A warming trend is expected on Wednesday with possible sunny periods. This recipe recently produced large, destructive avalanches on a buried weak layer. Don't be complacent -- these are not typical spring snowpack conditions.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain
Weather Forecast
WEDNESDAY: Mostly cloudy with light snowfall and afternoon clearing, accumulation 1 to 3 cm, light southeast winds, alpine temperature -6 C, freezing level 1400 m.THURSDAY: Mostly sunny, light southwest winds, alpine temperature -5 C, freezing level 1600 m.FRIDAY: Cloudy with light snowfall, accumulation 2 to 5 cm, light to moderate southwest winds, alpine temperature -5 C, Freezing level 1500 m.
Avalanche Summary
On Monday, small slabs and loose wet avalanches were triggered by skiers and the sun. The slabs were most often in lee terrain features near ridges.On Sunday, a large (size 3) persistent slab avalanche was remotely triggered by a snowmobile on a north aspect in alpine terrain after substantial air temperature and solar warming over the day. The avalanche likely released on the weak layer described in the Snowpack Summary. Another large avalanche was observed close to Whistler, on a north aspect around 2000 m, with a depth of about 1 m. It was suspected to have released during last weekend’s storm followed by sun and likely on the same weak layer. Be particularly cautious with this layer on Wednesday, as the sun may periodically shine along with rising freezing levels.
Snowpack Summary
10 cm of new snow has been redistributed from strong north winds followed by southwest winds. This overlies variable surfaces, including old wind slabs at high elevations, a sun crust on south aspects, and a temperature crust on all aspects. Ridgelines are heavily corniced and if they fail, they could trigger slabs on slopes below.A weak layer consisting of surface hoar, facets, and/or a melt-freeze crust from late March is now buried about 60 to 140 cm. This layer is spotty in its distribution and has recently produced large and destructive avalanches. It is mostly likely to be problematic on west, north, and east aspects between 1900 m and 2250 m. Professionals are treating it seriously and avoiding terrain because of it.Below this, the snowpack is well-settled and strong.
Problems
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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.