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 possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

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

On Wednesday night, high pressure should allow for further stabilizing of recent storm and wind slab layers.

A weak to moderate front should move across the Mt Hood area near midday Thursday, causing a period of moderate to briefly heavy 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.

Remember that closed ski areas without avalanche mitigation are equivalent to backcountry terrain!

Snowpack Discussion

Avalanche and Weather Summary

The great start to the 2017/18 PNW Winter took a giant step backwards last week. Warm, wet weather caused wet snow and glide avalanches and snowpack consolidation with total snow depths decreasing by 50% or more from their mid-November peak depths at many NWAC stations including Mt. Hood.

On Tuesday, a front brought a brief bump in snow levels to about 5000 feet, with SW to W winds in the 10 mph range with gusts in the 20's. By Wednesday morning, 24 hour snow totals of 8-10" and 48 hour totals in the 10-15" range at Mt. Hood.

Observations

On Wednesday, a professional ski patroller from Mt Hood Meadows toured to about 6600 feet, finding about 8-12" of recent storm snow well bonded to the crust from last week. Storm snow was deepest on N-E facing terrain and no avalanches were noted. Some rollerballing was noted on steep solar terrain due to direct sun warming.

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.