25-35cm of storm snow expected overnight Wednesday will increase the Avalanche Danger and may require traffic interruptions Thursday for control work on Highway 93N. Maligne Lake Road will be closed Wednesday evening until Friday evening.
Weather Forecast
Warm, wet, pacific air is meeting cooler arctic air over the forecast area with snowfall intensifying overnight. This system may bring 25-30mm of precipitation with moderate winds and temperatures near -5C. The snowfall event is expected to last into Friday morning when a ridge dominates with cooler temps and blue skies into the weekend.
Snowpack Summary
Moderate SW winds will add the new snow to last week's windslabs in the usual lee locations. Mid-pack persistent weak layers, most notably Jan 7 surface hoar down 40-60cm and rotten Dec 15 facets sitting just below, remain the primary snowpack concern for larger avalanches.
Avalanche Summary
On Monday a sz 3 natural slab avalanche was observed from a thin snowpack area in the Maligne valley on the NE side of Proposal peak at 2500m. Notable avalanches this past week were generally windslabs in the alpine to sz 2.5 and a few persistent slabs in the low alpine and treeline elevation band.
Confidence
Forecast snowfall amounts are uncertain on Wednesday
Problems
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.
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.