High danger expected for areas where new snow or rain, warm temperatures and high winds come together. Some parts of this region may see less precipitation, in which case the danger may be indicated slightly too high.
Confidence
Fair - Intensity of incoming weather systems is uncertain
Weather Forecast
A warm, wet storm is expected to carry through from Friday night into Saturday morning. Friday night is expected to see 15-20 mm snow or rain, with freezing levels between 1500 and 2000 m. Saturday: during the day it looks as though there will be a brief clearing, before the next storm starts again in the afternoon. A further 5-10 mm is expected by the end of Saturday with freezing levels around 2400 m. Sunday: a further 10-20 mm is expected, with freezing levels remaining around 2400 m. Very strong southwest winds expected througout the period.
Avalanche Summary
Forecast snow, rain, warm temperatures and high winds are a recipe for an avalanche cycle, especially when you throw in a couple of prominent weak layers. I anticipate an active cycle this weekend. Previous avalanche activity during and after the last storm system was widespread, with avalanches releasing up to size 3 on a variety of aspects and elevations.
Snowpack Summary
Moist new snow and rain forecast for this weekend will fall on a moderately dense upper snowpack. A weak layer that comprises feathery surface hoar crystals up to 15 mm lies buried between 40 and 60 cm below the surface, and may sit above a crust, particularly on south-facing slopes. This "mid-January" weak layer is widespread and has been the main focus for recent avalanche activity and I anticipate it to be active through the weekend as it is loaded by new snow and rain.The still problematic mid-December surface hoar/crust layer is now buried anywhere from 60 to 110cm below the surface and continues to produce clean shears at and below treeline. Avalanches were reported to have stepped down to this layer during and after the most recent storm cycle. With additional loading and warm temperatures, I'd continue to treat this layer with respect and suspicion, particularly in the 1700-1900 m elevation band. A release on this layer would have very high consequences.
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.