Avalanche Forecast

Low - Uncertainty is due to the limited number of field observations. Uncertainty is due to the fact that deep persistent slabs are particularly difficult to forecast.

THURSDAY NIGHT: 5-10 cm of snow as the storm tracks south of the region, strong southwest winds, freezing level drops form 1200 m to 500 m with alpine temperatures dropping to -8 C.

FRIDAY: Broken cloud cover with some isolated flurries, moderate southwest wind in the morning, light variable wind in the afternoon, alpine temperatures around -5 C.

SATURDAY: Clear skies, light north wind, alpine high temperatures around -5 C.

SUNDAY: Cloudy with light flurries and 2-5 cm of snow, moderate southwest wind, alpine high temperatures around -5 C.

We're short on observations that depict the change in conditions since the storm entered the forecast area. Observations are 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.

Prior to the storm (Saturday, Sunday and Monday), there were several reports of skier triggered avalanches also failing on the February 19th surface hoar. The slabs were size 1-2 with crowns that ranged from 10 to 60 cm thick. These slabs seemed 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. 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 as we get 5 to 10 cm of snow and strong southwest wind consistently through Friday.

• Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
• Be aware of the potential for larger than expected storm slabs due to the presence of buried surface hoar.
• Storm slabs in motion may step down to deeply buried weak layers resulting in very large avalanches.
• The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.