A warm storm is approaching for the weekend that is likely to drop rain onto an already weak snowpack. Eeek!
Confidence
Fair - Intensity of incoming weather systems is uncertain on Saturday
Weather Forecast
A succession of storms progressively get stronger and warmer through the weekend. Friday: Light rain or snow becoming heavier in the afternoon. Daytime amounts around 3-5 mm, with a further 10-15 mm likely overnight. Freezing levels 1000 m in the morning, climbing rapidly towards 2000 m in the afternoon. Ridgetop winds 40-60 km/h from the southwest. Saturday: Rain to all but the highest areas. 10-15 mm is the best current estimate. Freezing levels around 2300 m. Strong southwesterly winds. Sunday: More rain. Models currently indicate 15-25 mm. Freezing levels around 2500 m. Strong southwesterly winds.
Avalanche Summary
On Tuesday, which was after the storm had ended, there were a number of natural and human-triggered avalanches reported up to size 3. In some places the natural avalanches were thought to have been triggered as the sun started to shine. During the storm, avalanches up to size 3.5 were reported on a variety of aspects and elevations, most running on the most recent mid-January surface hoar layer. At least one avalanche in this region was reported to have stepped down to the mid-December layer.
Snowpack Summary
Between 30 and 50cm of new snow fell last weekend that was initially distributed into wind slab deposits. Since then, warm temperatures have settled the upper snowpack into a moderately dense slab. The new snow overlies a layer of surface hoar which may sit on top of a crust, particularly on sunny aspects. This "mid-January" weak layer is widespread and has been the main focus for most of the recent avalanche activity. It has also been consistently producing easy "pop" type failures in snowpack tests.The still problematic mid-December surface hoar/crust layer is typically down 90-130cm below the surface. The reactivity of this persistent weak layer appears to be quite variable but the potential for large, destructive avalanches still exists in some areas, particularly in the 1600-1900 m elevation band. The layer appears to be the most reactive in the south of the region and in zones where the surface hoar was observed to sit directly above a hard crust.
Problems
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.