Recent storm snow and wind are adding to a snowpack with several buried weak layers. Best to make conservative terrain choices and avoid wind-loaded areas.
Confidence
Moderate - Due to the number of field observations
Weather Forecast
TONIGHT: Flurries. Accumulation Trace. Ridge wind strong, southwest. Temperature -5. Freezing level valley bottom.WEDNESDAY: Mainly cloudy, flurries. Accumulation 2-5 cm. Ridge wind strong, southwest. Temperature near 0. Freezing level rising to 1600 m.THURSDAY: Mix of sun and cloud, flurries. Accumulation Trace. Ridge wind moderate to strong, southwest. Temperature -4. Freezing level 1100 m. FRIDAY: Mix of sun and cloud. Ridge wind moderate, west. Temperature -5. Freezing level valley bottom.
Avalanche Summary
no new reports of avalanches since saturday whenexplosive control in the Castle area produced several Size 2-2.5 storm slabs around 40 cm deep. Ski cutting in the same area produced numerous smaller (Size 1) releases also confined to the recent storm snow.Looking forward, expect ongoing strong winds to promote lee loading as well as slab formation and reactivity even as snowfall tapers off.
Snowpack Summary
Up to 30 cm of recent storm snow now covers both a new layer of feathery surface hoar in sheltered areas as well as sun crust on solar aspects buried mid-January, and about 50 cm below the surfaces lies the early-January surface hoar layer.About 60-80 cm below the surface a weak layer of feathery surface hoar crystals and/or sun crust buried mid-December is found at treeline and below treeline elevations. Deeper in the snowpack, another weak early-season layer of rain crust and sugary facets buried late-November exists. Although the snowpack structure is variable across the region, these persistent weak layers are generally widespread. Snowpack depths are also variable across the region and typically thinner in northern areas than in areas to the south.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.