Heavy snow followed by warming temperatures and rain will push avalanche danger to HIGH by Monday and tip off a natural avalanche cycle.
Confidence
Fair - Freezing levels are uncertain on Monday
Weather Forecast
Synopsis: A strong Pacific frontal system will impact the Coast beginning on Sunday morning. Precipitation should start as snow at low elevations as the moist airmass clashes with the entrenched Arctic air. On Sunday we could see 20-40 cm of snow with ridge winds increasing to moderate or strong from the SW. On Monday temperatures are expected to climb, but how warm it will get is still somewhat uncertain. The freezing level could jump to around 2000 m. Heavy precipitation will continue with another 40-60 mm (or cm at higher elevations) possible. Tuesday looks even warmer with freezing levels rising as high as 2500 m. Moderate precipitation should continue.
Avalanche Summary
On Friday there was one report of a couple intentionally triggered size 1 wind slabs from 35 degree north-facing slopes at treeline. Avalanche activity will increase over the next few days as snow accumulates, winds increase, and temperatures rise. We could see natural storm slab avalanches step down to deeper weaknesses creating very large avalanches in some areas.
Snowpack Summary
Forecast snowfall will cover a highly variable snow surface that includes wind affected surfaces (crust, hard and soft slabs), a freezing rain crust or sun crust, and surface hoar or sugary facets in sheltered shady terrain. How well the new snow bonds to this surface is the question. Below this we also have to contend with the mid-December surface hoar or crust/facet weak layers. These are buried between 40 and 80 cm deep on average. The surface hoar layer may be found in sheltered shady areas at and below treeline, while the crust/facet layer could be more widespread but also more variable. Reports suggest is could be between 0 and 120 cm deep depending on exposure to wind. Heavy loading in the coming days could trigger these weaknesses creating very large avalanches.
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.