Regions
Northwest Inland.
As snow and wind continue to build wind slabs, beware this is also loading a weak layer deep in snowpack. Skiers reported remotely triggering a large avalanche from 200 m away on Tuesday. Be sure to read and post your observations to the MIN.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
WEDNESDAY: Flurries, accumulation 5-10 cm. Strong southwest wind. Alpine temperature -5. Freezing level 1100m.THURSDAY: Flurries, accumulation 5-10 cm. Moderate to strong southwest wind. Alpine temperature -7. Freezing level 700 m.FRIDAY: Flurries, accumulation 5 cm. Moderate gusting to strong southwest wind. Freezing level 700 m.
Avalanche Summary
Skiers in the Seaton area on Tuesday reported remotely triggering a large (Size 2.5) avalanche from 200 m away as they approached a north-facing, treeline bowl at 1650 m elevation. The avalanche was triggered from a shallow snowpack area on the ridgeline and failed on a weak layer of sugary, snow crystals just above the late October crust near the base of the snowpack. Last week several estimated Size 2.5-3 avalanches were observed on the Kathlyn face of Hudson Bay Mountain and are also suspected to have failed on the late October crust. With a layer of sugary crystals above it, this "crust/facet combo" layer is capable of producing very large avalanche and is expected to remain a layer of concern for some time. Please post your observations to the Mountain Information Network. (MIN)
Snowpack Summary
Snowpack depths over the south of the region are above average for late November, with total depths of about 100-120 cm present at treeline elevations and above. Approximately 30-40 cm of recent snow now overlies the supportive November 23 crust that formed as a result of the warm, wet weather and is suspected to extended well into alpine elevations. Below this crust is moist settled snow and the widespread late October crust, which is present near the base of the snowpack at all elevations and features a layer of weak, sugary grains above it. This "crust/facet combo" layer remains an ongoing concern for step-down potential from large triggers such as a storm slab release or, the potential of triggering from shallow snowpack areas. The late October crust is less of a concern below treeline where it is broken up by vegetation near the ground.
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.
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.