This is a tricky forecasting period, the region's not getting a ton of wind or snow in one shot, but it's consistent and it could stack up enough to touch off another deep slab avalanche cycle in our tenuous snowpack. Human triggered avalanches remain possible at all elevations.
30 to as much as 90 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.
We're not getting a ton of snow in one shot, but consistent dribs and drabs of snow this week accompanied by strong southwest wind could reactivate the deep persistent slab problem. It's a bit of a moving target as we're not exaclty sure how much snow it will take to wake this problem up, but it needs to be on our minds this week. Some very large and destructive avalanches have been sporadically failing on deeply buried weak layers, predominantly on north through east aspects in the alpine.
Bits of storm continue to trickle into the region
WEDNESDAY NIGHT: Freezing level around 1200 m, strong southwest wind, 1 to 5 cm.
THURSDAY: Overcast, freezing level around 1100 m, strong southwest wind, 5 to 10 cm of snow with another 5 to 10 cm Thursday night.
FRIDAY: Broken cloud cover, freezing level holding around 1000 m, moderate southwest wind in the morning, light variable wind in the afternoon, 1 to 5 cm of snow.
SATURDAY: Clear skies, freezing level starting at valley bottom rising to around 1000 m, light north wind, no snow expected.
We're short on observations that depict the change in conditions since the storm entered the forecast area and there was no new activity to report from Tuesday. 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. A skier triggered size two avalanche was reported from a southeast facing feature at 1600 m, the avalanche ran on the Feb 19th surface hoar. On Sunday human triggered avalanches to size 2 were also failing on the February 19th surface hoar, crowns were 10 to 60 cm in depth.
On Saturday storm slabs resting on surface hoar failed naturally and were sensitive to skier triggering producing slabs up to 60 cm in depth. These slabs seem to be more sensitive at and below treeline which is the reverse of how we normally think about avalanche hazard.
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. Last Tuesday, a natural size 2.5 was observed. It is suspected to have started as a wind slab in a thin rocky area before stepping down. Last Monday, two deep persistent slab avalanches size 2-3 were triggered from thin snowpack areas by very large loads pushed by snowcats.
30 to 90 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 as we get 5 to 10 cm of snow and strong southwest wind consistently through Friday.
