Avalanche Forecast
Regions: Mt Hood.
You can still trigger a large wind slab at higher elevations, but the likelihood of triggering is gradually decreasing. Avoid wind loaded slopes greater than 35 degrees. Strong march sunshine and warm temperatures should revive Loose-Wet avalanches on steep sun-exposed slopes. Watch for rollerballs or wet surface snow as signs of potential wet avalanches.
Discussion
Snow and Avalanche Discussion
As we transition into the springtime, snow conditions change quickly. Soft snow persists on non-solar aspects and a thin sun crust is likely to break down on solar aspects to allow for Loose Wet avalanche activity. It’s important to make frequent observations and check whether the conditions line up with the forecast.
A recent observation from 03/13 illustrates Loose Wet activity that has already occurred. Roller ball activity was observed by Mt. Hood Pro Patrol on Thursday. Following several days of partial sunshine, it is difficult to predict how much Loose Wet activity will occur on Friday.
The recent storm hit the Mt Hood area the hardest with the greatest new snow and strongest winds. If traveling to higher elevations, be prepared for wildly varied surface conditions as a result of the strong to extreme winds. Large hard wind slabs may be hard to trigger but would be potentially deadly if triggered from a thinner portion. Best to give the higher terrain additional time to allow these wind slabs to heal.
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Snowpack Discussion
March 14, 2019
It’s starting to feel a lot like spring. The sun is stronger. Temperatures are warmer. The days are longer. Each year March brings a period of transition for the snowpack. Despite the feel of spring, the transition to a spring snowpack could take quite a while. Right now, it’s still winter in the mountains.
A northeast through southeast facing bowl in Tunnel Creek, Stevens Pass. The red line shows the transition from northeast to east aspects. Crusts were on the surface of east aspects (right of the line). Surface snow on northeast was soft and dry. March 11th, 2019. Photo: Josh Hirshberg
Aspect dependent snowpacks
A common trend this time of year is the distinct difference between snowpacks on shaded slopes and those on sunny slopes. We might refer to this as aspect dependent snowpacks. The strong March sun can create crusts and send melt-water through the snowpack on sunny slopes.
That process is beginning, right now. As of the 14th, sun-exposed aspects have a crust at the surface. Otherwise, these slopes hold mostly dry layers in the upper 3 feet of snow. The sun now affects more aspects than in the earlier half of the winter. You may find sun affected and melt-freeze crust on any slope that received direct or indirect sunshine. This could include some northwest or even northeast aspects.
Shaded slopes are still holding onto cold winter like snowpacks. That doesn’t mean you won’t find any melt-forms in these areas, but the snowpack looks, feels, and acts more like mid-winter. Avalanche problems can follow a similar trend by aspect. You may see wet avalanche problems on sunny aspects and lingering dry slab problems on shaded slopes.
Two photos taken on March 10th in the Snow Lake area. The left is N facing terrain on Chair Peak with cold, dry surface snow. The right photo shows loose wet avalanches on E facing slopes of Mount Roosevelt. Photo: Ian Nicholson
Balancing problems
So, how do we balance these sometimes opposing snowpacks? In a word, observations. Transitional periods like this require diligence and planning. It seems obvious, but observations you make on a south-facing slope aren’t applicable to N facing terrain. Consider this as you plan your day. Secondly, you’ll want to make observations to verify the avalanche forecast. As we’ve discussed, snowpack structures can diverge during these transitional periods. When your observations line-up with the information you find in the daily avalanche forecast, the forecast is likely valid. If you see observations that don’t match, it’s time to stop, reevaluate, and possibly choose more conservative terrain options.
Making snowpack observations to verify the forecast. Photo: Pete Durr
Less elevated danger and changing trends
You may not find low avalanche danger on every avalanche forecast, but spring is a time of year when less significant hazards can allow backcountry travelers to explore deeper into the mountains. The longer days and often lower avalanche hazard can support stepping into bigger terrain. Remember that low danger doesn’t mean "no danger." Additionally, avalanche conditions can change quickly during spring due to rapid warming over the course of a day or snowfall from potent storms.
We forecast for very large regions. You travel on individual slopes. You may encounter anomalies in the landscape where conditions don’t match the forecast. As with any time you travel in snowy mountains, you can stay safe by using standard travel protocols to minimize your exposure to avalanche terrain and the hazards of the day. Keep your eyes open and be ready to adapt to changing conditions.
What's next?
Winter is far from over. Keep checking the forecast, tracking conditions, making observations, and telling us what you find.
Avalanche Problems
Wind Slabs
Snow and Avalanche Discussion
Strong to extreme winds during the recent storm reshaped the new snow in many locations and created large wind slabs in higher terrain. Mt Hood Meadows patrol observed a very large recent natural slab avalanche Wednesday that ran more than 1000 vertical ft. While the start zone was well above our forecast terrain, it shows how quickly large wind slabs can form in open terrain. While wind slabs were becoming much more stubborn to trigger on Thursday, we think you can still trigger them, and natural loose wet avalanches may be able to trigger a large wind slab.
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.
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.
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
Loose Wet
Friday is expected to deliver strong March sunshine and elevating freezing levels. While a few days of partial solar radiation may have lessened the impact of the full blast of sunshine, we don't think we're done yet. If you see new rollerballs, find new fan-shaped avalanches, or experience moist surface snow, loose wet avalanches may occur. With all the new snow, if the slope is big enough, any loose wet may entrain significant new snow and become large. If you see signs of loose wet avalanches, steer away from steep slopes receiving any sunshine.
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: Possible
Expected Size: 1 - 1