Up to 60 cm of new snow could arrive by end-of-day Tuesday if we see the high end of forecast amounts. Good news for the snowpack, but also a good day to avoid avalanche terrain!
A crust / facet layer from mid-November was buried 50 to 100 cm before this storm. Storm slabs have potential to step down to this layer to produce very large, very destructive avalanches.
30 - 60 cm of new snow is expected to build a serious storm slab problem for Tuesday. Snowfall rates, temperatures, and danger all peak toward end-of-day.
Wet loose avalanches will become increasingly likely if the snow surface becomes moist or wet at lower elevations, especially where new snow sees rain.
Last Wednesday, a size 3 avalanche stepped down to a persistent weak layer near Allison Pass. See photo captions for more details.
Although new observations are scant, it's likely the atmospheric river has caused more persistent slab activity where this layer exists up high, and wet loose activity everywhere else there is snow.
Looking forward, a switch to heavy snowfall at higher elevations should form a reactive storm slab problem for above-threshold areas on Tuesday.
30 - 60 cm of new snow should accumulate above 1500 m by end-of-day Tuesday, with amounts increasing with elevation. This will covering a rain-soaked snowpack that was 80 - 140 cm deep ahead of the storm. There's not much snow below 1500 m.
A crust with facets, formed in mid-November, exists about 50 to 100 cm deep. Although all the rain ought to have cleaned out this problem where it existed, it's hard to gain much confidence in it with limited observations and as loading continues.
Monday Night
Cloudy with easing flurries bringing 5 to 20 cm of new snow. 40 - 60 km/h southwest ridgetop wind, easing. Treeline temperature -4 °C. Freezing level falling to 1200 m.
Tuesday
Cloudy with increasing snowfall bringing 25 to 40 cm of new snow. 25 - 45 km/h southwest ridgetop wind, increasing. Treeline temperature -2 °C. Freezing level rising to 1500 m.
Wednesday
Mostly cloudy with easing flurries bringing around 10 cm of new snow after 30 to 50 cm overnight (rain below about 1500 m). 50 - 60 km/h southwest ridgetop wind, easing. Treeline temperature -4 °C. Freezing level 1200 m.
Thursday
Mostly cloudy. 5 to 10 mm of mixed precipitation (rain below about 1800 m). 40 - 70 km/h southwest ridgetop wind, increasing. Treeline temperature 2 °C. Freezing level peaking at 2100 m.
More details can be found in the Mountain Weather Forecast.
