The forecast storm is expected to be short and intense. The avalanche danger is expected to reach high by the afternoon or early evening.
Confidence
Fair - Forecast snowfall amounts are uncertain on Wednesday
Weather Forecast
A low pressure system is forecast to move into the region on Wednesday morning from the south bringing heavy precipitation combined with strong southeast winds. Strong southwest winds are expected to diminish to moderate southeast at the beginning of the storm, and then become light easterly by Wednesday evening. Moderate precipitation is expected to continue overnight and into Thursday. About 20 mm of precipitation are expected on Wednesday, and another 10-15 mm is expected overnight and Thursday. The high freezing level from Tuesday should begin to drop overnight and hover about 1200 metres on Wednesday, and stay there for most of Thursday.
Avalanche Summary
There were some explosive controlled avalanches up to size 3.0 that released in the storm snow down about 70 cm on various aspects. Avalanches releasing on the deeply buried mid-February surface hoar are becoming less likely. Avalanches may step down to the weak layer of facets or depth hoar on a crust that was buried in October.
Snowpack Summary
The recent storms have added up to about a metre of snow in the region. High freezing levels have consolidated the storm snow into a cohesive slab that may be propagated remotely. The storm snow slab is sitting on top of a sun crust that was buried on March 27th on southerly aspects, and above wind affected surfaces that may be facetted on northerly aspects. Forecast new snow will add another load that may cause another cycle of natural avalanches or easily triggered avalanches. Cornices are reported to be very large in the region, and new cornice growth is expected with forecast new snow and strong southerly winds. The mid snowpack is well settled and strong. There is a weak layer of facets or depth hoar in shallower snowpack areas that may fail with initial warming of the snowpack. There is also a weak layer of of facets or depth hoar above a crust at higher elevations where there was already snow in October.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
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.