A rapidly rising freezing level, rain, and strong warm wind are expected to initiate large natural avalanches that could run into areas we normally think of as "safe" in the valley bottom. Travel in or below avalanche terrain is not recommended.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain on Tuesday
Weather Forecast
Tuesday delivers a significant warm up to the Purcells, precipitation is expected to fall as rain as high as 2000 m, possibly even 2500 m. Wednesday is ever so slightly cooler and then we should return to a more seasonal temperature regime on Thursday. TUESDAY: Scattered cloud cover, freezing level beginning around 1000 m, may rise as high as 2500 m, moderate to strong southwest wind, 1 to 3 mm of precipitation possible.WEDNESDAY: A few clouds in the morning building to scattered cloud cover in the afternoon, freezing level beginning around 1500 m rising to about 2000 m, light west/southwest wind, trace of precipitation possible.THURSDAY: Broken cloud cover, freezing level beginning around 500 m rising to about 1500 m, light variable wind, 2 to 6 mm of precipitation possible.
Avalanche Summary
On Sunday ski cutting produced storm slabs to size 3 on north facing terrain between 2100 and 2300 m. Loose wet avalanches were reported from all aspects between 1500 and 2600 m to size 2.5. Explosive control work resulted in cornice failures to size 2.5. Loose wet avalanche activity is expected in increase significantly on Tuesday. On Saturday a skier got off line on an east/southeast facing alpine feature in the Invermere area and triggered a size 2.5 wind slab with a ski cut. They were able to ski out of the feature unharmed.
Snowpack Summary
Recent wind and cold snow have formed wind slabs 10 to 50 cm in depth in the alpine that sit on a mix of crusts and surface hoar. Test profiles show resistent planar failures at this interface. In wind sheltered terrain Saturday's new snow rests on a surface crust that exists on all aspects aside from true north below 2000 m. This crust extends up to around 2400 m on south facing slopes. The mid-March Persistent Weak Layer (PWL) is now 40 to 90 cm below the surface. This layer is composed of a crust on all aspects at low elevations. At upper elevations the layer presents as a crust on solar aspects (south and west) and buried surface hoar on polar aspects (north and east). The likelihood of triggering a deeper persistent slab avalanche is steadily decreasing but the consequence of doing so remains high. Solar aspects are expected to be the most suspect as the freezing level begins to rise Tuesday and Wednesday.Deeper persistent weak layers from January and December are still being reported by professional observers, but are generally considered dormant.
Problems
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.
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.