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 1200 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 southwest wind, trace of snow possible. SATURDAY: Broken cloud cover, freezing level beginning at valley bottom rising to around 1300 m in the afternoon, light to moderate southwest wind, trace of snow possible.
Avalanche Summary
Tuesday was rather quiet. A size 1.0 human triggered avalanche was reported on a NE facing slope at 2100 m. The slab was 20 to 50 cm in depth suggesting it failed on the early January interface. Natural wind slabs to size 2.5 were reported from extreme north facing terrain at 2400 m.On Monday the early January interface continued to produce both natural and human triggered avalanches to size 2.5 on a variety of aspects above 1700 m. Crown depths were 30 to 75 cm and a size 1.5 avalanche was triggered by a skier on a north facing slope with a 22 to 32 degree incline. 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. There was one report of a size 1.5 human triggered avalanche failing on the mid-December interface on a south facing slope at 1100 m. A second human triggered avalanche was reported from a gently inclined slope at 2100 m, aspect unknown. The size 2.5 avalanche ran 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. This snowpack is currently quite complex; 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 70 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 60 to 140 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 90 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.