A buried persistent weak layer may roar back to life over the next few days as the freezing level rises and the sun comes out. South facing slopes are especially suspect and are best avoided at this time.
Confidence
Moderate - Timing or intensity of solar radiation is uncertain on Monday
Weather Forecast
Monday, most of BC will receive partly cloudy conditions with a few lingering flurries in the Columbia mountains. A warm system makes landfall Tuesday morning, which should open the door to rain and wet snow into the alpine on Tuesday and Wednesday.MONDAY: Broken cloud cover, freezing level beginning around 1200 m rising to about 1800 m, light to moderate west/southwest wind, 1 to 4 mm of precipitation possible. TUESDAY: Scattered cloud cover, freezing level beginning around 1400 m rising to about 2500 m, light to moderate south/southwest wind, trace of precipitation possible.WEDNESDAY: Scattered cloud cover, freezing level beginning around 1500 m rising to about 1700 m, light to moderate southwest wind, 2 to 5 mm of precipitation possible.
Avalanche Summary
No new avalanche activity to report from Saturday.On Friday a size 1.5 persistent slab avalanche with a 60 cm crown was skier triggered on a north facing slope at 2000 m. Natural loose dry and loose wet avalanches were observed on north and west facing slopes above 2000 m.On Thursday warming temperatures initiated large loose wet avalanches to size 3 on south facing aspects between 2100 and 2700 m in the neighboring Glacier National Park. A small shallow wind slab was accidently triggered by a skier on a steep/unsupported east/southeast facing feature at 2200 m too.
Snowpack Summary
5 to 10 cm of new snow fell Saturday with moderate southerly wind. This snow rests on a crust that is present on all aspects below 2000 m, and extends up to about 2500 m on south facing aspects. The storm snow be be sitting on surface hoar on polar aspects (north and east) at upper elevations. The main concern is the mid-March Persistent Weak Layer (PWL) which is now 60 to 110 cm below the surface. This layer is composed of a crust on all aspects at low elevations. At upper elevations the layer presents as a crust on solar aspects (those that face south and west) and buried surface hoar on polar aspects. The reactivity of this interface has been steadily decreasing and while the likelihood of triggering a deeper persistent slab avalanche is lowering, the consequence of doing so remains high. Solar aspects are expected to be the most suspect as the freezing level begins to rise over the next few days.Deeper persistent weak layers from January and December are still being reported by professional observers, 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.
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.