Avalanche Forecast

Issued: Mar 12th, 2018 11:28AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs and Loose Wet.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cooling Tuesday will limit wet snow avalanches to steep slopes at lower elevations. Watch for fresh shallow wind slabs forming by afternoon at higher elevations. Lingering hard wind slabs may still be found on lee slopes above treeline. Avoid steep slopes where recent winds have deposited snow.

Summary

Detailed Forecast

Clouds and mild temperatures Monday night will maintain shallow wet surface snow and limit a hard re-freeze. A weak front should spread light rain or wet snow over the area Tuesday but amounts should be light. The precipitation should come with cooling through the day. This should allow for decreasing chance of Loose-Wet avalanches Tuesday and limit any Loose-Wet activity to below treeline. Stay away from slopes where even a small Loose-Wet avalanche may have higher consequences such as above cliffs, gullies, and creeks.

Fresh shallow wind slabs may form by late Tuesday on some steep lee slopes so watch for wind transported snow at higher elevations.

Lingering hard wind slabs may still exist at higher elevations. You are most likely to find and trigger these avalanches on shaded slopes not receiving direct sunshine. Firm wind slabs can lure you far onto the slope before failing, creating a particularly dangerous situation. To avoid wind slabs, stay away from any steep slopes where you believe winds have recently deposited snow. If the snow below you feels hollow, seek lower angle terrain.

Limit your exposure to Mt Hood’s very large avalanche paths Tuesday. Very large and difficult to predict wind slabs have formed high on the mountain above the forecast area. A very large avalanche initiating high above you may travel to your location. Similarly, limit your exposure to cornices above you.

Snowpack Discussion

Sunshine and temperatures in the 40's to 50's over the past few days has allowed for significant snowpack settlement and for a variety of surface conditions to develop. Wet surface snow in the daytime and firm melt-freeze crusts are forming overnight on all but true north facing slopes which are barely hanging on to some settled old snow that fell Thursday.

Clear weather over the past few nights has allowed for surface hoar or near surface faceted snow to develop on many slopes. These surfaces are easily destroyed by wind or sun and warming, so may end up being widely variable when future snowfall occurs. 

Six to eight inches of new snow fell with moderate winds Thursday night and Friday. Before cooling Thursday night, rain reached up to 6600 ft. Winds throughout the storm reshaped the snow surface. Firm wind slabs, snow drifts, and wind scoured surfaces have all been reported.

While we are tracking several layers deeper in the snowpack, there are currently no significant layers of concern.

Observations

Mt. Hood Meadows pro-patrol reported warmth and wet snow conditions on Sunday and again Monday, with generally small loose wet avalanches on steep sunny slopes. 

NWAC Pro-Observer Laura Green traveled in the Mt Hood backcountry Friday. Laura found moderate to strong winds transporting snow throughout the day. Plumes were seen high on Mt Hood during the morning. She observed a variety of wind featured snow surfaces including large deep snow drifts, hard wind slabs, and uneven snow surfaces near and above treeline.

Problems

Wind Slabs

An icon showing 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, North West.

Elevations: Alpine.

Likelihood

Possible

Expected Size

1 - 1

Loose Wet

An icon showing Loose Wet

Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Elevations: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 13th, 2018 11:28AM