Danger ratings may go higher than forecast in areas affected by the combination of warming temperatures and solar radiation. Stay well clear of terrain affect by cornices. Conservative terrain choices are essential now.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
The high pressure ridge over the interior of the Province will remain through most of Sunday. Southerly winds will continue, with the freezing level forecast to climb to 2700 m late Saturday afternoon and continue through Sunday, then begin to drop late on Sunday afternoon to around 2200 m. No overnight freeze and temperature recovery expected. Flurries are forecast for Tuesday that should to bring 10mm of rain at lower elevations, and a chance of 10 to 15cm above 1200m. Tuesday night the freezing level is forecast to lower to 500 m then climb again on Wednesday to the 2000 m level. No precipitation is forecast for Wednesday.
Avalanche Summary
Recently the reports of avalanches have been limited to small loose dry, or loose wet releases running in steep terrain on the storm snow. Earlier in the week, explosive control work in the far north of the region produced numerous large to very large persistent slab avalanches on north through east facing features between 2200 m and 2400 m. Cornices are reported to be very touchy and sensitive to triggering. Over the course of the last week we have received many reports of cornice failure (some of them quite large), but no reports of avalanches being triggered when falling cornices impacted slopes below.
Snowpack Summary
Earlier in the week, 15 to 25 cm of new snow above 1700 m. Northeast wind over the last 48 hours has formed soft wind slabs in upper elevation wind exposed terrain. These wind slabs may be sitting on the mid-March crust, which is down around 20 cm below the snow surface. An earlier March crust can be found down around 50 cm below the surface. Both of these crusts are reported to be present from valley bottom to around 2300 m, after which they begin to disappear. There are thin snowpack areas in the South Rockies region where deeply buried weak layers near the ground remain sensitive to triggering. Huge cornices still hang over many ridge-lines and with solar radiation and warming temperatures may now be on the brink of failure.
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.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.