Watch for fresh storm slab formation at upper elevations and continue to seek out conservative terrain in an effort to avoid tangling with the late February persistent weak layer.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
A weak upper low moves into the region Monday displacing the ridge that was in place over the weekend. With the ridge collapsed, the door is open to a series of fast moving storms interspersed by the odd ridge. SUNDAY NIGHT: Freezing level around 1600 m, 5 to 10 cm of snow above 1500 m, rain below, moderate to strong southwest wind. MONDAY: Freezing level around 1600 m, 1 to 5 cm of snow, light to moderate southwest wind. TUESDAY: Freezing level starting near valley bottom rising to about 1600 m, light to moderate northwest winds, no significant precipitation. WEDNESDAY: Freezing level starting near valley bottom rising to about 1500 m, 1 to 5 cm of snow possible, moderate southwest wind.
Avalanche Summary
On Saturday a wind event triggered a size 3 avalanche that started near 2300 m, failing on a steep un-skiable east facing piece of terrain. Over the last week we have received 5 reports of very large (size 3) avalanches on north, east and south aspects between 1700 and 2400 m. Some have failed after being hit with a large chunk of falling cornice, others have released without a large trigger.
Snowpack Summary
Large cornices loom over many ridge lines and many are teetering on the brink of failure. Old wind slabs may remain a problem on high elevation north facing features. The make up of the late February persistent weak layer is an aspect dependant mix of surface hoar, facets and/or a thick crust down around 70 to 100 cm below the snow surface. Large triggers like natural cornice fall and explosive control work continue to initiate avalanches failing on this interface. The recent warm to cool temperatures should give the overlying slab a bit of strength, but it has yet to prove itself trustworthy. Unfortunately there's not much of a reliable pattern telling us what exact aspects are most suspect. As a precautionary measure, we recommend remaining suspicious of steep unsupported features at and above treeline.
Problems
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.
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.
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.