We should see sunny breaks over the next few days. Brief periods of solar radiation could be enough to trigger stubborn persistent slabs or cause cornice failures. Conservative route selection is crucial at this time.
Weather Forecast
Thursday: Cloudy with flurries and sunny breaks – up to 10 cm in places. The freezing level is around 600-800 m and ridge winds are light gusting moderate from the NW. Friday: A mix of sun and cloud. The freezing level is around 600-800 m and ridge winds are light from the NW. Saturday: Mainly cloudy with sunny breaks and a chance of flurries. The freezing level is around 800 m. Ridge winds are light.
Avalanche Summary
On Tuesday there were several reports of natural slab avalanches up to size 2.5 that all appeared to fail on the mid-March interface or within the new storm snow. There were also a couple reports of cornice falls, in some cases triggering slabs below. We're still seeing relatively small inputs, like periods of strong winds or brief sunny breaks, trigger deeper persistent slab avalanches. Many of these events have occurred on southerly aspects from slopes in the alpine or at treeline. Extra caution is advised when the sun is out.
Snowpack Summary
Currently there is around 30-60 cm of recent storm snow sitting on the mid-March interface, which is commonly described as a sun crust/surface hoar sandwich. Recent snowpack tests show variable results on this weakness, with mostly hard shears where the storm snow overlies surface hoar. Periods of gusty SW-NW winds have redistributed some of the new snow forming soft wind slabs in exposed lee terrain.The early March persistent weak layer, which is a crust mixed with facets or surface hoar in places, is now down around 80-120 cm. This layer has been most reactive on southerly aspects. The mid-February crust/facet/surface hoar layer is widespread throughout the region, but the depth varies significantly. It still producing sudden "pops or drops" results in snowpack tests where it is less than 150 cm deep. The biggest concern with this interface is being able to trigger it where it is less deep and having it propagate into thick snowpack areas producing very deep and large avalanches. Cornices are also large and fragile at this time and could pop off during sunny breaks.
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.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.