South Columbia

A storm on Sunday will increase the Avalanche Danger. Conservative terrain selection will become increasingly important.

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 1400m on Sunday and Monday, and then drop to valley bottom on Tuesday.

Natural and human-triggered wind slab activity to size 2 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.

Up to 20cm 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 snow and wind forecast for Sunday will increase the size and reactivity of the developing storm slab. The problematic mid-December surface hoar/crust layer is typically down 80-120cm below the surface, and may still be sensitive to human triggering. 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 in the south of the region at and below treeline.