Static danger ratings belie a complex set of avalanche problems in our region. Old and new wind slabs on a variety of aspects as well as persistent slabs all demand your careful evaluation.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Monday
Weather Forecast
Sunday night: Flurries delivering 5-10cm of new snow with light westerly wind.Monday: Continuing flurries bringing another 4-7 cm of new snow. Winds moderate gusting to extreme from the southwest. Alpine temperatures to -8.Tuesday: Mainly cloudy with scattered flurries and up to 5 cm of new snow. Winds light gusting to moderate from the northwest. Alpine temperatures to -13.Wednesday: A mix of sun and cloud with no new snow. Winds light to moderate from the northeast. Alpine temperatures to -17.
Avalanche Summary
No new avalanches were reported on Saturday, but explosives control in the southern Alberta Rockies produced a Size 2 result in a heavily wind loaded start zone on Friday. A pocket of wind slab released to Size 1 under skier traffic in the same area on Friday. Wind slabs are expected to remain reactive to human triggers with new snow and ongoing winds keeping slabs touchy, as well as highly variable snowpack depths keeping trigger points easy to strike.
Snowpack Summary
New snow and wind over Sunday and Monday will recharge our wind slab problem, feeding existing slabs with additional load and forming new slabs in the lee of exposed terrain features. Forecast winds are from the southwest, but recent wind slabs formed under the influence of northerly winds. Yet another layer of wind slabs were formed on north and east aspects below our existing surface slabs. This mix of old and new wind slab problems will require backcountry travelers to be aware of slabs on virtually any aspect. Newer wind slabs will probably be easier to trigger, but the old wind slabs may continue to release with the added load of a rider, especially where they are sitting on a shallow, weak, sugary base. The mid snowpack is generally right side up, with the mid-December interface down 40-80cms, giving inconsistent results in snowpack tests. There is some faceting below this interface but resistances are good and showing signs of rounding (stabilizing). Travel conditions have been challenging (especially at lower elevations) and little change is expected until a significant warm up helps to settle the snowpack.
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.