Heads up, avalanche danger is rising tomorrow due to the forecast snow accumulations and winds.
Confidence
Poor - Intensity of incoming weather systems is uncertain
Weather Forecast
The deep offshore low pressure system should start to affect the region tonight bringing increasingly milder temperatures and increasingly more moisture throughout the bulletin period. The first pulse of moisture could leave around 5 cm of snow by tomorrow morning and up to 10 cm by the end of the day tomorrow with freezing levels rising but remaining at valley bottom for most of the time. Another wave of moisture should bring heavier precipitation on Friday but with freezing levels considerably higher (up to 2200m…). This pattern is expected to continue through Saturday. Forecast winds for the entire period are in the moderate to strong range from the southwest.
Avalanche Summary
No new avalanche reported today. Expect avalanches to be easily triggered and possibly some natural activity as the snow keeps accumulating during the day tomorrow.
Snowpack Summary
5 - 10 cm of new snow is expected to fall overnight till the end of the day tomorrow. This new snow will fall over another 5 cm which is sitting on a variety of surfaces; on a surface hoar and crust combo (that I will call the “late January surface hoar/crust” further down) in more sheltered areas, on a thin breakable crust in more wind affected areas or over windslabs lee of recent southwest winds in the alpine. The windslabs in the alpine and at treeline will keep building as the snow accumulates and gets transported on lee features by the southwest winds tomorrow. I suspect the bond between the storm snow and windslabs with the underlying late January surface hoar/crust layer will be poor. The mid-December crust/facet/surface hoar layer is down 40-85cm and is suspected to be reactive in isolated areas, especially where a slab of stiffer snow has consolidated above it. These areas are most likely located in the alpine, where windslabs and variable snowpack depth exist.
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.
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.