Last week's snow and wind has activated the deep persistent slab, the potential for these large destructive avalanches needs to be accounted for in your travel plans this weekend. It feels like our snowpack is primed for human triggering right now.
30 to 50 cm of storm snow rests on a widespread layer of buried surface hoar. These slabs have been most sensitive to triggering at lower elevations (at and below treeline) which is a setup that can catch us off guard. Wind fueled storm slabs can be found in the alpine and may be most sensitive on solar aspects where they rest on a sun crust.
Consistent dribs and drabs of snow this week accompanied by strong southwest 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.
Out of the frying pan and into the fire, it looks like we’re in for another series of significant storms this week.
SATURDAY NIGHT: Freezing level at valley bottom, light west/northwest wind, trace of snow possible.
SUNDAY: Overcast, freezing level rising to about 1100 m, strong west/northwest wind, 1 to 5 cm of snow possible during the day with another 1 to 5 cm Sunday night.
MONDAY: Overcast, freezing level rising to about 1300 m, strong west/southwest wind, 1 to 5 cm of snow possible during the day, 5 to 10 cm of snow possible Monday night.
TUESDAY: Overcast, freezing level around 900 m, strong southwest wind, 1 to 5 cm of snow possible during the day, 1 to 5 cm of snow possible Tuesday night.
It's been an active period for avalanche activity. On Friday a natural cornice failure produced a size 3.5 deep persistent slab on a northeast facing feature at 1800 indicating that the deep persistent slab problem continues to be active. Human triggered wind slab avalanches up to 30 cm in depth were running on the February 19th surface hoar too.
On Thursday natural avalanches to size 2 were observed running on the February 19th surface hoar.
Observations were limited to a few small (size 1) skier triggered slabs on Wednesday. These were triggered on wind loaded slopes and produced 30 cm thick slabs that ran on the February 19 surface hoar.
Some very large and destructive avalanches have been sporadically running on deeply buried weak layers near the bottom of the snowpack, predominantly on north through east aspects in the alpine. On February 17 and 18 there were three large (size 2.5-3) deep persistent slab avalanches in thin rocky terrain. Steady loading over the past week has once again aggravated these deeper layers.
30 to 50+ cm of snow now 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 alpine has been heavily wind affected as evidenced by this MIN submission. The surface hoar may be very sensitive to human triggering at lower elevations 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 and by large loads or from thin, rocky areas. We're now in another one of those periods that could wake up the deep slab.
