While it feels like spring in the valley, the alpine has returned to winter. Stay on your toes as a potentially touchy wind slab likely exists at upper elevations.
Weather Forecast
SUNDAY: Freezing level rising to 1500m. Moderate W/SW winds at treeline, strong W/SW winds at ridge-crest. Overcast skies, no significant precipitation expected.MONDAY: Freezing level rising to 1700m. Strong south winds at all elevations. Few clouds in the morning, quickly building to overcast by lunch. No significant precipitation expected during the day, 5 to 10cm of snow possible Monday night.TUESDAY: Freezing level constant around 1400m. Moderate NW/W winds at treeline, Strong W winds at ridge-crest. Broken cloud cover, no significant precipitation.
Avalanche Summary
No new activity to report from Friday. A few natural cornice failures were observed Wednesday & Thursday, but slabs were only triggered in steep, unsupported rocky features and even then had minimal propagation. On Tuesday cornice failure triggered a size 3 avalanche on a North facing feature at 2700m.
Snowpack Summary
The region picked up around 5 cm of snow Friday night accompanied by strong SW winds burying the old surface which consists of facets, surface hoar and crust. Prior to Friday nights storm the 15 to 30cm that fell the weekend of April 4th remained dry on high elevation polar aspects but had turned moist on east and west facing aspects. South facing features were moving into the spring corn cycle. Much of the snowpack has been reported as isothermal at mid and lower elevations.Two significant persistent weak layers composed of crust and facets exist in the snowpack. Although they appear to have gone dormant for the time being, we will continue to monitor them closely. Mid-March is down 40 to 80cm below the surface and Mid-February is down 80 to 200cm. Down at the bottom of the snowpack a weak layer of basal facets exists. Large loads like cornice/ice fall or even sustained warming could initiate an avalanche on this very deeply buried weak layer.
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.