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Archived

Avalanche Forecast

Dec 18th, 2017–Dec 19th, 2017
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Mt Hood.

Look for shallow wind slab forming on lee slopes mainly above treeline and watch for early season terrain hazards.

Detailed Forecast

Moist warm frontal moisture impacting the Washington Cascades will largely miss Mt. Hood, with only some light rain and snow expected Sunday night and during the day on Monday.  However W-SW transport winds will be quite strong especially on the upper mountain. Watch for shallow wind slab on wind loaded lee slopes mainly above treeline. 

Generally small loose wet avalanches are possible in isolated areas at lower elevations in steep terrain.  Be especially careful of even small loose wet avalanches around terrain traps. 

Remember that beneath any new snow received during this storm, a myriad of early season terrain hazards exist, especially at lower elevations. Expect terrain hazards with poorly covered rocks, vegetation and creeks, particularly on south-facing terrain, at lower elevations, and on exposed ridges where wind events have stripped much of the seasons snowcover.

Snowpack Discussion

Long stretches of above freezing temperatures, windy conditions, and periods of light rain over the weekend have likely erased persistent grain types that were observed prior to Friday's storm on Mt. Hood.

The warm and sunny weather over the past 12 days has done a number on the early season snowpack throughout the region, returning many areas to very shallow early season conditions. The most consistent snowpack remains on shaded northerly aspects in the upper below treeline to near treeline.

The overall snowpack is stable with settled old snow sitting over the strong Thanksgiving rain crust, buried about 1-3 feet.

No human-triggered avalanches have been reported in over a week. 

Observations

No recent observations. 

 

Avalanche Problems

Wind Slabs

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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1