This is a tricky forecasting period, the region's not getting a ton of wind or snow, 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.
10 to as much as 60 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.
Batten down the hatches, the storm train is coming.
TUESDAY NIGHT: Freezing level around 500 m, strong southwest wind, 2 to 6 cm of snow.
WEDNESDAY: Overcast, freezing level rising to about 1300 m, strong southwest wind, 3 to 6 cm of snow during the day with another 4 to 10 cm Wednesday night.
THURSDAY: Overcast, freezing level holding around 1300 m, strong southwest wind, 2 to 8 cm of snow during the day with 5 to 10 Thursday night.
FRIDAY: Overcast, freezing level holding around 1000 m, moderate southwest wind, 1 to 6 cm of snow.
On Monday in the neighbouring Northwest Coastal 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.
In most of the region 10-15 cm of recent snow has been redistributed by strong wind, the alpine is starting to look pretty wind hammered as evidenced by this MIN submission. Snow totals are deeper in the far north of the region where there is 30 to 60 cm of recent storm snow. All the new snow rests on surface hoar in sheltered locations and a sun crust on solar aspects. 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 30-80 cm deep. A couple of weak layers that formed in January are buried in proximity to one another 80-140 cm below the surface and continue to give hard results in snowpack tests. 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 coming into 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 for a couple of days.
