Regions
Northwest Inland.
The storm continues to develop storm slabs above a weak shallow snowpack. Storm slab avalanches in motion may step down and result in large avalanches.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
Freezing levels dropping to near valley bottoms overnight combined with moderate southwest winds and 3-5 cm of new snow. Overcast with flurries or light snow on Thursday and moderate southwest winds. Freezing near valley bottoms and -5 in the alpine. Clearing and cooler on Friday with light winds and alpine temperatures near -10. Overcast with moderate southeast winds and a chance of flurries on Saturday.
Avalanche Summary
Expect developing storm slabs to be easy to trigger where they are sitting on a weak layer of surface hoar and/or a crust left behind after the cold and windy weather. Full depth avalanches continue to be a concern due to the weak facetted snowpack below the hard wind slabs left behind after strong winds from several directions. This deep persistent weak layer may take prolonged warm weather with consistent snowfall to settle and bond.
Snowpack Summary
20-25 cm of new storm snow that has been transported by the southwest winds now sits on a mix of old surfaces left behind after the recent cold and windy weather. Expect the new storm snow to bond poorly where it is sitting on a weak layer of buried surface hoar and/or a hard old wind polished surface. Most areas continue to have a shallow and weak snowpack that is about 120-150 cm at treeline. In general, the snowpack above treeline has been heavily wind effected; this has resulted in hard wind slabs above weak facettted crystals. The snowpack below treeline in sheltered areas may be unconsolidated with foot penetration close to the ground.
Problems
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.
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.
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.