Southern areas are expected to receive most of the new snow and wind. Where there's more than 20 cm of new snow Friday, Avalanche Danger Ratings may be higher than forecast.
Confidence
Moderate - Intensity of incoming weather systems is uncertain
Weather Forecast
A weak front is stalling on a north to south axis over the region setting up a southerly wind with most snow expected in the southern parts of the region.Friday: The important part of the weather forecast is new snow and wind overnight Thursday: up to 10-15 cm of new snow that fell with moderate south or southeast winds. Calmer and drier through the day. Temperatures staying on the negative side of zero. Possible sunny breaks, especially in the north.Saturday: Mix of sun and cloud (sunnier in north, cloudier in south) with continued flurries or light precip (drier in the north, snowier in the south). Little wind and temperatures at treeline staying around or below -5 C.Sunday: Compared to Saturday, Sunday looks a little bit sunnier, dry throughout the region, a bit windier (light from the SW), and similar temperatures.
Avalanche Summary
On Wednesday only sluffing (Loose Dry) or "No New Avalanches" were reported by avy pros. Prior to that avalanche activity consisted of mainly wind slabs (skier triggered and skier remote) in the size 1-1.5 range. However, we've also received reports of persistent slab avalanches (to size 3) where recent accumulations have been the highest. These avalanches, were skier-triggered or remotely triggered (from a distance) on Monday and naturally occurring on Tuesday with wind loading on cross-loaded slopes. We suspect the buried sun crust buried mid-February. Deep persistent avalanche activity has become less frequent over the past week.
Snowpack Summary
Up to 40 cm of recent storm snow (in the south and west parts of the region) was redistributed into wind slabs in wind-exposed terrain by south and southwest flow. Up to 60cm below the surface there's a layer buried mid-February that's a sun crust on solar aspects, and spotty surface hoar on sheltered slopes. This layer looks most concerning on on solar aspects where it's associated with with small facets or surface hoar above.There are several deeper layers that have shown signs of improving but still remain suspect as low probability - high consequence avalanche problems. I'm talking about surface hoar layer buried back in December and January. Near the base of the snowpack is a November crust combined with loose sugary snow. These layers may "wake-up" with strong inputs such as sustained warming, sustained snowfall, large triggers (e.g. cornice fall or smaller avalanches coming down from above); human triggering is also possible in shallow snowpack areas with convoluted terrain.
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.
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.