We're still managing a midwinter snowpack and the persistent slab problem requires thoughtful terrain selection.
Weather Forecast
A seasonably cool weather pattern is expected to stick around for the foreseeable future and we may pick up a few centimeters of snow over the next few days too. THURSDAY: Scattered cloud cover, freezing level holding near valley bottom, very light northerly wind, no snow expected. FRIDAY: Broken cloud cover, freezing level holding near valley bottom, light southerly wind, trace of snow possible. SATURDAY: Broken cloud cover, freezing level beginning near valley bottom rising to about 700 m, light variable wind, trace of snow possible.
Avalanche Summary
On Tuesday reported activity was limited to sluffing in steep terrain and a natural cornice failure that produced a size 2.5 slab avalanche on a northeast facing slope at 2500 m.Explosives control in the region yielded several persistent slab (size 3) and shallower storm slab (size 1.5) releases on north to northeast aspects in the alpine on Monday. Persistent slabs had 90 cm fracture depths with the shallower storm slabs at 40 cm.Reports from Sunday included several large (size 2) recent natural persistent slab releases. These were noted on south aspects from 2000 to 2300 metres, with failure planes at the late-march layer mentioned in our snowpack discussion.Reports from last week showed evidence of a natural avalanche cycle during and following the storm, with storm slabs in the size 2-3 range reported on all aspects between 1700 and 2500 m. South aspects were the most reactive with numerous large and very large (size 2.5-3.5) avalanches running on the buried sun crust.
Snowpack Summary
Tuesday night's storm brought up to 10 cm of cold snow to the region with very little wind.Last week's storms brought totals of 60-100 cm of new snow to the region. The snowfall was initially accompanied by strong south winds and then followed by strong north winds, so a mix of old and stubborn and newer, more reactive wind slabs can now likely be found on a range of aspects at higher elevations. In sheltered areas, this storm snow has been gradually settling into a slab above a persistent weak layer buried in mid-March that consists of crusts at low elevations and on south aspects, and surface hoar on shaded aspects at higher elevations. The structure of storm snow above this layer isn't uniform, however, and recent storm slabs have been observed running on an interface down about 40 cm as well as at the late-March crust. Other persistent weak layers from early January and mid-December are still being reported by local operators, but are generally considered dormant.
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.
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.