New snow is bringing a critical load to buried persistent weak layers. The consequences of triggering an avalanche could be much higher than surface instabilities suggest.
There are indications of a destructive persistent slab problem waking up in the Cascades with forecast new snow and wind set to increase its sensitivity and destructive potential.
Moderate snowfall and high southwest winds will form a storm slab prolem that wil need to be managed carefully on Monday. Expect the greatest hazard in high elevation, leeward terrain.
Periods of sunshine, where they emerge, will destabilize new snow on sun-exposed slopes. Loose avalanches may run naturally or with a human trigger.
A natural size 3 persistent slab was observed in Allison Pass on Sunday, strong evidence of the persistent slab problem seen in adjacent regions emerging in the Cascades. It occurred on a 37°, northeast-facing slope at treeline (1740m) and had a 240 m-wide crown fracture. While it's clear it was a persistent slab, whether it failed on the late January crust or a more recent layer of faceted snow is uncertain. Storm loading is increasing concern for this type of activity.
Around 20 cm of new snow and high southwest winds Sunday night will layer a fresh storm slab problem across the region and otherwise bring storm totals to about 50 - 60 cm overlying a weak layer of faceted snow or surface hoar existing in higher elevation, particularly shaded terrain.
Storm loading has woken this layer up decisively in neighbouring regions and it produced a troubling size 3 avalanche in Allison Pass on Saturday. High elevation rain bought moisture to the facet layer at treeline on Saturday, both a possible trigger and a suggestion that the layer may strengthen soon.
A crust from December is buried 80 - 140 cm deep, with facets around it in shallow areas. Otherwise, the lower snowpack is strong.
Sunday Night
Cloudy with snow showers bringing about 20 cm of new snow, increasing with elevation, rain below about 1000 m. 20 to 30 km/h southwest ridgetop wind. Freezing level 1700 m.
Monday
Clearing over the day before clouding over again in the afternoon. 10 to 30 km/h southwest ridgetop wind. Freezing level 1300 m - 1700 m.
Tuesday
Cloudy with scattered wet flurries bringing less than 5 cm of new snow, including overnight. 10 - 30 km/h variable south ridgetop wind. Freezing level 1200 - 1600 m.
Wednesday
Becoming sunny. 10 - 30 km/h south ridgetop wind. Freezing level shooting to 2700+ m.
More details can be found in the Mountain Weather Forecast.
