Winter is returning to the alpine and with it comes new storm instabilities. Storm slabs may be touchy, especially in wind loaded areas. Buried persistent weak layers remain reactive and still have the potential to produce very large avalanches.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
A cold front crosses the interior on Tuesday bringing light precipitation with cooler temperatures. Models are currently showing 5-10mm of precipitation. Freezing levels are expected to stay high Monday overnight and progressively fall to around 1500m by Tuesday afternoon. Alpine winds are forecast to be moderate-strong from the SW switching to the NW. On Wednesday, mainly dry and cloudy conditions are expected with light flurries and possible sunny breaks. Freezing levels are expected to drop below 1000m Tuesday overnight and reach around 1500m on Wednesday afternoon. Alpine winds are forecast to be moderate from the NW. Similar conditions are expected for Thursday with a mix of sun and cloud, light scattered flurries, and freezing levels around 1500m.
Avalanche Summary
On Saturday and Sunday, isolated natural size 2-3 persistent slabs were reported. These occurred on a variety of aspects and mainly above 2000m. At lower elevations, lots of loose wet avalanches were reported. On Thursday and Friday, a widespread avalanche cycle occurred in response to wet and windy conditions. These were a mix of storm slab, loose wet avalanches and persistent slabs up to size 3. Natural avalanches are possible on Tuesday as new snow falls at higher elevations and rain falls down low. Human-triggered storm slabs and persistent slabs are a major concern, especially on steep, unsupported slopes. At lower elevations, be cautious of sluffing from steep slopes when it is raining.
Snowpack Summary
5-15cm of recent snow overlies a wet layer or a crust that formed on Friday when it rained into the alpine. This new snow is moist to around 2100m. In the alpine, strong winds have redistributed the new snow and wind slabs can be expected in leeward terrain features. Several persistent weak layers exist in the upper 1m of the snowpack and remain a serious concern. The mid-March crust/facet layer is down around 60cm and remains reactive to light triggers. Down around 80cm is the mid-February crust/facet/surface hoar layer which has also been reactive to heavy loads like avalanches in motion stepping down, cornice failures, or explosives. Both layers have the potential for wide propagations and very large avalanches are possible.
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.
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.