A storm on Sunday will increase the Avalanche Danger. Conservative terrain selection will become increasingly important.
Confidence
Fair - Intensity of incoming weather systems is uncertain on Sunday
Weather Forecast
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 1100m on Sunday and Monday, and then drop to surface on Tuesday.
Avalanche Summary
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.
Snowpack Summary
Up to 25cm of new snow fell on Friday night. The new snow, which has been distributed into deeper deposits in higher wind-exposed terrain, overlies widespread 5-15mm 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 add to the size and reactivity of this developing storm slab. The problematic mid-December surface hoar/crust layer is typically down 50-80cm below the surface 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.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.