Make a plan to step back from avalanche terrain and away from overhead hazard as the storm progresses and avalanche danger increases over the day. Then stick to it! Surface slides entraining recent accumulations or stepping down to a deeper weak layer could result in surprisingly large and destructive avalanches on Thursday.
In higher snowfall parts of the region, deep accumulations of new snow that haven't formed into a slab are likely to continue producing large sluffs with rider traffic on steep slopes.
Up to 25 cm of new snow by end of day and strong southwest winds will create an increasingly hazardous storm slab problem over the day on Thursday. Storm slabs may also entrain recent accumulations from the last storm to create larger, more destructive avalanches.
A weak layer of facets exists near the base of the snowpack. The likelihood of human triggering is low given the layer's deep burial depth, but storm slab avalanches in motion have the potential to step down. Suspect terrain for human triggering includes steep, thin, shallow, and rocky terrain, which is often found near ridgelines.
Improving observations from operators in the region observing up to 25% of their tenures since the weekend storm generally show no new avalanches. Reports closer to the immediate aftermath of the storm described numerous small dry loose avalanches and several storm slabs releasing with skier traffic and ski cutting, generally below treeline and limited to size 1 or smaller.
We expect some degree of a natural avalanche cycle also took place during the storm at higher, wind affected elevations, especially in areas that were on the higher end of our new snow totals. This MIN report from Westridge gives a good indication of conditions that were likely fairly widespread in the region.
A few very large (size 3.5 to 4) deep persistent slab avalanches were also observed over last week, which likely occurred during the very windy arctic outflow event.
Looking forward, moderate snowfall and high winds are expected to trigger an uptick in natural avalanche activity and human triggering potential through Thursday.
10-20 cm of new snow is expected to accumulate in the region by end of day Thursday. For the most part it will add to low density storm snow from the weekend, but it may also bury recent wind slabs in more exposed areas and likely a thin sun crust on sun-exposed aspects.
The roughly 50 cm of new snow from the weekend storm sits on an interface that includes small surface hoar in lower elevation sheltered areas and more widespread faceted (sugary) snow and heavy wind effect. This interface remains in question with storm slabs having been generally slow to form over it in recent days.
A small layer of surface hoar crystals may be found about 70 to 130 cm deep, particularly in areas sheltered from the wind around treeline. This layer had been gaining strength prior to the last storm and is currently considered dormant. We are still light on observations showing whether it did or didn't wake up in isolated areas from the weight of the recent snow.
Large and weak facets that formed in November are found near the base of the snowpack. Although the likelihood of triggering this layer is low at this time, the consequence of doing so remains very high. This layer is most likely to be human-triggered in thin, rocky slopes at alpine and upper treeline elevations.
Wednesday night
Cloudy with increasing flurries bringing 5-10 cm of new snow. Strong to extreme southwest winds.
Thursday
Cloudy with continuing snowfall bringing another 5-15 cm of new snow. Strong southwest winds easing over the day. Treeline high temperatures around -8.
Friday
Cloudy with easing flurries bringing up to 5 cm of new snow and storm totals to 20-30 cm. Light southwest winds. Treeline high temperatures around -9.
Saturday
A mix of sun and cloud. Light east winds. Treeline high temperatures around -8.
More details can be found in the Mountain Weather Forecast.
