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Avalanche Forecast

Archived

Feb 3rd, 2017–Feb 4th, 2017

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
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.

An avalanche warning has been issued for Mt Hood for Saturday. Natural or triggered wind and storm slab avalanches are very likely at Mt Hood on Saturday. Rain at the lower elevations may also trigger loose wet or wet slab avalanches on Saturday.  Travel in back country avalanche terrain is not recommended at Mt Hood on Saturday.

Detailed Forecast

A very important pattern change is taking place over the Northwest Friday and Saturday. The dry weather and strong cold E-NE winds seen Wednesday and Thursday are being replaced by increasing strong southwest flow aloft, increasing snow and warmer temperatures Friday and Saturday.

Saturday at Mt Hood should be a day of strong southwest alpine winds, moderate to heavy snow with snow changing to rain at lower elevations, and warming temperatures.

At Mt Hood at higher elevations from Friday to the end of the day on Saturday there should be 1-2 feet of warmer, denser new snow. This will generally build new upside down wind and storm slab layers. This new snow may build over weak or faceted snow from the cold weather mid-week on some or many slopes.

This is a recipe for avalanches.

An avalanche warning has been issued for Mt Hood for Saturday.

New sensitive wind slab is very likely on NW to SE slopes on Saturday.

Wind slab formed by strong E-NE winds on Wednesday and Thursday may linger on Saturday. The primary aspects for these wind slabs should be W facing slopes.

New sensitive potentially deep storm slab will continue to build on sheltered slopes on Saturday in areas that see rapidly accumulating snow. The warming trend will help build upside down layers and promote storm slab formation.

Natural or triggered wind and storm slab avalanches are very likely at Mt Hood on Saturday. Wind or storm slab avalanches may step to deeper layers. Rain at the lower elevations may also trigger loose wet or wet slab avalanches on Saturday.  Travel in back country avalanche terrain is not recommended at Mt Hood on Saturday.

Snowpack Discussion

Weather and Snowpack

An atmospheric river arrived Tuesday 1/17 with rain up to about 7000 feet on Mt. Hood.

Then from Thursday 1/19 through Sunday 1/22, generally 15-20 inches of snow accumulated at the NWAC Mt Hood stations.

A large upper ridge and warm air mass built over the Northwest last Thursday to Saturday 1/26-1/28. By Saturday temperatures had reached the 40's at the NWAC Mt Hood stations.

Strong NE-E winds were seen Wednesday and Thursday with very cold temperatures. Fresh, touchy wind slabs formed with large plumes of snow visibly transporting loose snow from exposed terrain.

Recent Observations

NWAC pro-observer Laura Green was out near Barlow Pass on Friday at 4100-5200 feet and found limited wind transport, low quality shears in tests in the upper snowpack and no direct signs of instability. But she noted the warming and increasing intensity snowfall.

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.