Purcells

Light new snow may create thin but touchy new wind slabs, but also obscure our stubborn, older ones. Since low visibility will likely funnel you into sheltered areas, be aware that buried surface hoar may be lurking in steep openings.

Saturday: Cloudy with isolated flurries and a trace of new snow. Light west winds, strong in the alpine.

Sunday: Cloudy with continuing isolated flurries and up to 5 cm of new snow (total) by end of day. Light to moderate west winds (strong in the alpine), shifting northwest. Alpine high temperatures around -8 with freezing levels rising to about 1000 metres.

Monday: A mix of sun and cloud. Light northwest winds. Alpine high temperatures around -8.

Tuesday: A mix of sun and cloud with cloud increasing. Light to moderate southwest winds (strong in the alpine). Alpine high temperatures around -6.

Over the course of the week following Tuesday's storm, we received a range of reports showing explosives control yielding many size 2 (large) storm slabs that were confined to the depth of new snow, but also of avalanches reaching size 3 in the central part of the region, lending support to the idea that some larger releases may have involved persistent weak layers.

Recent observations from the Quartz Creek area include several skier and rider triggered storm slabs to size 2 (large), particularly reactive on north-facing slopes around treeline. This is good evidence of a reactive layer of preserved surface hoar beneath our storm snow. Check out these MIN reports from the Quartz zone on Thursday.

Looking forward to Sunday, it's worth continuing to give recent wind slabs a wide berth as you approach wind affected terrain, and to remain suspicious of steeper sheltered slopes where new snow may have slabbed up over the above-mentioned layer of surface hoar. Using small test slopes to investigate the bond of new snow is a good plan, but be cautious about extending your observations to more committing terrain.

15-40 cm of snow fell in the region during Tuesday's storm. In exposed areas, elevated winds have redistributed much of this snow into wind slabs while adding to cornices. The storm snow buried more wind affected surfaces at upper elevations, but also surface hoar in wind sheltered areas and a thin crust on solar aspects. 

In most places, the storm snow seems to be forming a good bond with the previous surface. Exceptions are likely in areas harbouring surface hoar. Think of shaded upper below treeline and lower treeline areas, above where any thick surface crust exists and below more wind-affected elevations.

A couple of other persistent weak layers exist in the upper to mid snowpack:

• The upper layer (from late December), now about 60-100 cm deep, is a spotty layer of surface hoar at treeline and below. 
• The lower layer (from early December), found 90-140 cm down, is widespread and may present as decomposing surface hoar, but more likely as a combination of crust and faceted snow.

Avalanche activity on these layers has dwindled since last week, with snowpack tests results showing some variability between sudden and more resistant propagation.

The lower snowpack is characterized by more crusts, the most notable is a rain crust from early November that is near the base of the snowpack, surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers in steep, shallow, rocky areas with a thin to thick snowpack.

• Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
• Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
• Use small low consequence slopes to test the bond of the new snow.
• Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
• When a thick, melt-freeze surface crust is present, avalanche activity is unlikely.