There is much uncertainty surrounding the March 10 Surface Hoar and how it will respond to Wednesdays storm. Hedge your bet and stick to simple terrain free of overhead hazard.
Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain for the entire period
Weather Forecast
Synopsis: A low pressure system approaches from the SW which should deliver strong wind and significant precip. A trailing cold front in it’s wake will bring the freezing levels down to valley bottom on Thursday. Wednesday: Freezing Level: 1500m. Precipitation: 15 /20mm – 30/40cm Wind: Strong SWWednesday night: Precipitation: 10/15mm – 20 – 30cm.Thursday: Freezing Level: Valley Bottom. Precipitation: Isolated convective flurries. Wind: Moderate to strong NW.Friday: Freezing Level: Valley Bottom. Precipitation: Isolated convective flurries. Wind: Moderate NW.
Avalanche Summary
Skier triggering to size 1.5 continued to be a problem Monday on steep unsupported features. A widespread slab avalanche avalanche cycle to size 2 took place on Sunday. The activity occurred in response to new snow and variable winds at all elevations. One notable occurrence was a size 3 slab avalanche which took place in the southeast corner of the region. The avalanche occurred on an east-northeast aspect at 2500m and was likely triggered by rapid loading from strong northwest winds. Observations were limited, but its size may suggest that a persistent weakness was the failure plane.
Snowpack Summary
Light to locally heavy amounts of new snow fell over the weekend and have have been distributed by northerly winds into deeper windslabs in lee terrain. These recent accumulations overlie a thick rain crust which is reported to extend up to 2000m. About 1m below the surface is a layer surface hoar or a sun crust buried on March 10th. Natural avalanche activity dropped off in the wake of the last storm but isolated natural activity and remote triggering is still occurring in some areas. I expect another round of natural activity on this layer as the coming storm adds a significant new load on Wednesday. At lower elevations where last week's heavy rain saturated the snowpack, subsequent cooling has formed crusts and has dramatically strengthened the snow.Weaknesses deeper in the snowpack have become unlikely to trigger.Widespread cornice development has also taken place, and cornices are reported to be very large and weak.
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.