Avalanche Forecast

Archived

Nov 27th, 2017–Nov 28th, 2017

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, human triggered possible.

Regions

Olympics.

Watch for the development of new storm and wind slab layers, especially if you venture to higher elevations. Make more conservative terrain choices as the day progresses.

Detailed Forecast

On Tuesday a front crosses the Olympics in the morning, causing moderate to strong shifting alpine winds, with initial warming followed by cooling. Hurricane will transition from mixed precipitation to snow on Tuesday.

This weather is likely to build new denser storm and wind slab on previous snow from Sunday night. Storm snow avalanches should primarily involve snow from Sunday night and new snow. These layers lie on a moist to wet older snowpack, which may provide a bed surface. Underlying older snow should continue to gain strength.

The avalanche danger should increase during the day Wednesday, mainly near and above treeline.

Watch for the numerous travel hazards such as open creeks, barely buried rocks and trees, and glide cracks, creating poor and challenging travel conditions, especially below treeline. A lack of snow at the lowest elevations should reduce the avalanche danger there.

Snowpack Discussion

Avalanche and Weather Summary

The great start to the 2017/18 PNW Winter took a giant step backwards over the last week. During an extended period of warmth and wet weather in the days leading into Thanksgiving, an initial round of wet snow and glide avalanches occurred, especially on steep unsupported slopes and rock faces. Another round of rain Saturday night through mid-day Sunday likely did not cause widespread wet snow avalanches on an already beat up and saturated snowpack. This warm, wet and windy period melted significant snow with total snow depths decreasing by 50% or more from their mid-November peak depths at many NWAC stations.

A front and a splitting upper trough crossed the US west coast Sunday and Sunday night. At Hurricane Ridge, the passage of a sharp cold front on Sunday caused south winds in the 20's with gusts to the 40's before changing to westerly in the 10-20 mph range. Temperatures generally dropped from the 40's into the 20's on Sunday afternoon. New snow at Hurricane Ridge was 5" on Monday morning.

There was some partial clearing at Hurricane Ridge on Monday.

Observations

NWAC pro-observer Matt Schonwald was out in the Hurricane Ridge area on Saturday. Matt found pockets on wind slab up to 15 cm (6") thick on north aspects of Mt. Angeles near 5000 feet. Wind slabs were stubborn to trigger and did not propagate. A few glide cracks were visible in the Hurricane Ridge area, but the only large slide observed from the recent rain events was noted off the south side of Mt. Angeles. Windward slopes above treeline were largely stripped of snow.

The ranger at Hurricane Ridge on Monday Morning reported no avalanche activity.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.