Regions
Northwest Coastal.
A complex pattern of wind slabs has developed at higher elevations. Below it, a persistent slab is waiting for the right trigger.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Saturday
Weather Forecast
Significant change is on the way, and it will be warming up as well. We're at the start of a stormy period for the Northwest, and the snow could really ramp up Sunday night. FRIDAY: Flurries bringing approximately 10-20 cm of new snow to the south and 20-30cms to the Stewart area (more in the western portions). Winds strong (60 Km/hr +) from the southwest. Alpine temperatures around -5 Celcius and rising overnight. SATURDAY: An additional 10-20 cm of new snow, with higher amounts in the north of the region. Winds strong (60 Km/hr) from the southwest. Freezing level rising to 1200 metres with Alpine temperatures around -2 Celcius. SUNDAY: A brief lull in the action - some flurries and up to 5cm snow. Freezing levels temporarily dropping to 1000m. Winds moderate to strong southwesterly. Alpine temperatures near -3 Celcius.
Avalanche Summary
With the temperature inversion yesterday (warming almost to 0 degrees at 1400m) there were natural avalanches observed on northern aspects around 1300m, running on the Jan 5/6 surface hoar layer.
Snowpack Summary
Recent strong winds (southeast through northeast) have redistributed the 20-40 cm of snow from Friday-Saturday at all elevation bands. This snow sits on a variable interface composed of hard wind slabs, weak surface hoar (Jan 5/6 layer) and faceted snow. The net result is touchy slabs on wind-loaded features that are giving easy to moderate sudden results in snowpack tests. Below the new snow, a well settled slab sits above the Christmas surface hoar layer which is well preserved in southern areas. This surface hoar is now buried 60-100 cm deep, and is still reactive in sheltered areas and steep open features at and below treeline. Deeper weak layers have only been reactive in areas with thin snowpacks. This includes a facet layer from early December that has been reactive in snowpack tests at lower elevations in the southern part of the region, and weak facets near the ground that have produced avalanches in the northern part of the region.
Problems
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.
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.