Very heavy, coastal-style snowfall amounts are falling on a cold, interior-style snowpack. Unusual and dangerous avalanche conditions are anticipated.
Confidence
Fair - Intensity of incoming weather is uncertain
Weather Forecast
Friday night: Up to 30-40 cm new snow expected. Strong to extreme ridgetop winds will blow from the SW.Saturday: Further heavy snowfall is expected, associated with the passage of a cold front. Amounts of 20-30 cm expected. Winds veering to strong westerly. Freezing levels spiking briefly to around 1500 m.Sunday: Another winter storm arrives bringing around 15-20 cm additional snow late in the day and strong winds. Temperatures should be slightly cooler, with freezing levels around 1000 m.Monday: Continued heavy precipitation is likely in localized areas with unstable convective air that moves in following Sunday's storm system. Strong SW winds. Freezing levels remaining around 1000m.
Avalanche Summary
Only sluffing in steep terrain was observed on Thursday and Friday. However, I anticipate avalanche activity will rapidly increase in response to heavy forecast snow amounts and warming temperatures.
Snowpack Summary
Dense new snow is forming a fresh storm slab on the snow surface. The underlying snow was cold and low density--it will likely act as a weak layer, enhancing the likelihood of slab avalanches during and after the current winter storm. A rain crust lies buried approximately 40 cm below the snow surface at lower elevations (up to around 1800 m). Reports indicate the bond at this interface is quite good, although the slick nature of the crust may still provide a sliding layer in some steeper locations. Concern for lower snowpack layers has diminished and remains only in shallow snowpack areas. Facets associated with a crust from mid-December and/or sugary facets at the base of the snowpack may still be a concern in shallow areas. Fragile cornices are still looming.
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.
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.