Use increased caution during afternoon warming, especially on sun exposed slopes.
Confidence
Fair - Timing or intensity of solar radiation is uncertain on Wednesday
Weather Forecast
Synopsis: The South Coast should remain mostly dry under the ridge of high pressure until Thursday evening when a more potent front makes landfall.Wednesday: Freezing Level: 1400m - 1700m; Precipitation: Nil; Wind: Treeline: Light, Variable | Ridgetop: Moderate, NWThursday: Freezing Level: 1100m - 1600m; Precipitation: Trace; Wind: Treeline: Light, S | Ridgetop: Moderate, SWFriday: Freezing Level: 1100 - 1800m; Precipitation: 1:10mm - 1:15cm; Wind: Treeline: Moderate, SW | Ridgetop: Strong W/SW
Avalanche Summary
Warming temps drove a natural avalanche cycle to size 3 on Monday. It's important to note that many of these large avalanches were running on shady N-NE facing slopes between 1900 and 2500m. Included in this cycle was a size 3 at 2500m on a steep north facing slope that is suspected to have released on one of the deep persistent weak layers. Several smaller loose snow avalanches to size 2 were running in the recent storm snow on all aspects too.
Snowpack Summary
The weekend storm produced 30 to 50cm in the north and around 30 cm in the south. Mild temperatures and solar radiation on Monday/Tuesday are likely causing settlement of the storm snow and melting of the snow surface on most slopes expect maybe steep north aspects in the alpine. This settlement is helping this snow to bond to the underlying melt freeze crust that exists below 2100m, on all aspects with the exception being north facing terrain above 1500m where the crust is not present. The first part of the storm was accompanied by strong southerly winds that formed isolated wind slabs immediately below ridge crest.The persistent weak layers from early March and early February continue to be a concern. The March melt-freeze crust/surface hoar is reported to be down 60 - 100cm. The February crust/facet/surface hoar layer is now deeply buried down 150 - 250cm and may still be triggered by heavy loads like cornice falls or storm slab avalanches in motion. Avalanches releasing on these layers may step down to the ground during periods of strong solar radiation or warm spring rain.
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.