Regions
Banff Yoho Kootenay.
Major warming event for Tuesday - freezing levels to 3500m under sunny skies. Avoid avalanche terrain and DON'T CLIMB UNDER SUNNY START ZONES! Avalanche control planned for Bourgeau, Whymper and Simpson paths - no access to these areas on Tuesday.
Weather Forecast
The most significant warming event of the winter will occur on Tuesday, as freezing levels are forecast to reach 3500 m and temperatures in the high alpine could reach +5 - warmer in sheltered areas when combined with clear skies and sunshine. Strong west winds (50 km/hr) in alpine areas.
Snowpack Summary
A 50-100 cm slab now overlies the January 6 weak layer of surface hoar, facets and sun crust and snowpack tests indicate an unstable bond between the two. Extreme warming and sunshine forecast for Tuesday will provide the necessary trigger, and slab avalanches are likely in many locations along with cornice failures and wet sluffs in gullies.
Avalanche Summary
The past 48 hours saw a widespread avalanche cycle of natural and human triggered avalanches up to size 3. Evidence of recent avalanches exists in many avalanche paths - most are slabs that failed 50-70 cm deep on the Jan 6 weak layer. Today we observed one fresh size 2 on Mt Whymper on a south facing, windloaded slope at 2400 (crown depth ~100 cm)
Confidence
Freezing levels are uncertain on Tuesday
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.
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.