There is uncertainty with precipitation amounts Wednesday night through Thursday. This forecast is based on the upper end of estimates.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Thursday
Weather Forecast
WEDNESDAY NIGHT: Snow, accumulation 10-15 cm / Moderate southwest wind / Alpine temperature -7 C THURSDAY: Snow, accumulation 10-20 cm / Moderate to strong southwest wind / Alpine temperature -3 C / Freezing level 1300 mFRIDAY: Flurries, accumulation 5-10 cm / Moderate to strong south wind / Alpine temperature -7 C SATURDAY: Isolated flurries / Light to moderate west wind / Alpine temperature -6
Avalanche Summary
Expect to see avalanche activity increase on Thursday as a storm passes through the region Wednesday night through Thursday.Reports from Sunday show a few very large (size 2.5 and size 3) deep persistent slabs running during the last storm. These occurred in alpine terrain and are suspected to have failed on deep weak layers formed early in the season. A convincing pattern of deep persistent slab activity has yet to emerge, but these events highlight lingering hazards in thin or variable snowpack areas in the alpine.
Snowpack Summary
An additional 10-15 cm of new snow falling Wednesday night will add to last weekend's storm snow totals of 20-30 cm.A weak layer that formed during the dry spell in early December is now 80 to 110 cm deep. The layer is composed of facets (sugary snow), surface hoar (feathery crystals), and a sun crust (on south aspects). Another similar weak layer is buried 90 to 150 cm deep. Finally, the base of the snowpack has weak faceted layers at alpine and treeline elevations. These basal weak layers have been producing large avalanches as recently as December 31st. Human triggering any of these layers is most likely on slopes that didn't previously avalanche and on slopes that have variable snowpack depth (such as rocky alpine features).
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.