The new storm is expected during the day. Danger levels are expected to increase during the storm. These danger ratings may be a little high for the morning unless the storm comes early.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Overnight and Wednesday: A strong frontal system is expected to move into the region from the Northwest during the day and bring 15-20 cm by evening. Strong West-Northwest winds are expected during the storm and the temperature should rise, but the freezing level should remain at valley bottom.Thursday: Continued moderate precipitation bringing another 10-15 cm. Temperatures continue to rise slightly and moderate Westerly winds continue.Friday: Light precipitation and moderate Northwesterly winds.
Avalanche Summary
Expect increased sluffing of dry surface snow and near surface facets before the new storm snow arrives. The new storm snow may become easy to trigger where it is sitting on buried surface hoar and/or wind crusts.
Snowpack Summary
A new storm is forecast to add moderate new snow amounts above the dry cold surface snow that recently buried the very cold and facetted snow that was on the surface during the arctic outbreak. There is a great deal of variation in the snowpack with respect to elevation and exposure to winds. Shallow snowpack areas may be weak and facetted, and may not support the additional load of storm snow. The old storm snow (~30-50cm) overlies the late-November interface which typically consists of sun crusts on steep south facing slopes and surface hoar in sheltered areas. These weak layers are expected to be persistent, although in many areas the upper slab is not yet stiff enough for slab avalanches. When temperatures rise, and we get additional snow, expect this interface to become more reactive.
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.