Destructive persistent slab avalanches remain a concern in the north of the region. Extra caution is required in the Duffey Lake, Birkenhead and Hurley areas.
Confidence
Moderate - Freezing levels are uncertain on Wednesday
Weather Forecast
On Wednesday expect mainly overcast skies. On Wednesday night and Thursday the region may see 3-5cm of moist snow at higher elevations with similar amounts accumulating again on Friday. On Wednesday the freezing level should drop from about 2800m to 2000m, and should remain at that elevation for the rest of the forecast period. Ridgetop winds should remain mainly moderate from the southwest.
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. On Monday, widespread loose wet avalanche activity to size 3 was observed in steep, sun-exposed terrain. The gradual cooling trend will make heat-induced surface avalanches less likely; however, very large persistent slab avalanches are still possible in the north of the region.
Snowpack Summary
Extremely high freezing levels and solar radiation have left most surfaces moist or wet. This warming has also had a significant destabilizing effect on the snowpack. Cornices are reported to be huge and collapse has become more likely with daytime warming. 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
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.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.