HEADS UP!!A storm is to bring 15-30cm with up to 90km/hr winds in the alpine. This will create extensive wind slabs sitting on a weak base. Perfect recipe for avalanches.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Wednesday
Weather Forecast
Big change on its way. We are expected to have 10-15cm of new snow by Wednesday morning with additional amounts of 5-15cm during the day on Wednesday. The big kicker is that the winds will be strong from the west. We are talking of winds up to 90km/hr. Temperatures will be steady around -10c.
Avalanche Summary
No recent avalanches were observed on Tuesday Dec 11. We are likely to see some with this storm coming.
Snowpack Summary
The strong westerly winds have been blowing since Monday morning. On Tuesday, these winds were fairly constant in the 70km/hr range. The few cm of snow that has fallen on Tuesday with whatever was kicking around has now turned into widespread wind slabs in the alpine and at tree line. With the 10-15cm of snow forecast by Wednesday morning and the strong west winds, expect extensive wind slab development on all exposed slopes. More snow and 90km/hr winds for Wednesday will continue to grow the wind slabs. Expect these new slabs to be touchy and if they do release, they will probably step down into the weak base layers and involve the whole snowpack.
Problems
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.
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.