Avalanche Forecast
Issued: Mar 16th, 2019 11:31AM
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
Expect continued warming and sunny skies for Sunday. You can trigger small loose wet avalanches on sun-exposed slopes as temperatures warm through the day. Plan your travel to move off of steep, sunny slopes before the surface snow gets weak and wet.
Summary
Discussion
All but the highest, shadiest slopes are feeling the effects of the warm temperatures and strong sunshine. Both gradual warming and ongoing cloud cover have prolonged the wet loose cycle, which has been slow to run its course. Wet loose slides (up to D1.5) have occurred on E-S-W aspects at all elevations each of the past 4 days. We are beginning to feel the rhythm of a diurnal melt-freeze cycle. Be intentional with the timing of your travel through the terrain. The safest conditions will exist in the early morning hours when snow surfaces are cold and frozen. Stability will deteriorate throughout the day as it warms up. Wet avalanches have been particularly active late in the afternoon. Be wary of any large cornice formation as well - these could fail naturally and pose a significant hazard. If you venture high into the alpine, you may find lingering wind slabs in isolated areas, primarily on northerly aspects. Pay attention as you transition into wind affected terrain and back off consequential slopes if you find strong-over-weak layering and stiff, hollowing sounding slabs.
Warm Spring weather and a changing snowpack in the Mt. Baker Backcountry 03/16/19 Pete Durr Photo
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, this transition could take quite a while. Right now, its still winter in the mountains and most of the snowpack is cold and dry.
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.
Problems
Loose Wet
Wet loose avalanches may begin on east and southeasterly aspects early in the day and continue onto southwest aspects by afternoon. With significant warming and freezing levels rising to 8000ft, you may see these avalanches start to occur at higher elevations and even on shaded slopes. The potential for large wet loose slides exists. A small wet sluff could quickly grow as in entrains snow and gouges into the snowpack. As you travel through the mountains, take note of how sun and warming are affecting the snow surface. Look for signs of loose wet instability such as wet, heavy surface snow, rollerballs, and fan-shaped avalanches. If you notice these clues, avoid travel underneath steep slopes where loose wet slides may run naturally.
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, North West.
Elevations: All elevations.
Likelihood
Expected Size
Wind Slabs
A handful of slab avalanches (all D1) with unknown triggers occurred on Saturday, primarily in north facing terrain and appear to have failed on the old/new snow interface. Some of these avalanches fit the profile of a wind slab, while others are harder to characterize. We have uncertainty about the distribution of the problem, but it appears isolated. Carefully evaluate the snowpack, especially if you venture into more remote areas in the forecast zone. Use caution around wind loaded features steeper than 35 degrees, especially on convex rollovers, under cornices, and on unsupported slopes.
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
Expected Size
Valid until: Mar 17th, 2019 11:31AM
