Avalanche Forecast

Archived

Nov 29th, 2017–Nov 30th, 2017

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

Watch for recently and newly forming storm and wind slabs, especially on wind loaded slopes near ridges and if venturing to higher elevations. Watch for early season terrain hazards, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Detailed Forecast

A weak to moderate front should move across the Olympics Thursday, causing a period of light to moderate snow and moderate ridge level winds. This weather should build new areas of storm and wind slabs, especially in wind affected terrain, such as higher elevations and near ridges.

Watch for storm slabs in areas that received more than a few hours of moderate or heavy snowfall intensities.

Watch for firmer wind-transported snow on lee slopes near ridges, generally northwest through southeast aspects, but expect wind deposited snow on a variety of aspects in areas of more complex terrain.

Storm or wind slab avalanches may entrain recent snow down to the firmer old refreezing snow from last week.

Although not listed as a current avalanche problem, loose dry avalanches are possible Thursday, particularly on steep slopes.

Watch for the numerous travel hazards such as open creeks, barely buried rocks and trees, creating poor and challenging travel conditions, especially below treeline.

The avalanche danger should generally increase for all elevation bands on Thursday.

Snowpack Discussion

Avalanche and Weather Summary

The great start to the 2017/18 PNW Winter through mid November, retreated last week. Warm, wet weather caused wet snow and glide avalanches and snowpack consolidation with total snow depths diminishing by about 50% or more from their mid-November peak depths.

On Tuesday, a front brought a period of moderate snow with a brief warming trend at Hurricane Ridge. Winds were generally south in the 10's with gusts in the 30's. By Wednesday morning 24 hour snow totals were about 5" and the 48 hour storm snow was about 10".

A break in snowfall Wednesday with some clearing at times and light winds allowed for recent storm and wind slab layers to slowly stabilize.

Observations

NWAC pro-observer, Matt Schonwald was out in the Hurricane Ridge area on Saturday. Matt found pockets of 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.