Regions
Northwest Coastal.
There is dramatic variability across the region, and this forecast represents the areas hardest hit by the recent storms.
Confidence
Fair - Due to limited field observations
Weather Forecast
Monday: Expect a lull in the storm activity, with cloudy skies, light variable winds and alpine temperatures reaching -6. No precipitation.Tuesday & Wednesday: Several weak disturbances should make their way across the region, giving isolated bursts of precipitation under consistent cloudy skies. Expect light to moderate southerly winds and alpine temperatures should reach -5 each afternoon.
Avalanche Summary
Avalanche activity has picked up over the last couple days with the increase in load and temperatures. Many loose wet avalanches up to size 2.0 have occurred below treeline, where the recent precipitation has fallen as rain. Higher in the terrain both explosive and rider controlled triggering has been observed, predominantly in wind loaded areas. A single, large, cornice triggered natural was reported (size 3.0) on a north facing slide path in the Bear Pass region.
Snowpack Summary
Up to 40mm of water equivalency has fallen in the past 24-36hrs. Below 1000m, much of this precip has fallen as rain and we have reports in some areas that the upper 50cm of the snowpack is rain soaked. Above 1000m, the new snow is settling rapidly. Windslabs are forming in lee zones and behind ridges with the consistent SW winds. Cornices are growing rapidly under the current condition.A thin melt-freeze crust buried on January 17 is widespread at lower elevations. The combo of rainsoaked surface snow and this slick sliding layer may magnify the loose sluffing at lower elevations in the short term.Lingering deeper (down 50-80 cm) sits a persistent weakness comprising of surface hoar and/or facets. Recent tests on this layer are showing both sudden and resistant planar results. We have also seen an extended column test at 30/100 (end) indicating the propensity for propagation if this layer were triggered. Keep these layers on your radar in regards to distribution and reactivity. The mid and lower snowpack is generally well settled and strong, although basal facets remain a concern in the northern part of the region.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.