Storm slabs are highly reactive to human-triggering and conservative terrain selection remains critical for the next few days. Extra caution is required on south-facing slopes in the afternoon if the sun pokes out.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Saturday
Weather Forecast
FRIDAY: Cloudy with sunny breaks and light precipitation starting late in the day. Freezing levels reaching 1600 m late in the day with strong southwesterly ridgetop winds. SATURDAY: Periods of snow bringing another 5-10 cm. Freezing levels steady around 2000 m with moderate to strong southwesterly ridgetop winds. SUNDAY: Periods of snow. The freezing level is around 1600-1800 m and ridge winds ease to light and variable.
Avalanche Summary
On Wednesday, there were reports of a few natural slab avalanches up to size 2.5 failing on the late Feb. sun crust/surface hoar combo. These slides were from S-E aspects at treeline and showed impressive propagation. Reports from Tuesday include more evidence of the widespread natural avalanche cycle early Monday, with avalanches up to Size 2.5. These fresh storm and wind slabs should increase in size and sensitivity and become much more widespread as they build throughout the week.
Snowpack Summary
30-60cm thick storm slabs are bonding poorly to a crust on previously sun-exposed slopes and surface hoar on shady and sheltered slopes. Thicker and touchier wind slabs are lurking throughout exposed terrain at and above treeline. A couple sun crusts might exist in the upper 50-70cm on southerly aspects. The surface hoar and/or crust layer which was buried February 10 is now down around a metre and is still producing isolated hard sudden results in snowpack tests. Large cornices will be getting weak with warmer temperatures.
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.
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.