This warm, wet and windy storm is expected to form a touchy slab that rests on crust and surface hoar. There is potential for rain as high as 1700 m and winds will quickly form potentially deep slabs. Choose simple objectives without overhead hazard.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Thursday
Weather Forecast
A classic winter storm smashed into the coast Wednesday morning and that system spilled into the interior Wednesday afternoon. By Wednesday night strong southwest flow aloft will allow the freezing level to rise to around 1700 m as waves of convective precipitation move through the region. Snowfall should persist into Thursday and the freezing level is expected to fall back towards valley bottom Thursday night. Friday and Saturday look cool and calm with another system set to move into the interior on Sunday. WEDNESDAY NIGHT: Freezing level between 1500 and 1700 m, 5 to 15 cm of snow across the region, favored convective pockets could see 20 cm or more, strong south/southwest wind. THURSDAY: Broken cloud cover, freezing level around 1400 m, moderate to strong southwest wind, 5 to 10 cm of snow, favored convective pockets could see 15 cm or more. FRIDAY: Broken cloud cover, freezing level beginning at valley bottom rising to around 1400 m in the afternoon, light to moderate west/southwest wind, trace of snow. SATURDAY: Broken cloud cover, freezing level beginning at valley bottom rising to around 1300 m in the afternoon, light to moderate west/southwest wind, trace of snow.
Avalanche Summary
Tuesday was much more quiet. Explosive control work produced cornice failures to size 2 on north and northeast facing ridge lines. A "tree bomb" initiated a size 1.5 persistent slab avalanche on an east facing feature at 2200 m.On Monday warm temperatures and direct solar induced natural avalanches to size 3.0 on solar aspects, the largest ran on a steep south/southwest face in the alpine. Human triggered avalanches to size 1.5 were reported on north and northeast facing terrain above 2000 m, likely running on the early January interface.On Sunday there was a widespread natural avalanche cycle to size 3.5. These very large avalanches were running on southwest, south and southeast facing alpine features and were triggered by a combination of above freezing alpine temperatures and direct solar input. Three size 2.5 avalanches likely failing on the early January interface were reported to have ran naturally on a southwest facing slope from 1900 m down to 450 m, covering a snowmobile access road south of Revelstoke. Another remotely triggered size 2.0 avalanche was reported on a northeast facing feature at 2050 m that failed on the early January weak layer.
Snowpack Summary
Spotty freezing rain Tuesday may have left a slight "glaze" on the surface which adds to the crust that is being widely reported from many locations, high elevation north is likely one of the few crust-free zones. Fresh surface hoar 3 to 10 mm in size has been reported to at least 2100 m, possibly higher. We can probably expect a touchy storm slab to form on this interface as storm totals exceed 10 cm.There are two active Persistent Weak Layers (PWL) that we are monitoring.The first PWL is known as the early January interface, it is 30 to 90 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects and is present at all elevation bands. Recent snowpack tests have shown sudden results with moderate loads.The second PWL is the mid-December facet/surface hoar/crust combination that is now buried 70 to 150 cm deep and is most problematic at and below treeline. This interface is not thought to be present in the alpine.The November rain crust down 125 to 200 cm is thought to have gone dormant for the time being.
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.
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.