Regions
Northwest Coastal.
Weather Forecast
Synopsis: A windy pacific storm is heading your way Saturday, clearing out Sunday.Saturday: 20 to 30 mm additional precipitation bringing storm totals in the range of 35 to 55 mm of water (somewhere around 40 to 70 cm of snow); Strong SW winds easing and veering W late in the day; freezing level below 500 m with alpine temperatures around -5Sunday & Monday: drier with less than 5 cm of snow; cooler temperatures with freezing level at the valley bottom and around -8 in the alpine; light SW wind Saturday becoming light E wind on Monday.
Avalanche Summary
In the north, only sluffing was reported recently; however, reports from a few days ago (after the previous storm) may provide insight into what might happen with this incoming storm. I'm thinking about a few big avalanches on the basal facets / rain crust / glacier ice. Smaller storm-snow slides or dropping cornice noses could be the "heavy load" that "steps down" to this weakness.
Snowpack Summary
Storm snow will continue to get deeper in the region, likely creating some short-term unstable layers but also hiding some previous wind slabs. Because the wind shifted around the clock (outflow and onshore),slabs could be on most aspects in alpine and exposed treeline slopes. Previously, spotty surface hoar was reported in the upper snowpack which may remain under this new snow in sheltered treeline areas. A key concern is a November facet/crust layer which can be found near the base of the snowpack. This layer, which is widespread, can be triggered from thin-spot trigger points, or with a heavy load, such as storm slabs stepping down, cornice fall or a sled track trenching.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.