Convective flurries and moderate southwesterly winds may create wind slabs in lee features below ridge tops.
Confidence
Low - Due to the number of field observations
Weather Forecast
WEDNESDAY: Cloudy with sunny periods / Moderate south wind / Alpine temperature -3 / Freezing level 1800mTHURSDAY: Mix of sun and cloud / light west wind / Alpine temperature -3 / Freezing level 1800 mFRIDAY: Cloudy with sunny periods / Light west wind / Alpine temperature 2 / Freezing level 2300mNOTE: The convective flurries which are common during this time of year can result in widely varying snowfall amounts throughout a region. These spring squalls routinely drop 20+ cm of snow in one valley while the adjacent drainage remains dry. For this reason, the distribution of avalanche problems associated with new snow, such as wind slabs, can vary greatly within a region.
Avalanche Summary
Reports from Monday indicate a few loose wet avalanches running on solar aspects at tree line and a cornice fall triggering a deep persistent weakness on the slope below to size 2.5 from very steep north facing terrain in the alpine. On Sunday two size 2.5 slab avalanches were reported in this region on Sunday. One was triggered by a falling cornice and the other by a small loose wet avalanche. Falling cornices triggering large avalanches is currently our primary concern.
Snowpack Summary
Recent new snow tree line and above has been redistributed by west and south winds. Cornices remain large and continue to become more overhanging. The middle of the snowpack is generally well settled and continues to transition into a spring melt/freeze regime except for high elevation northerly aspects. Expect the entire thickness of the snowpack to be moist or wet at lower elevations. At upper elevations a deep persistent layer consisting of facets sitting on a crust that was buried back in November has been sporadically reactive to heavy loads i.e. a cornice fall or the weight of a smaller avalanche running. Cornices remain large and continue to become more overhanging.
Problems
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.
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.