Forecast new snow amounts are uncertain. Watch for developing hazard due to new snow and wind.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Overcast with light snow, light northwest winds, and freezing levels down to 800 metres overnight. Light snow with increasing southwest winds on Wednesday and daytime freezing levels up to 1800 metres. Moderate precipitation (10-15 cm) starting early Thursday with moderate to strong southwest winds and overnight freezing levels around 1300 metres. Freezing dropping back to valley bottoms by Friday morning. A weak ridge is expected to build on Friday, but we are un-certain regarding the amount of sunshine we will see.
Avalanche Summary
No new reports of persistent slab avalanches on Monday. Loose wet solar induced activity in most areas up to size to size 2.0, as well as some thin wind slabs in the high alpine on shaded aspects. On Sunday we had reports of natural cornice falls up to size 3.0 that triggered persistent slab avalanches on the slopes below, and solar triggered persistent slab avalanches up to size 2.5 in the alpine from slopes steeper than 40 degrees. On Saturday cornice fall produced avalanches to size 3 on north and northeast facing features between 2000 and 2400 m, failing on the late February persistent weak layer. Small natural storm and wind slabs were also reported on northwest, northeast and east facing slopes between 2100 and 2400 m.
Snowpack Summary
Freeze and thaw conditions have developed a crust on solar aspects at all elevations, and up to at least 2000 metres on shaded aspects. Recent storm snow has released as loose wet avalanches on steep solar aspects. Cornices continue to be described as large and fragile. 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 60 to 110 cm below the snow surface. Commercial operators continue to see hard sudden planar results in snowpack tests, which suggests that wide fracture propagations are possible. Forecast new snow may not bond well to hard crusts on solar aspects, and pockets of wind transported snow may develop in the alpine and at treeline. Wind transported snow may trigger fragile cornices.
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.
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.
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.