Expect to see sunny breaks over the next few days. Brief periods of solar radiation could be enough to trigger stubborn persistent slabs or cause cornice failures. Conservative route selection remains crucial at this time.
Weather Forecast
A cool and unstable NW flow should bring light scattered flurries to the region on Friday. A ridge of high pressure builds on Saturday before a weak upper trough crosses the region Saturday night into Sunday morning.Friday: Cloudy with sunny breaks, scattered flurries 1-3cm, freezing level am: surface, pm: 500m, ridgetop wind: light NW-NSaturday: A mix of sun and cloud, mainly dry, freezing level am: surface, pm: 500m, ridgetop wind: light variableSaturday Night: Flurries 1-4cm, freezing level surface, ridgetop wind: light variableSunday: Cloudy with sunny breaks, mainly dry, freezing level pm: 800m, ridgetop wind: light W-NW
Avalanche Summary
On Wednesday, we received reports sluffing from steep terrain features. Also reported were four natural size 2 storm slab avalanches at treeline. These released down 30-40cm.We continue to see large natural avalanches stepping down to the deeply buried persistent weak layers. On Tuesday there was one report of a size 2.5 remotely triggered avalanche where a group of skiers felt a massive whumpf at the top of a slope and realized that they triggered an avalanche on an adjacent slope almost 500m away. We're still seeing relatively small inputs, like periods of strong winds or brief sunny breaks, trigger deeper persistent slab avalanches. Many of these events have occurred on southerly aspects from slopes in the alpine or at treeline. Extra caution is advised when the sun is out.
Snowpack Summary
Around 30-40 cm of recent storm snow has fallen in the past few days. There may be a variety of layers within the storm snow from thin sun crusts, to graupel balls, and maybe even recently buried surface hoar from brief clearings overnight. Also, expect fresh wind slabs in exposed lee and cross-loaded terrain. This new snow now sits on a surface hoar and/or sun crust layer. Some recent snowpack tests give moderate "pops or drops" shears on the interface between the recent storm snow and previous surface. A couple other notable persistent weaknesses remain a concern in the mid-snowpack. The surface hoar/sun crust combo buried in early March is now down around 60-80 cm. The early-February surface hoar/facet/crust combo is down 90-150cm and widespread throughout the region. Surface avalanches in motion and cornice fall have the potential to initiate an avalanche on this deeply buried weak layer. The mid and lower snowpack remains weak and facetted in areas with a relatively shallow snowpack.
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.
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.