Purcells

New storm problems combined with the persistent slab problem means tricky conditions for the weekend. Use a conservative approach if you are heading into the mountains.

Natural and human-triggered wind slab activity to size 1.5 was observed in response to new snow and wind on Friday night. With more snow and wind forecast for Sunday, I would expect the developing storm slab to increase in size and reactivity. An avalanche in motion may also "step down" to a deeper, more destructive weak layer which was buried in mid-December.

Fair - Intensity of incoming weather systems is uncertain on Sunday

A moist pacific frontal system will cross the region on Sunday bringing moderate to locally heavy amounts of snow. Weather models are in high disagreement with forecast snowfall amounts ranging from 15cm to 30cm falling between Sunday morning and Sunday night. Unsettled conditions with light flurries are expected on Monday before a dry ridge of high pressure builds on Tuesday. Winds are expected to be strong from the southwest on Sunday, dropping to light and westerly on Monday and Tuesday. Freezing levels will peak at about 1300m on Sunday and Monday, and then drop to surface on Tuesday.

Between 10cm and 30cm of new snow has buried widespread 10-20mm surface hoar, a sun crust on steep sun exposed slopes, and/or wind affected surfaces in the alpine. More wind and snow forecast for Sunday will add to the size and reactivity of the developing storm slab. The problematic mid-December surface hoar/crust layer is typically down 40-80cm below the new snow and remains sensitive to human triggering is some areas. The reactivity of this persistent weak layer appears to be quite variable but still has the potential for large, destructive avalanches in some areas. The layer appears to be the most reactive at and below treeline. The mid-Nov weak layer is near the bottom of the snowpack but has generally become inactive.