Extreme winds and locally intense snowfall is expected Monday evening. If the system arrives earlier, danger ratings will quickly ramp up to High at Treeline.
Confidence
Fair - Intensity of incoming weather is uncertain on Tuesday
Weather Forecast
A trough of low pressure anchored over SW BC allows for moist SW flow for the forecast period. Snowfall picks up again around 10 pm Sunday night but totals stay meager, I expect 2 - 4cm by Monday morning. Monday morning should be cool, crisp and beautiful as dawn is greeted by clear skies with freezing levels at valley bottom. Winds are expected to be strong at ridge top out of the W, dropping down to moderate speeds at treeline. Freezing levels begin to creep up around lunch time topping out around 1000m Monday afternoon. Remnants of a deep pacific low slide through the region Monday afternoon/evening bringing strong SW winds at ridge top and moderate winds at treeline. I expect 5 -10 cm Monday evening. Snow continues through the day Tuesday, another 5 - 10 cm are expected with winds out of the SW; Moderate to Strong at ridge top, light below treeline. Snowfall should taper a bit Tuesday afternoon. A brief ridge of high pressure is in place Wednesday morning, but, the current models indicate more snow arriving Wednesday afternoon.
Avalanche Summary
Limited visibility Saturday meant that we didn't see many avalanche observations. It sounds like the new snow was susceptible to skier & sledder triggering on steep convex features, producing avalanches to size 1. The following observations are from Thursday/Friday: In the dogtooth backcountry, observers reported a size 2.5 avalanche with a crown 1m+ in depth, that was likely human triggered. Explosive control work once again produced large avalanches to size 3.5 on all aspects with crowns ranging from 40 - 120 in depth.
Snowpack Summary
Another 6 cm of snow fell Saturday night bringing the weekend storm snow total up to 16cm of moist snow. Much of the snowfall has been accompanied by light to moderate SW winds. Freezing levels topped out around 1800m Saturday, with the top 10 cm moistening as a result. This new snow covers old pencil hard wind slabs which were created by last week's strong W/SW winds. These wind slabs are getting more stubborn, and it likely takes a big trigger, like a falling piece of cornice to get them going. That being said, they may still be sensitive to human triggering in areas where the snowpack goes from thick to thin, rock outcroppings being the classic example of this kind of structure. The big story remains the early February Surface Hoar that is 90 - 160cm deep. The snow above this weak layer has been under the influence of warmth and time which has settled the snow into a thick cohesive slab 90 -160 cm in depth. Obviously, when a slab almost as tall as the average Canadian releases, the consequences are severe. Operators in the region have been diligently gathering data on this weak layer; In snowpack tests, the layer fails in a sudden planar fashion indicating that it has lots of energy in it, and showing that when it does fail, it has the potential to propagate across large distances. These tests mesh with the large avalanches that have been observed in the region recently. Conditions have been favorable for cornice growth recently, as a result many ridge lines are sporting large cornices. Basal facets have only been reactive on steep, shallow, and rocky slopes, but operators continue to monitor this layer in tests. Triggering this deep persistent weak layer is unlikely, but shallow snowpack areas or shallow weak areas adjacent to deeper wind loaded slopes are the most suspect locations.
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.
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.