This week's snow and wind could be aggravating 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 as 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 may be aggravating the deep persistent slab problem. It's a bit of a moving target as we're not exactly sure how much snow it will take to wake this problem up, but it needs to be on our minds this weekend. Some very large and destructive avalanches have been sporadically failing on deeply buried weak layers, predominantly on north through east aspects in the alpine.
A brief respite...
FRIDAY NIGHT: Freezing level at valley bottom, light wind generally out of the north, trace of snow possible.
SATURDAY: Scattered cloud cover in the morning building to broken cloud cover in the afternoon, freezing level holding around valley bottom, light northwest wind, trace of snow possible.
SUNDAY: Overcast, freezing level rising to about 1000 m, strong west/southwest wind, 1 to 5 cm of snow possible during the day, another 1 to 5 cm Sunday night.
MONDAY: Scattered cloud cover in the morning, overcast in the afternoon, freezing level rising to about 800 m, strong west/southwest wind, trace of snow possible during the day, 5 to 10 cm of snow possible Monday night.
We're short on observations that depict the change in conditions since the storm entered the forecast area.
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 Monday in the neighbouring Northwest Coastal region in the Ningunsaw zone a natural cornice failure resulted in a very large (size 3.5) deep persistent slab avalanche on a south/southeast facing feature. On February 17 and 18 there were three large (size 2.5-3) deep persistent slab avalanches in thin rocky terrain (one triggered naturally by a smaller wind slab and two by heavy loads pushed onto the slope by snowcats). Steady loading over the past week could be aggravating these deeper layers again.
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.
