Warming and solar radiation will continue to drive the Avalanche Danger on Tuesday. Avalanches will become more frequent and destructive throughout the day.
Weather Forecast
On Tuesday expect mainly clear skies, and freezing levels at about 3500m. On Wednesday the region should see increased cloud and freezing levels dropping to about 2200m. Moist snowfall (5-10cm) is forecast for Wednesday night and Thursday. Ridgetop winds should remain mainly moderate from the southwest for the forecast period.
Avalanche Summary
On Sunday, a size 3 persistent slab avalanche was triggered naturally in the Birkenhead Lake area. The aspect and elevation are unknown; however, the mid-January surface hoar was the likely culprit. This speaks to the ongoing touchy persistent slab problem in the north of the region. On the same day, 2 people were partially buried and 1 person was fully buried in a size 2.5 slab avalanche on a south-facing alpine feature in Marriott Basin. The exact failure plane of the slide was unknown, but warming was thought to have played a major role in initiating the human triggered avalanche. Hats off to everybody who pulled-off a successful rescue. Check out the great Mountain Information post for more details. At the time of publishing this bulletin on Monday, observations were extremely limited but I suspect there was a very widespread natural loose wet avalanche cycle in response to warming and solar radiation. Similar action is expected on Tuesday. Warming will also increase the likelihood of triggering wet slabs and deep and destructive persistent slabs.
Snowpack Summary
Continued high freezing levels and solar radiation will leave most surfaces moist or wet on Tuesday. This warming will also continue to have a significant destabilizing effect on the snowpack. On Friday between 25-40cm of new snow fell. Strong southwest winds shifted these accumulations into deep and dense storm slabs. Cornices were already large before the storm, and new growth is expected to be fragile. About 50-80cm below the surface, you'll likely find a rain crust which formed on January 28th. This crust is widespread and exists up to about 1900m. Previous wet and heavy storms have likely flushed out the deeper mid-January surface hoar in the south of the region; however, this weakness which lies between 60 and 130cm below the surface is still reactive at higher elevations in the Duffey, Hurley and Birkenhead areas. This layer continues to produce sudden results in snowpack tests and has been responsible for recent destructive avalanches.
Problems
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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.
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.