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Archived

Avalanche Forecast

Jan 19th, 2017–Jan 20th, 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
2: Moderate
The avalanche danger rating below treeline will be moderate
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
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Mt Hood.

Existing wind slab on easterly aspects above treeline is the primary wind slab hazard, but shifting winds may build new wind slab on a variety of aspects near and above treeline Friday. Shallow storm slabs may form Friday if periods of intense shower activity persist. We are still getting a handle on the snowpack post storm, so be prepared to evaluate the snowpack carefully in your local area. 

Detailed Forecast

Light snow showers on Thursday should taper down overnight. A frontal band rotating up from the south Friday should renew shower activity by late morning over Mt. Hood.  

Existing wind slab on easterly aspects above treeline is the primary wind slab hazard, but shifting winds may build new wind slab on a variety of aspects near and above treeline. Look for recent wind effects near and below ridgelines.

Shallow storm slabs may form Friday if periods of intense shower activity persist. Give these layers time to settle and approach steeper slopes with caution.

We are still getting a handle on the snowpack post storm, so be prepared to evaluate the snowpack carefully in your local area. 

Snowpack Discussion

Weather and Snowpack

An arctic air mass was over the Northwest with fair, cold weather last week. Surface hoar and near surface faceted crystals formed in wind and sun sheltered areas during this period. Observations over the next few days should help determine to what extent any of these weak persistent grain types might have survived the atmospheric river event and remain relevant to the forecast moving forward.

An atmospheric river moved over the Northwest Tuesday and Wednesday. Heavy rain was seen up to about 7000 feet at Mt. Hood. 3 day precipitation totals through noon Thursday were about 1.5 - 2 inches of water at the Meadows and Timberline stations. Light snow showers and cooler temperatures were seen Thursday.

Recent Observations

The Mt Hood Meadows pro-patrol on Wednesday reported that rain penetrated about 10 inches into the snow with no results during avalanche control using explosives. Visibility was low on Wednesday and it could not be determined if there had been any natural avalanches on the upper mountain.

Visibility remained poor on Thursday but Meadows pro-patrol reported wind slabs releasing on NE aspects well above treeline control work with explosives. Wind slabs became less reactive near tree-line and no avalanche activity or layers or concern were reported below treeline.  

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Possible

Expected Size: 1 - 1