New snow and cornice growth receiving direct sun on Thursday morning will be particularly sensitive to triggering. In your route selection, account for the potential for human triggered storm slabs as well as the low probability/high consequence deep persistent slab problem.
Mild temperatures, strong winds and new snow have been a recipe for significant cornice growth recently. The new growth is still brittle and weak and will be especially sensitive to seeing the sun for the first time.
20-30 cm of snowfall was highly wind affected by strong southwesterlies as it fell Wednesday, with moderate northwesterlies overnight. Watch for deep wind-loaded pockets in lee features at alpine and treeline elevations.
As recently as Tuesday, explosives testing continues to periodically trigger a weak layer of snow at the bottom of the snowpack. This deep persistent layer has proven that if triggered, it will produce very large and destructive avalanches.
Wednesday night: Clear skies. Light to moderate west to southwest wind. Alpine low -8 C. Freezing level 800 m, valley bottom further inland.
Thursday: Mix of sun and cloud with flurries starting in the afternoon bringing 5-15 cm new snow. Southwest winds increasing to strong. Alpine high -4 C. Freezing level 1100 m.
Friday: 20-50 cm new snow with rain below 1300 m. Strong southwest winds, approaching extreme at ridgetop. Alpine high 0 C. Freezing level rising 1300-1900 m.
Saturday: 40-100 cm of new snow in the alpine. 60-160 mm water equivalent in precipitation will start as rain below 1600 m, turning to snow through the day for elevations above 500 m. Strong to extreme southwest winds easing to light. Freezing level peaking at 1900 m in the early morning, dropping to valley bottom by Sunday morning.
Explosive control work Sunday through Tuesday produced mostly size 1-1.5 storm slabs and size 2 cornices with up to 75 m wide propagation. Natural windslab and cornice failures up to size 2.5 were observed in the Whistler backcountry on lee and cross-loaded alpine terrain.
The deep persistent problem again reared its head on Monday with large explosive loads producing a size 2.5. in shallow, rocky terrain.
20-30 cm of new snow has been highly wind affected in the alpine and open areas at treeline. Suspect wind slabs in leeward terrain and cornice growth at ridgetops. Below treeline, fluctuating freezing levels have left a combination of moist and crusty surfaces.
Weak faceted grains and crusts near the base of the snowpack continue to be a concern in inland parts of the region such as the Spearhead Range.
