Avalanche Forecast

Archived

Dec 8th, 2016–Dec 9th, 2016

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

Mt Hood.

Expect very dangerous avalanche conditions to develop Thursday night and continue through the day Friday on Mt. Hood. Travel in avalanche terrain is not recommended especially near and above treeline Friday.

Detailed Forecast

A first round of moderate snow Thursday night along with shifting winds and a warming trend should be followed by a second round of increasing shower activity and additional warming Friday afternoon translating to a rapidly unstable snowpack in the Mt Hood area Friday.

Expect dangerous avalanche conditions to develop Thursday night and continue through the day Friday on Mt. Hood. Travel in avalanche terrain is not recommended especially near and above treeline Friday.

Wind slabs previously formed on westerly aspects will shift back to more traditional easterly aspects as W-SW winds increase Friday near and above treeline.

Storm slabs will become increasingly sensitive and may fail due to weaknesses between storm layers or if poorly bonded to previous weak surface snow found in less wind affected areas.

Even though the lower part of the below treeline band is filling in, watch for early season travel hazards such as barely covered rocks and open creeks.

Snowpack Discussion

Weather and Snowpack

An active and cool weather pattern has resulted in a snowy December thus far with Mt. Hood NWAC stations recording about 2-4 feet of lower density snow.

Given the favorable snowpack profile, good bonds between older snow layers and a lack of deeper layer concerns, avalanche problems should be confined to the upcoming old snow/new snow interface or within new storm layering.

Moderate easterly winds have shifted to the west along with a warming trend Thursday evening along with rapidly accumulating new snow. There remains ample low density snow available for transport.

Recent Observations

On Monday and Tuesday the pro-patrol at Mt Hood Meadows reported only isolated wind slabs in exposed terrain at the upper elevations. These avalanches were fairly shallow within the upper 4-8 inches of snow mostly and released with explosives only. In general, most areas have surface snow conditions lacking slab character with low cohesion snow providing some excellent conditions.

Lastly the indefatigable pro-patrol at Mt Hood Meadows on Wednesday reported light winds that were not causing wind transport. The upper snowpack remains generally right side up with good skiing and riding conditions. Some surface hoar was seen in sheltered locations.

On Thursday shifting winds was producing significant wind transport near and above treeline on Mt. Hood.

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.

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.