Variable snowpack conditions, particularly at upper treeline and alpine elevations. The snowpack is proving slow to recover following the recent big storm.
Confidence
Fair - Due to variable snowpack conditions
Weather Forecast
Synopsis: Northwest flow will bring flurries/squalls and cooler temps until Tuesday, when a low pressure system is expected to make landfall.Sunday and Monday: Mostly light convective snowfall. Some areas may see nothing, others 2-5 cm. Some sunny breaks in the cloud giving daytime warming both days. Afternoon freezing level around 800 m. Winds mostly light nothwesterly, but occasionally gusty.Tuesday: Moderate, maybe heavy snowfall starting in the afternoon. Models currently disagree with track and intensity of this system. Freezing level going to around 1500 m. Southwest winds to 60 km/h at ridgetop.
Avalanche Summary
There was evidence of a natural avalanche cycle up to size 3 during the recent storm. Typically, soft slabs were failing within the storm snow with crowns to 40 cm. On Friday (after the storm had ended), loose natural avalanches up to size 2.5 were reported from treeline and below treeline due to warm daytime temperatures. Two skier-triggered slab avalanches and one vehicle remote triggered slab avalanche were also reported from north aspect slopes between 1800 and 2000 m.
Snowpack Summary
Precipitation totals for the storm that ended Thursday were impressive: up to 130 mm. This fell as rain in the valleys, a mix of rain and snow at elevations up to 1900 m, and as dense snow above that. On Thursday night, things cooled just enough to set up a weak, breakable crust, but Friday's daytime temperatures were enough to melt this out in many areas. The lack of a good freeze-up contributed to avalanche activity continuing into Friday. On Friday night, another skiff of snow fell on top of this interface. This insulating layer will further delay the re-freezing process, despite cooler temperatures. Below the recent storm snow lies one or two weak layers buried approximately 60 cm below the surface. These comprise faceted snow, surface hoar and/or a crust. As a result of all these factors, the snowpack structure is highly 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.