Avalanche Forecast

Archived

Dec 30th, 2015–Dec 31st, 2015

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

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A MODERATE danger means human triggered avalanches are possible. Watch mainly for possible wind slab mainly on north to southeast aspects and for possible loose wet avalanches on steep solar slopes.

Detailed Forecast

Sunny weather should be seen on Thursday. Warmer temperatures should be seen at higher elevations in the north Cascades by Thursday afternoon. Cool temperatures with possible low clouds should be seen at lower elevations east of the crest.

This weather should allow older wind slab to continue to slowly settle and stabilize. Wind slab will most likely be found on north to southeast aspects near and above tree line. Any wind slab avalanches should be confined to surface layers and cool temperatures east of the crest will slow the stabilizing of these layers. Watch and test for inverted strong over weak storm snow.

The sunny weather and warmer temperatures at higher elevations mainly in the north Cascades should bring the possibility of loose wet avalanches Thursday afternoon on steep solar slopes. Watch for surface wet snow deeper than a few inches, initial roller balls and natural loose wet avalanches on steep solar slopes where snow may shed from rocks or cliffs.

Snowpack Discussion

A cool very snowy storm cycle brought 2-4 feet of snowfall to the east slopes the week ending December 24th. Light additional accumulations in the past week have allowed the snowpack to slowly settle with over 2 meters of snow at Washington Pass and good skiing reported.

A weak weather system Sunday and Monday helped create some new mostly small and mostly shallow wind slab in many areas.

An observation posted to the NWAC observations Sunday described a shallow triggered wind slab on a SSE facing slope at 4500 feet, well below treeline, on a wind loaded feature near Merritt Lake off Highway 2.

The guides at Washington Pass on Monday reported that ski cuts off ridge crests above 7000 feet gave unremarkable small 5-10 cm wind slab with little or no propagation. Little to no slab structure was seen in non wind affected areas.

The guides were out again at Washington Pass on Tuesday and reported stubborn to trigger shallow wind slab near ridge crest in the above tree line and right side up, bonded, settled snow in the near tree line.

The southeast zone has a shallower snowpack versus areas further north, but after a snowy few weeks, has more than enough snow to warrant an avalanche risk. No snowpack observations have been received from this zone.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.