The weather is changing and so is the avalanche danger. An Atmospheric River over BC and AB will deliver strong winds, warming and up to 20 cm of snow (rain in the valley) = chinook conditions on the east slope and pounding snow and rain to the west. Avalanche danger rises on Thursday, again on Friday and peaks on Saturday. Expect windslab development and a reawakening of the deep persistent problem.
With up to 20 cm of loose snow available for wind transport, Thursday's increasing winds will create fresh, sensitive windslabs in leeward areas. Expect this to build by Thursday afternoon, growing even larger on Friday as the winds continue.
The lower snowpack is comprised of weak facets, with a slab formed over this weakness at treeline and in the alpine. This weak basal layer is uniform across most of the region and has the potential for human triggering, long propagations and remote triggering. This problem may wake up over the next few days with warming and wind.
The sun was out! Two natural avalanches reported on S aspects on Wednesday: Mt. Ball S face had a size 3 natural (observed from a distance) and Bourgeau Left-hand waterfall released a size 2 natural that started up high in the rocks. Minimal results from avalanche control at the ski areas. No new avalanches observed by our field team on the Ogden Bench.
10-20 cm of loose snow sits over a denser mid-pack which has formed a slab over the weak facet layers below - this loose snow will get blown into windslabs starting Thursday. The December 17 weak layer is down 25-60 cm and becoming less reactive. The November 16 deep persistent layer of facets and depth hoar is near the base of the snowpack, continuing to show sudden failures. Puzzle Peak snow profile from Tuesday.
The weather pattern is changing as an Atmospheric River establishes itself across southern BC and AB. The coast and interior ranges will see heavy rain, snow, wind and warming. Here in the Rockies, we expect the winds to start on Thursday and reach 80 km/hr by Friday. Some snow and perhaps rain at low elevations with 5-10 cm by Friday. Expect warm and windy weather!
