Avalanche Forecast
Issued: Apr 19th, 2018 11:10AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
You may be able to trigger lingering wind slab avalanches Friday. Use visual clues such as firm snow surfaces, snow drifts, and cornices to identify and avoid steep wind-loaded terrain. The sun and warm air temperatures will create wet surface snow conditions as the day progresses. If you see new roller balls, observe fresh fan-shaped avalanche debris, or experience wet surface snow deeper than your ankle, expect loose wet avalanches on similar slopes.
Summary
Detailed Forecast
It may feel spring, but avalanche danger still exists in the backcountry around Mt Hood. Wind slabs formed on many slopes near and above treeline Monday through Wednesday. You are most likely to trigger a lingering wind slab at higher elevations in very steep terrain, near convex rollovers, or on an unsupported slope. Use visual clues such as uneven snow surfaces, drifts, and cornices to identify and avoid wind-loaded terrain.
As the sun comes out and air temperatures warm, expect snow surfaces to soften and become wet. This will occur first on steep E-SE slopes then move aspects as the day progresses. You may also find sufficient loose wet snow on northerly aspects late in the day. If you see new rollerballs, observe fresh fan shaped debris, or experience wet surface snow deeper than your ankle, it’s time to change slopes. With the recent snow, Loose Wet avalanches may entrain additional snow and become large.
Cornices over the winter have grown very large. As air temperatures warm and the sun comes out these large hanging blocks of snow will begin to droop and fail. Predicting when and where they will collapse can be extremely difficult. Do not linger areas below large cornices and choose travel routes to limit your exposure to these potential ticking time bombs.
Snowpack Discussion
Warm and generally sunny weather affected the snow in the Mt Hood area Thursday. On slopes receiving sunshine and on lower elevation slopes, moist surface snow conditions developed resulting in numerous large rollerballs. Cold drier surface snow may still be found on shaded slopes at higher elevations. The mild weather allowed many of the recently formed wind slabs time to gain strength.
A significant snowfall impacted the Mt Hood area earlier in the week. Around 16-20 inches (40-50cm) of snow fell Sunday (4/15) through Tuesday (4/17). Strong west-southwesterly winds during the storm transported the new snow forming wind slabs on many slopes at higher elevations.
While numerous layers can be found wihin the snowpack, there are no significant layers of concern at this time.
Observations
On Thurday, NWAC professional observer Laura Green traveled in the Mt Hood backcountry. Laura found wind slabs on NE-E aspects near treeline. These wind slabs were still reactive in some snowpack tests. On south-facing slopes, she observed large rollerballs up to 3 feet (90cm) in diameter.
On Wednesday, NWAC received reports through Mt Hood Meadows ski patrol of a skier triggered wind slab avalanche outside the ski area. The avalanche was triggered by a backcountry user and estimated to be about 2 feet deep.
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
Expected Size
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: South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Apr 20th, 2018 11:10AM
