If you can, get out and enjoy the cooler temps and lower avalanche danger ratings Friday. All bets are off Saturday and Sunday as the region heats up in a big way.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
The ridge remains in place Friday offering one last day of seasonal normal temperatures before southwesterly flow injects a substantial amount of warm air into the South Rockies Saturday and Sunday. FRIDAY: Freezing Level climbing to around 1600 m, returning to valley bottom overnight, light northwest winds, no significant precipitation expected, mostly clear skies. SATURDAY: Freezing level starting at valley bottom then rapidly climbing to around 2800 m by lunch time, moderate west/southwest winds, no precipitation. No overnight temperature recovery expected. SUNDAY: Freezing level holding around 2800 m, moderate southwest winds at ridge top, no precipitation expected. Increasing cloud cover will likely trap warm air leading to a greenhouse situation.
Avalanche Summary
On Wednesday 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 were 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 subsequent slab avalanches being triggered when falling cornices impacted slopes below.
Snowpack Summary
On Wednesday the field team was in the Crowsnest north area around Window Mtn, where they found 155 cm of snow on the ground at 2250 metres on a north aspect. The foot penetration was 35 cm, but the light winds were not transporting this available snow today. Ski penetration was about 15cm. In Smith Basin on Tuesday, our field team found a thinner snowpack with only 140-200 cm on the ground. They found about 10 cm of recent storm snow above a 2 cm breakable crust that was not supportive. The March 7th crust in Smith basin was down about 20 cm, with decomposing snow below that becoming facetted weak crystals deeper in the shallow weak snowpack. There were no notable test results, and evidence of aggressive slope testing on east-south-west aspects that did not trigger any releases. On Monday at 1920 metres in the Crown Mtn area there was 17 cm of new snow above a breakable 2 cm crust. Below the crust there was dry snow at this elevation. We estimate the crust extends up to about 2000 metres. Strong southwest winds have created widespread wind slabs at treeline and above.
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.