Danger could rise sooner than indicated if we see more than 10 cm of new snow (with wind) before the end of the day on Wednesday.
Weather Forecast
WEDNESDAY: Mainly cloudy with snow developing late in the day. The freezing level should be around 1500 but will rise to 1800 m overnight. Ridge winds increase to strong or extreme from the SW-SE with the arrival of the storm. THURSDAY: Periods of snow – 40-70 cm Wednesday night through Thursday. The freezing level drops to around 1400 m and winds are moderate to strong from the south. FRIDAY: Cloudy with sunny breaks. The freezing level could rise to 1800 m during the day and winds ease to light.
Avalanche Summary
We've had reports of skier and explosive controlled avalanches up to size 2 (mostly size 1-1.5) each day for the past few days. Most of these have involved only the most recent snowfall (~24 hours) and were limited to steep wind-loaded slopes. On Sunday a loose dry sluff triggered a size 3 wet slab at 1400 m. This slide released on the mid-Feb crust. On Saturday there was evidence of widespread natural storm slab avalanche activity to size 3.5. The avalanches failed in response to heavy storm loading on Friday and possibly warming on Saturday.
Snowpack Summary
Storm totals from the past week were well over 100cm with more currently falling and much more on the way. Previous strong S-SW winds have shifted the new snow into deeper slabs on lee and cross-loaded alpine and treeline features. With the exception of shady alpine slopes, 30-50 cm of storm snow overlies a melt-freeze crust that formed on Sunday from high freezing levels and sun. Ongoing stormy weather has also encouraged extensive cornice growth. Weaknesses below the recent storm snow are expected to have gained considerable strength while the mid and lower snowpack are also strong and well-settled.
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.