Regions
Northwest Coastal.
Thursdays danger ratings are based on the storm arriving fairly late in the day. If it arrives earlier, the danger could quickly rise to high.
Confidence
Fair - Timing of incoming weather systems is uncertain on Thursday
Weather Forecast
The first of three distinct waves of well organized precipitation accompanied by strong S/SW winds should impact the North Coast Thursday afternoon. The second wave should arrive mid-day Friday with the final wave colliding into the region Saturday. Cold Arctic air mixing should keep the freezing level right around 500m throughout the storm. With 25 to 95 mm of water expected from the storm, total alpine snow totals in excess of one meter aren't out of the question.
Avalanche Summary
On Monday and Tuesday wind and storm slab avalanches to size 1.5 ran both naturally and with human provocation. Sluffing from steep terrain was also reported.
Snowpack Summary
30- 50cm of new snow has fallen in the last few days. Strong winds have redistributed this snow in exposed terrain forming winds slabs in leeward features and stripping exposed slopes. The late-January crust is typically down 100-140cm in the south of the region and about half this depth in the north. Tests are indicating that this interface is generally well bonded but isolated avalanches have been reported to have released on this layer recently. The mid-January rain crust and/or surface hoar layer is down over 150cm in the south and has generally become inactive. It may still be a concern in thinner snowpack areas. The November crust/facet combo near the bottom of the snowpack is thought to be generally well bonded. However, this deep and destructive layer is still reported to be reactive in the far north 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.