Avalanche Forecast
Issued: Mar 11th, 2018 12:07PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
You will be able to trigger wet avalanches Monday due to a weak overnight refreeze, warm daytime temperatures, and sunny skies. Stay off of steep sunny slopes where you see signs of wet surface snow, especially slopes sheltered from the wind. 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
Wet snow conditions will develop Monday due to a poor refreeze Sunday night followed by warm temperatures and sunshine. However, E-SE winds will locally cool the snow surface, creating a more variable loose wet problem on Mt. Hood than Sunday. You will find wet snow first on steep rocky slopes receiving direct sunshine and especially on more wind sheltered terrain. Stay off of any steep slope where you see signs of wet surface snow.
Expect unstable conditions to spread as the day warms and freezing levels extend to the top of the forecast area. Expect new roller balls, pin wheels, and loose wet avalanches. As water reaches deeper into the snowpack, wet slab avalanches could occur. If you see signs of any new slab avalanches, step back your terrain use and stay out of avalanche terrain.
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 Monday. 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. These may fail without warning due to warm temperatures and sunshine.
Snowpack Discussion
Temperatures gradually warmed over the weekend under mostly sunny skies, topping out in the 40s at Mt. Hood weather stations near and below treeline Sunday afternoon. Wet snow conditions were reported on E-W aspects Sunday afternoon.
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, 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
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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.
Likelihood
Expected Size
Valid until: Mar 12th, 2018 12:07PM
