Confidence
Fair - Timing of incoming weather is uncertain
Weather Forecast
A steady stream of weather systems will continue to deliver moderate to heavy precipitation, strong winds, and fluctuating freezing levels. Tuesday night through Wednesday: 30-50cm of snow. Very strong SW-W winds. Freezing level (FL) rising to 1600m overnight. Thursday: Snow beginning later in the day; 15-20cm. Moderate to strong SW-SE winds. FL lowering to 1000-1200m. Friday: Drier during the day but could see another system arrive in the evening. FL around 1000-1200m.
Avalanche Summary
There is one new report of skier triggered slab avalanche from Monday. It was intentionally triggered on a steep northerly slope near treeline, and may have released on buried surface hoar. I expect the size and likelihood of natural and human triggered avalanches to increase with the next weather system arriving Tuesday night.
Snowpack Summary
Wind-exposed slopes are predominately scoured and pressed, with pockets of wind slab from 30 to 60cm deep on North to East aspects at treeline and in the alpine.This new storm snow overlies a weak rain crust that formed last weekend. This crust extends to treeline elevations and overlies buried surface hoar and/or facets 10cm below. Generally speaking, confidence in this interface is growing, especially since it has'nt seen a significant load at the elevations where it exists. In the alpine where the crust does not exist, a stiffer wind slab may over-ride surface facets that formed during the early December dry spell. Persistent weaknesses seem to be less of a concern in the Cascades (Coquihalla and Allison Pass).The mid and lower snowpack are generally well-settled and strong.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
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.