Avalanche Forecast

Moderate - Timing or intensity of solar radiation is uncertain on Monday

The normal decrease in temperature with height has once again been replaced by an inversion. This classical pattern allows for cooler air to remain in the valleys while alpine temperatures rise to a degree or two above freezing. This setup should stick with us until Tuesday when warm air is expected to begin mixing into the valleys as an offshore front impacts the coast. The interior should remain mostly dry until a more robust system hits the south coast late Wednesday with precipitation spilling into the interior on Thursday.MONDAY: Scattered clouds in the morning, clearing throughout the day, freezing level at valley bottom with above freezing temperatures between 1600 m and 3300 m, moderate south/southeast wind, no snow expected. TUESDAY: Increasing cloud cover, freezing level at valley bottom with above freezing temperatures between 800 m and 2000 m, moderate south wind, no snow expected during the day, 1 to 10 cm possible Tuesday nightWEDNESDAY: Broken cloud cover, freezing level at valley bottom, rising to 1500 m in the afternoon, moderate to strong southerly wind, trace of snow possible.

On Saturday natural avalanches up 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. Reports from the last few days include several size 1 to 1.5 persistent slab avalanches that were triggered remotely (from a distance) with slabs 30 to 100 cm in depth.  Both the early January and mid-December surface hoar layers were noted as failure planes. These reports all came from the south of the region.Considerably more avalanche activity has been reported from the adjacent North Columbias, where numerous persistent slabs and storm slabs have recently been observed running naturally and with remote triggers up to size 3.0 with slab depths from 30 to 100 cm. This activity has been observed on all aspects and elevation ranges, although the deeper avalanches running on the mid-December surface hoar have been at treeline and below.

The last series of storms left 20 to 50 cm of snow in their wake with more snow falling in the south of the region. Winds from a wide variety of directions have formed wind slabs in northeast through south facing alpine and upper treeline elevation bands. This snowpack is currently quite complex, and there are three active Persistent Weak Layers (PWL) that we are monitoring. The first PWL is known as the early January interface, it is 20 to 50 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects. It is thought to be widespread at all elevations bands and has produced numerous recent large avalanches. The next PWL is the mid-December facet/surface hoar/crust combination which 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.The most deeply buried PWL is the late November rain crust. It is down 70 to 100 cm below the surface and has been less reactive in recent snowpack tests but it may still be susceptible to human triggering, especially in alpine terrain features with a thin or widely variable snowpack.