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
Light flurries Sunday morning are expected to taper off early in the day. A weak ridge of high pressure will build Sunday night and should persist for most of Monday. Some weak disturbance may bring light flurries for Monday night but Tuesday is expected to be mainly dry. The next organized system is expected Tuesday night or Wednesday morning. Sunday: Mostly cloudy, flurries 0-2cm, freezing level am: surface pm: 600-800m, ridgetop wind: light NWMonday: A mix of sun and cloud, freezing level am: surface pm: 700-1000m, ridgetop wind: light variableMon. Night/Tuesday: Flurries overnight 3-8cm, freezing level am: 800m pm: 1600m, ridgetop wind: 20-40 km/h SE-SW
Avalanche Summary
Reports from Friday include a natural size 2.5 storm slab avalanche, a natural size 3 wind slab, natural cornice releases, and solar triggered sluffing from steep features. Generally, we are 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
A 40-70cm storm slab sits on the mid-March crust/surface hoar layer. There may be a variety of layers within the storm slab including thin sun crusts, graupel balls, and maybe even small surface hoar from brief clearings overnight. Expect wind slabs in exposed lee and cross-loaded terrain from recent SW-W winds. A couple other notable persistent weakness layers remain a concern in the mid-snowpack. The surface hoar/sun crust combo buried in early March is now typically down around 1m. The early-February surface hoar/facet/crust combo is down around1.5m and widespread throughout the region. Smaller avalanches stepping down and cornice falls 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.