Regions
Northwest Inland.
Fresh wind slabs could be touchy and deep persistent slabs are lurking in shallow snowpack areas.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
SATURDAY: Mainly cloudy with scattered flurries bring another 3-5 cm, temperature around -20, and moderate easterly ridgetop winds.SUNDAY: Cloudy with sunny periods, temperatures reaching -10 C with a strong alpine temperature inversion, and light southeasterly ridgetop winds.MONDAY: A mix of sun and cloud with isolated flurries, temperature around -15 and light to moderate easterly ridgetop winds.
Avalanche Summary
Reports from Wednesday and Thursday include two more deep persistent avalanches. One Size 2 and one Size 3 both running on facets at the base of the snowpack south facing alpine slopes. These deep persistent slabs will remain a concern, as it is possible to trigger large avalanches.
Snowpack Summary
A light dusting of snow has likely buried recently formed surface hoar and/or facets. Snow depth at treeline varies from about 1 m in most areas to 2 m at deeper snowpack areas in the south and west of the region, and up to 250 cm in the alpine. The shallow snowpack areas mostly consist of weak facetted or sugary grains beneath hard wind slabs. These wind slabs may produce surprisingly long fractures resulting in large avalanches, and in some instances they may step down to weak snow crystals near or at the ground. In addition to the facets, a thick layer of buried surface hoar may be found 20-30 cm below the surface. There is another persistent weakness that formed during the early December cold snap which is now about 40-60 cm deep. A crust from mid-November may be lurking close to the ground nestled in amongst the facets.
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.