Avalanche Forecast
Regions: Mt Hood.
Expect conditions to change during the day Wednesday as the snowpack transitions from a cold storm to warmer temperatures and sunshine. Fresh wind slabs will be reactive to human triggering, so keep your eyes open for places where the wind drifted the snow into deeper slabs, and avoid wind loaded slopes greater than 35 degrees. As the sun comes out, expect natural loose avalanches from steep sun-exposed slopes. Don’t get surprised by these easy to predict and avoid avalanches, and don’t linger under sunny slopes where natural avalanches may occur.
Discussion
Snow and Avalanche Discussion
Wednesday will be a day of transition for the snowpack in the Mt Hood area. As snow showers end, temperatures rise, and the sun comes out, the snowpack will change quickly. Be prepared to adapt to these changes during your day. Slopes you travel in the morning may be very different later in the day.
This storm hit the Mt Hood area the hardest with the strongest winds and significant new snow. Storm slab conditions may linger in areas Wednesday. Be suspect of any location that received more than 10 inches of new snow. In these areas, you may find conditions different that the avalanche forecast implies.
This new snow may have buried a variety of surfaces, including firm crusts, wind pack snow, facets, and areas of surface hoar, all reported in the Mt Hood area. This is in addition to a layer of buried surface hoar and/or facets from earlier this month. How will these layers fair under the new snow load? It’s tough to say. These weak older layers were less prevalent in the Mt Hood area than adjacent zones to the north but where present, there is the potential for larger avalanches to fail in unusual ways. The only way to observe these layers is to dig in the snow. If you take the time to look at these layers, keep your eye out for sugary facets or feathery surface hoar buried 1 and 2 feet below the snow surface.
Snowpack Discussion
March 10, 2019
February started stormy and cold with very active avalanche conditions on persistent weak layers. The month finished with continued cold temperatures but drier weather allowed avalanche activity to taper off. Persistent weak layers that were buried early in the month (Feb 8th) are now unreactive, though you can still find the grains.
Aside from the series of storms in early to mid-February, most regions have measured relatively light snow accumulation in the past 3 weeks. Snow has stayed soft especially on shaded slopes and faceting and surface hoar have been plentiful. While there has been plenty of sunshine since mid-February, very cold temperatures have kept melt-freeze crust to a minimum on sunny slopes.
A natural loose wet avalanche (D1.5) on a southeast aspect of Lichtenberg Mtn at 5,400ft. 3/10/2019. Photo: Josh Hirshberg
Recent Avalanches
Our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow and weak snow surfaces on both dry, shaded slopes and sun-exposed aspects. In some locations, a weak layer of surface hoar and facets was buried on March 6th. We’ll keep an eye on this interface as we move into the future.
A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo.Â
Moving Forward
As we move further into March, there are two points to consider:
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The strength of the March sun: As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.
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Weak surfaces: facets and surface hoar have blanketed snow surfaces in many zones. Any significant snowfall will bury well-developed and widespread persistent weak layers. Recently, most storms have not delivered enough precipitation to cause a problem.
Avalanche Problems
Wind Slabs
Strong winds during this storm likely reshaped the new snow in many locations. Observations from Mt Hood Meadows patrol Tuesday indicated that the winds were efficiently transporting recent snow, likely building new wind slabs on lee slopes. We expect these new wind slabs to still be reactive to human triggering Wednesday. Steer away from any open slope greater than 35 degrees where you see fresh cornices, drifted snow, or feel firm snow under you. If you find firm wind slabs, give these slopes a wide berth. Hard slabs can lure you on to them before failing, sometimes above your head.
In areas on Mt Hood, you may find lingering storm slabs. In these locations be particularly leery of convex rollovers, slopes greater than 40 degrees, and unsupported slopes where you are most likely to trigger a lingering storm slab avalanche.
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: Likely
Expected Size: 1 - 1
Loose Wet
As the sun comes out and temperatures rise, look for conditions to change quickly. You may see natural avalanches occurring on steep sun-exposed slopes. These avalanches may entrain significant amounts of snow as they move downhill. Don’t get surprised by these easy to predict and avoid avalanche problem. Don’t travel on our under steep sunny slopes, and be particularly cautious around gullies and cliffs where even small avalanches can harm you.
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: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1