Avalanche Forecast

Archived

Dec 12th, 2015–Dec 13th, 2015

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 unlikely, human triggered possible.

Regions

Mt Hood.

The strong storm Saturday should have created widespread new storm layers and new unstable wind slab on lee slopes near ridges by Sunday with further snowfall and strong winds expected during the day Sunday. Travel in avalanche terrain near and above treeline is not recommended Sunday.

Detailed Forecast

Snow showers will continue Sunday with moderate ridge top winds. Temperatures should remain cool.

Recent storm and wind slabs may continue to build and storm layers from Saturday and Saturday night will not have settled or stabilized by Sunday.

Either wind or storm slabs should remain sensitive and likely to trigger. North to southeast facing lee slopes will continue to be loaded throughout the day as well.

Dangerous avalanche conditions are expected near and above treeline requiring conservative decision-making and cautious routefinding.

Travel conservatively by staying off, or below steeper slopes and avoiding the wind loaded terrain near and above treeline.

Be aware of early season hazards below treeline. The heavy rains from last week have left many creeks open at lower elevations.

Snowpack Discussion

After the deluge earlier in this week with nearly a foot of rain below 7000 feet, winter has returned to Mt Hood with lower snow levels and new snow, especially Saturday afternoon. About a foot of new snow fell over the 24 hours ending 5 pm Friday, with close to another 12 inches of snow during the day Saturday and still snowing hard.

Avalanche and Snowpack Observations

The Mt Hood Pro Patrol on Wednesday reported a generally stable and draining snowpack with wet grains to the ground. By Thursday new wind slab was developing near and above treeline on lee easterly slopes, but was generally not found to be reactive with ski cuts or in snowpack tests. Ski area control work from Friday showed a few instabilities in the new storm snow, but these were unlikely to propagate. Lighter transport winds Friday should have limited new wind slab development above treeline. NWAC Mt Hood area observer Laura Green traveled to 6500 ft in the Bucket Creek drainage Friday, indicating about 40 cm (16 inches) of new storm snow had accumulated over the newly forming pencil hard crust. No significant storm snow related weaknesses were yet observed with only stubborn test results noted. However, it was felt those conditions would rapidly deteriorate with Saturday's expected storm, which has certainly arrived! Below treeline should still contain ample terrain anchors, but above 5000' there is enough snow to allow small avalanches in specific areas. No recent observations have been received for the bulk of the above treeline zone due to weather and visibility restrictions.

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.