A punchy storm makes landfall Tuesday night. Avalanche danger will increase in response to the storm.
Confidence
Fair - Intensity of incoming weather is uncertain on Wednesday
Weather Forecast
Synopsis: A punchy Pacific frontal system will bring heavy precipitation Tuesday night and Wednesday.Tuesday: Day starting clear, but clouding over then moderate snowfall starting in the afternoon. Southerly winds 40 km/h.Tuesday Night: 10-20 cm overnight. Freezing level around 1200 m. Southwest winds increasing to 60 km/h at ridgetop.Wednesday: 15-25 cm new snow. Freezing level around 1300 m. Southwest winds 60 km/h.Thursday: 5-15 cm new snow. Freezing level 800 m. Gusty westerly winds becoming northwest later on.
Avalanche Summary
On Monday, a cornice collapse triggered a size 3 slab avalanche on a NE aspect at 2200 m with a crown depth of up to 100 cm. On Sunday, there were a couple of size 2 natural avalanches on east aspect slopes around 2000 m. Also on Saturday, slab avalanches up to size 1.5 could be easily human-triggered, and there was a remote triggered size 3 avalanche on a southeast aspect at 2100 m. The presence of these large avalanches illustrates the low confidence with alpine snowpack conditions at this time.
Snowpack Summary
20-30 cm new snow now sits on a weak crust below 1900 m. Below the crust is moist snow, from the previous intense, wet storm. Freeze-up following this storm was slow, due to a slow decline in temperatures and the insulating skiff of snow on the surface. Below the recent new snow lies one or two weak layers buried approximately 60 cm below the surface. These comprise faceted snow, surface hoar and/or a crust. The snowpack structure is quite variable at this time. It may change dramatically with only a subtle change in elevation, aspect, wind or sun exposure. With further cooling, the upper snowpack should start to bond better. However, with the amount of moisture and the insulating effect of the new snow, expect this to take longer than usual. Mid and lower snowpack layers are well bonded.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.