Regions
South Coast Inland.
Freezing levels will rise over Monday along with sunny patches in the afternoon. The warming will weaken the snow and increase the likelihood of triggering slabs, cornices, and loose wet avalanches. Expect the hazard to rise over the day.
Weather Forecast
MONDAY: Cloudy with light morning snow and afternoon clearing, accumulation trace to 10 cm, light southwesterly winds, alpine temperature -4 C, freezing level 1500 m rising to 2000 m by late afternoon.TUESDAY: A mix of sun and cloud early morning then mostly cloudy with rain switching to snow at higher elevations, accumulation 10 mm, moderate to strong southerly winds, alpine temperature 0 C, freezing level 2600 m dropping to 1700 m over the day.WEDNESDAY: Mostly cloudy with light snowfall, accumulation 5 to 10 cm, light southeasterly winds, alpine temperature -4 C, freezing level 1600 m.
Avalanche Summary
Many loose wet avalanches were observed on all aspects up to treeline in steep terrain on Saturday.On Friday, a large (size 2.5) persistent slab avalanche was triggered by a cornice on a northeast aspect around 1900 to 2200 m. The slab likely released on the weak layer described in the Snowpack Summary. Small to large (size 1 to 2) wind slabs were also reactive to skiers and large loads, generally failing in alpine terrain in immediate lee terrain features. Loose wet avalanches were observed across the region on all aspects to size 1.5.
Snowpack Summary
Around 10 to 20 cm of recent snow and strong southwesterly winds created new slabs at higher elevations. This snow overlies a melt-freeze crusts on sunny aspects and all aspects to treeline, and dry snow on shaded aspects in the alpine. On ridges, cornices are reported to be large and fragile. At low elevations below treeline, precipitation fell as rain and created moist or wet snow conditions.A weak layer consisting of surface hoar, facets, or a sun crust from late March is now buried about 40 to 80 cm. This layer is spotty in its distribution. It is mostly likely to be problematic on shady aspects between 1900 m and 2250 m.The mid and lower snowpack are strong and well-settled.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
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.