Tuesday's Danger Ratings are based on 10-15cm of new snow. If more snow falls and treeline winds are strong, the treeline Danger Rating may also be Considerable.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Monday
Weather Forecast
TONIGHT: 10-20cm of snow / Strong southwest winds / Freezing level at 1500m TUESDAY: Mix of sun and cloud / Light to moderate southwest winds / Freezing level at 1500m WEDNESDAY: Light flurries / Light to moderate southwest winds / Freezing level at 1500m THURSDAY: 10-20cm of snow / Moderate southwest winds / Freezing level at 1500m
Avalanche Summary
Evidence of a natural wind slab avalanche cycle to size 2.5 was observed at treeline and in the alpine on Sunday. Most of the natural activity is thought to have occurred in the height of Saturday night's storm, while wind slab avalanches to size 1.5 were ski cut during the day. Of note, there was a report of a skier-triggered wind slab avalanche in Rogers Pass. The size 2 avalanche failed on a cross-loaded feature at 2150m. Two people went for a ride in the avalanche, one of whom sustained injuries and was rescued by helicopter on Monday morning. At the time of publishing this bulletin, observations were extremely limited; however, I'm sure there was similar activity observed on Monday. Increasing southwest winds and new snow on Monday night will spark a new round of wind slab activity in higher elevation lee terrain. Periods of sun on Tuesday may also promote loose wet avalanche activity in steep sun-exposed terrain, especially in spots where new snow is exposed to solar radiation for the first time.
Snowpack Summary
On Monday night, light to locally moderate amounts of new snow are expected to fall, and strong southwest winds are expected to form new wind slabs in lee features at treeline and in the alpine. The new snow will overlie older wind slabs and settling storm snow from the past few days. About 40-60 cm below the surface, you'll likely find a melt freeze crust. This crust exists in most places except for higher elevation shaded terrain. In general there seems to be a reasonable bond between the crust and the overlying snow. I'd still keep an eye on this interface as it has become reactive on some features. Surface hoar buried in early January now lies up to 200cm below the surface and has become less of a concern. Possible triggers for this deep and destructive layer include a large cornice fall or significant warming.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.