Avalanche Forecast
Issued: Mar 14th, 2019 11:00AM
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 clearing skies, sunshine, and warming temperatures on Friday. Loose wet avalanches may become easy to initiate. Move off of steep sun-exposed slopes before the surface snow gets weak and wet, and avoid wind loaded features steeper than 35 degrees.
Summary
Discussion
The storm snow that fell Monday-Wednesday is right-side-up and well bonded to old snow surfaces. Thursday was quiet in the Baker zone with increasing clouds throughout the day and no new avalanches reported. Loose avalanche activity (wet and dry) occurred on Wednesday. All loose avalanches were small (D1), but some ran far downslope and entrained quite a bit of snow - especially wet loose slides on steep solar aspects. No slab avalanches have been observed this week, and any concerning wind slabs are likely confined to upper elevation north facing terrain. Despite the cloud cover, the strong March sun has already affected the snow on all but the coldest, shadiest slopes. Expect to find variable snow surfaces, and be heads up around steep slopes facing E-S-W when the clouds break and the sun comes out.
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.
Problems
Loose Wet
Wet loose avalanches will continue to run their course with more sun and warming temperatures on Friday. You are most likely to encounter the problem on steep, rocky, sun-exposed slopes in the afternoon hours. With strong sun and the warmest temperatures since January, cold dry snow surfaces will change rapidly and lose strength. If you see new rollerballs, pinwheels, or experience moist surface snow, expect loose wet avalanches. With all the recent snow, any loose wet slide could entrain significant snow and become large. If you see signs of loose wet avalanches, steer away from large, steep slopes receiving direct 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
Expected Size
Wind Slabs
Strong southwesterly winds blew during the first several hours of the recent storm. Snow surfaces were sculpted and wind-drifts formed. Wind loading primarily occurred in upper elevation northerly terrain. Additional snowfall at the tail end of the storm on Wednesday came in light and without any wind. This last shot of snowfall covered evidence of previous wind loading and masked wind slabs. The lack of slab avalanches following this storm gives reason to believe that any lingering wind slabs are isolated and becoming hard to trigger. Use caution if you transition into wind affected terrain. Feel for stiff slabs under the new snow. Be aware of features in the terrain where you are more likely to trigger a wind slab: 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.
Likelihood
Expected Size
Valid until: Mar 15th, 2019 11:00AM
