Intense snowfall beginning late Saturday afternoon and overnight, will increase avalanche danger on Sunday. Conservative terrain with no overhead hazard is a good choice.
Confidence
Moderate - Forecast snowfall amounts are uncertain
Weather Forecast
TONIGHT: Snow. Accumulation 10-20 cm. Ridge wind moderate to strong, northeast. Temperature -16. Freezing level valley bottom.SUNDAY: Cloudy, light flurries. Accumulation trace. Ridge wind moderate to strong, northeast. Temperature -20. Freezing level valley bottom.MONDAY: Mix of sun and cloud. Ridge wind light, northeast. Temperature -16. Freezing level valley bottom.TUESDAY: Mix of sun and cloud. Ridge wind light, southwest. Temperature -12. Freezing level valley bottom.
Avalanche Summary
On Saturday explosive control work produced cornice failures up to size 2, triggering storm slabs on the slopes below.Reports of natural and skier triggered storm slab and loose snow avalanches up to size 1.5 were reported during the storm on Friday, primarily in leeward areas in the alpine and at treeline.On Thursday, natural storm slab and loose storm snow avalanches up to size 1.5 were reported to have occurred overnight with active loading during the storm, while explosive control work produced storm slab results up to size 1.5 on northerly slopes between 1700-1800 m.Earlier in the week, explosive control work and ski cutting produced widespread storm slab results up to size 2. Avalanche activity will increase as a result of Saturday's storm. Expect widespread storm slabs and new cornice growth from intense snowfall and wind-loading. Rapid loading also has the potential to wake up buried weak layers. Conservative terrain with no overhead hazard is recommended.
Snowpack Summary
About 40-60 cm of recent storm snow and wind have created storm slabs at upper elevations. This overlies old wind slabs on leeward slopes at treeline and above, as well as sun crusts on solar aspects and a mix of surface hoar and facets in sheltered areas near treeline. Below 1700m, this new snow covers a rain crust that was buried on February 8th.For the past month there has been regular avalanche activity on multiple weak layers in the mid and lower snowpack. A widespread weak layer from mid December composed of facets, crusts, and surface hoar is 120-160 cm deep. Also, a rain crust with sugary facets buried in late November is near the base of the snowpack.The take home message is that several weak layers are still lurking in the snowpack and they deserve a lot of respect.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
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.