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, 2 to 10 cm of snow across the region, favored convective pockets could see 15 cm or more, strong south/southwest wind. THURSDAY: Broken cloud cover, freezing level around 1200 m, strong southwest wind, 1 to 5 cm of snow, favored convective pockets could see 10 cm or more. FRIDAY: Scattered cloud cover, freezing level beginning at valley bottom rising to around 1300 m in the afternoon, light to moderate southwest wind, no snow expected. SATURDAY: Scattered cloud cover, freezing level holding near valley bottom, light south/southwest wind, 1 to 5 cm of snow possible.
Avalanche Summary
No new avalanche activity to report from Tuesday.On Monday warm temperatures and direct solar induced natural persistent slab avalanches to size 3.0 on solar aspects between 1600 and 2600 m. Loose wet avalanches were reported from all aspects to size 2.0. On Sunday loose avalanches were reported from all aspects to size 1.0. On Southwest, south and southeast facing terrain between 1500 m and 2700 m, loose wet avalanches to size 2.5 and persistent slab avalanches to size 2.0 were reported to have failed naturally. A small wind slab was human triggered on a north facing feature just below ridge crest. On Saturday natural avalanches up to size 2.5 were reported on northeast facing features between 1200 m and 1500 m. One small skier triggered avalanche was remotely triggered on a moderately inclined northeast facing alpine feature.
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 up to 10 mm in size has been reported to ridgetop . We can probably expect a touchy storm slab to form on this interface as storm totals exceed 10 cm.There are two significant and active Persistent Weak Layers (PWL) that we are monitoring.The first PWL is known as the early January interface, it is 20 to 60 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 easy to moderate planar results at this interface.The second PWL is the mid-December facet/surface hoar/crust combination that is now buried 50 to 80 cm deep and is most problematic at and below treeline. This interface is not thought to be present in the alpine. This interface has shown sudden collapse in recent snowpack tests with easy to moderate loads.The November rain crust down 70 to 100 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.