New snow on Tuesday will stress several weak layers, creating dangerous avalanche conditions. The potential for large destructive avalanches needs to be accounted for in your travel plans.
15-30 cm of new snow with strong to extreme wind from the west will form fresh and unstable wind slabs on Tuesday. Wind slab avalanches in motion could step down to buried weak layers and release very large persistent or deep persistent slab avalanches.
30 to 50 cm of settled snow rests on a widespread layer of buried surface hoar. While this weak layer is thought to be present at all elevations, these slabs have been most sensitive to triggering at treeline which is a setup that can catch us off guard. Watch for deeper slabs at ridgecrest too.
Recent loading from new snow and wind has aggravated the deep persistent slab problem. Some very large and destructive avalanches have been sporadically failing on deeply buried weak layers, predominantly on north through east aspects in the alpine. Cornice falls are a likely trigger for these slabs.
A series of fronts hit the region Monday night, Tuesday night, and Thursday.
MONDAY NIGHT: Moderate intensity flurries with 5-15 cm of snow, strong to extreme wind from the west, freezing level drops from 1000 to 500 m.
TUESDAY: Periods of heavy snow with 10-15 cm of snow, strong southwest wind, freezing level around 700 m, alpine high temperatures around -6 C.
WEDNESDAY: Another 5 cm of new snow by the morning and then mostly cloudy throughout the day, moderate southwest wind, alpine high temperatures around -6 C.
THURSDAY: Scattered flurries with 3-8 cm of new snow, strong south wind, alpine high temperatures around -3 C.
Poor visibility since the arrival of stormy weather on Sunday has limited observations, however natural wind slab avalanches are likely occurring in alpine terrain. Over the past few days there have been reports of wind slab, persistent slab (failing on the February 19 surface hoar layer), and deep persistent slab (failing on basal facets) avalanches. This pattern highlights how the current stormy weather is aggravating multiple weak layers in the snowpack.
Some of the recent activity includes:
Extreme wind from the southwest and northwest have hammered open terrain, scouring some slopes and forming hard wind deposits in other slopes. Another 15-30 cm of snow by Tuesday afternoon accompanied with strong wind will form more fresh wind slabs.
30 to 50+ cm of settled snow overlies the Feb 19 surface hoar in sheltered locations and a sun crust on solar aspects, with the deeper slabs being found in the north of the region. The surface hoar may be most sensitive to human triggering at treeline which is a bit different setup than we're used to.
There are few different persistent weak layers (PWL's) we're watching in the mid and lower snowpack. A patchy layer of small surface hoar crystals and a sun crust both from early February are now 40 to 100 cm deep. A couple of weak layers that formed in January are buried in proximity to one another 80-140 cm below the surface. Below that, an early season crust lurks at the base of the snowpack. Some large avalanches have been triggered on these layers, usually during periods of rapid loading by new snow or wind or from large loads such as collapsing cornices. We're now in another one of those periods that could wake up the deep slab problem.
