The possibility of a large persistent slab avalanche means continuing to avoid large, steep, open slopes.
These avalanches may be hard to trigger, but the consequences would be serious.
20-40 cm of recent snow fell with moderate to strong south and west winds, forming wind slabs. These slabs may be particularly reactive at upper treeline where they are sitting on a crust.
Persistent weak layers of crust, facets, or surface hoar are buried up to 250 cm deep. Triggering is most likely on northerly aspects, or later in the day on sun-exposed slopes when the upper snowpack has been weakened.
While details are lacking, a size 3 persistent slab avalanche with a wide propagation was remotely triggered on a north-facing alpine slope on Friday. It appeared to occur on a steep, rocky slope with a thin-to-thick snowpack.
On Tuesday, a variety of small to large (size 1-2) natural and explosive triggered avalanches were reported around the region. Wind and Storm slabs in in the alpine, and dry loose in steep terrain.
In the alpine, 20-40 cm of recent snow is settling over crusty surfaces across most terrain, except in high alpine north-facing areas, where dry, wintry snow can still be found. Previous moderate to strong winds (mostly from the southwest) redistributed this loose snow into deeper deposits on leeward slopes.
In most places below treeline, the upper snowpack is moist, and possibly even isothermal at low elevations.
Several persistent weak layers are buried up to 250 cm deep. While triggering these layers is becoming unlikely, they present a low-probability, high-consequence problem. A cornice fall (large load) may be enough to trigger this slab.
Wednesday Night
Partly cloudy. 15 km/h west ridgetop wind, picking up to 25-40 km/h through the night. Treeline temperature -3 °C.
Thursday
Mostly cloudy. 2 cm of snow. 15-35 km/h west ridgetop wind, easing slightly through the day. Treeline temperature -4 °C.
Friday
Mostly cloudy. 1 cm of snow. 20 km/h west ridgetop wind. Treeline temperature -3 °C.
Saturday
Mostly cloudy. 1 to 2 mm of precipitation as snow or rain at treeline. 30 km/h south ridgetop wind. Treeline temperature 0 °C.
More details can be found in the Mountain Weather Forecast.
