Northwest Coastal

Human triggering of large persistent slab avalanches remain possible; especially in rocky, alpine terrain with a shallow or highly variable snowpack depth.

High -

FRIDAY NIGHT: Cloudy with wet flurries; 2-5 mm. / Strong, southeasterly winds / Alpine low -2 C / Freezing level dropping to 1200 m.SATURDAY: Mostly cloudy / Light to moderate, southerly winds / Alpine high 0 C / Freezing level 1200 m.SUNDAY: Mostly sunny / Light, southeasterly winds / Alpine high 2 C / Freezing level 1500 m.MONDAY: Sunny / Light, northeasterly winds / Alpine high 2 C / Freezing level 1500 m.

Natural avalanche activity will taper off with the cloudy skies and cooling temperatures. Concern remains for human triggering persistent slab avalanches on a layer of facets that are down 50-100 cm. Explosive control work on Thursday produced several large avalanches, up to size 3, on this layer and there continue to be reports of skiers experiencing whumphing (collapsing) of this weak layer in shallow or variable snowpack areas in the alpine.

50-100 cm of settled snow forms the upper snowpack at higher elevations, the product of storms since the drought ended on March 10. At lower elevations this precipitation came as rain. With the help of warm temperatures, this recent snow has either settled into a slab on shaded aspects or become increasingly isothermal (slushy) on sun-exposed aspects. Where it exists as a slab, it covers a variety of old snow surfaces left in the wake of the drought. These include crusts on solar aspects, facets (sugary snow) on shaded aspects at higher elevations, and surface hoar (weak, feathery crystals) in shaded/sheltered locations. This variable layer has been the primary failure plane in recent slab avalanches.Not much further below the March 10th interface is a second weak layer of facets buried on February 19th. Recent loose wet and slab avalanches have been observed gouging into this faceted snow and entraining additional mass.The lower snowpack is generally considered to be strong in most areas, except for rocky areas in the alpine with a shallow or highly variable snowpack depth.