Tricky conditions are expected for Wednesday with a new storm slab sitting on a potentially touchy weak layer. Don't let the warm air and sunny conditions make you over confident.
Weather Forecast
A ridge of high pressure should keep the south coast dry for the next three days. Wednesday should be mostly sunny with light alpine winds and freezing levels around 3000m. A temperature inversion is expected and the warmest air (+5 deg C) is expected to sit around 2000m. Similar conditions are expected on Thursday with freezing levels around 3000m, a mix of sun and cloud, and light winds. On Friday, mostly cloudy conditions are expected with freezing levels beginning to fall progressively. The next weak storm system is currently forecast to arrive Friday night and it looks like freezing levels will be back down below 1000m.
Avalanche Summary
No new avalanches have been reported but this probably speaks to a lack of observers during the storm. Natural slab avalanches up to size 2.5 were reported in the Sea-to-Sky region on Tuesday. Skier triggered slabs remain the main concern for Wednesday with the storm slab possibly sitting on a touchy weak layer.
Snowpack Summary
At the end of the storm, freezing levels rose and rain fell on the new storm snow. The storm slab sits on a previously variable snow surface that included wind affected surfaces (crust, hard and soft slabs), a freezing rain crust or sun crust, and surface hoar or sugary facets in sheltered shady terrain. The bond between the storm slab and the layer below is currently unknown but is expected to remain reactive on Wednesday. Cornices are expected to be weak with the current warming. In the middle and lower snowpack we might find the mid-December surface hoar or crust/facet weak layers. These are now buried 1m or more. The surface hoar layer may be found in sheltered shady areas at and below treeline, while the crust/facet layer is expected to be more widespread but also more variable. Prior to the storm, reports suggested that the layer could be between 0 and 120cm deep depending on exposure to wind. The new storm load means these deep layers may be more reactive than prior to 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.
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.
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.