north through east aspects between 1500 and 1800m. On Sunday, natural storm slab avalanches up to size 2 were reported. These were typically 15-30cm thick slabs but the thickest reported was 70-80cm. Typical start zone elevations were 1400-1500m. Explosive control also produced numerous storm slab avalanches with the same characteristics. I expect we will see more natural storm slab activity on Tuesday and Wednesday with storm conditions.
Snowpack Summary
The new snow is adding to the storm slab that developed over the weekend which is about 50-70 cm thick at higher elevations. Strong Southwest winds have transported snow into windslabs that may be about 100 cm thick. Cornices are expected to be large and fragile due to new rapid growth. Deeper in the snowpack, weaknesses such as the mid-November crust-facet layer may still exist. Avalanches at this interface have become unlikely, although the consequences of a release remain high. This layer may be more sensitive to triggering in steep, unsupported high alpine terrain, or in the far north of the region.
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.