Unusual persistent slabs continue to produce very large human triggered avalanches. See the latest forecaster blog for a more in depth look at this situation: http://bit.ly/1HHQrK2
Confidence
Fair - Intensity of incoming weather systems is uncertain
Weather Forecast
A series of wet storms are set to impact the coast until Tuesday morning then a cooling trend. Saturday night: 5-10 mm precip can be expected between 4pm Saturday and 4am Sunday with freezing levels around 1700 m and strong southwesterly winds. Sunday: A further 10-20 mm precip is expected with freezing levels around 1900 m and southwest ridgetop winds gusting to 60 km/h. Monday: Another pulse looks to bring 10-15 mm precip with freezing levels around 1900 m and southwest ridgetop winds gusting to 60 km/h. Tuesday: Possibility of lingering storms that could bring highly variable precipitation. A cooling trend is expected to lower freezing levels to around 1000 m. Winds are expected to diminish.
Avalanche Summary
Over the past week, there have been a number of avalanches to size 2.5 occurring on the mid-March persistent weak layer. Some avalanches have occurred naturally and many have been remote triggered from up to 100m away.
Snowpack Summary
This region has a very serious persistent slab problem that exists under the upper layers of dense storm snow. Depending on how much recent snow has fallen, a facet/crust persistent weak layer buried mid-March lies anywhere from 40-120 cm below the surface. Avalanches have continued to run on this layer at upper treeline and alpine elevations, especially in the north of the region. Snowpack layers deeper than this critical interface are reported to be generally well-settled and strong.
Problems
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.
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.