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The normal decrease in temperature with height has once again been replaced by an inversion.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 1700 m and 3000 m, light south wind, no snow expected. TUESDAY: Increasing cloud cover, freezing level initially at valley bottom, possibly rising as high as 2000 m. There may be a slight inversion, stay tuned for details. Moderate to strong south/southwest wind, trace of snow possible.WEDNESDAY: Broken cloud cover, freezing level at valley bottom, rising to 1300 m in the afternoon, moderate to strong southwest wind, trace of snow possible.

We received observations Sunday of a natural avalanche cycle to size 3.5. These very large avalanches were running naturally on southwest, south and southeast facing alpine features and are thought to have been triggered by warming alpine temperatures and direct solar input. On Saturday natural avalanches to size 3 were reported from a wide variety of aspects and elevations. Some of the more notable occurrences were cornice failures on north and east facing aspects at upper elevations. Also of note was a size 2.5 avalanche that was remote triggered by a skier on a south/southeast facing feature around 2000 m, likely running on the early January weak layer.Widespread avalanche activity occurred Thursday and Friday with numerous reports of slab avalanches reacting to both explosive control work and human triggering. All aspects were involved as avalanches ran to size 3.0 on the early January interface. Some of the notable themes that are emerging from recent activity include accidental and remote triggering, 'step down' avalanches, avalanches on surprisingly low angle/supported terrain, and wide fracture propagations.

The last series of storms left 50 to 70 cm of snow in their wake. Moderate to strong southwest winds previously formed wind slabs in upper elevation terrain. This snowpack is currently quite complex; there are four 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. It is thought to be widespread at all elevations bands and has produced numerous recent large avalanches. Another surface hoar layer from late December is just below the early January interface.The next 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 most deeply buried PWL is the late November rain crust. It is found 125 to 200 cm below the surface and has gone dormant for the time being.