The snowpack may be reaching a breaking point as another day of above freezing temperatures allow more water to enter the snow. Now is the time to use more conservative travel option. Don’t linger on or under steep slopes that are receiving sunshine or during the warmest parts of the day.
Summary
Discussion
Determining how a snowpack will respond to a warm-up like this is tricky. Several days of above freezing temperatures and poor overnight refreezes makes us think we could be reaching the tipping point. We believe the size and scale of wet avalanches are increasing. Even though a cool East wind may help delay the thawing of the snow surface Monday, the sun and warm temperatures will win the battle, and wet snow concerns will abound.
Several large loose wet avalanches were reported from the Baker backcountry Sunday. This increase in size is notable from the other loose avalanche activity this week. Saturday evening we received a report of a large isolated wet slab near Excelsior Ridge on a S aspect at 5400’. Sometimes isolated events such as this are the precursor to more wide spread trends.
When we’re in spring-like patterns such as this, you can use timing to minimize your risk. Avalanche danger is generally lowest in the morning but can increase rapidly during the day. Travel early, and plan on changing conditions. Remember, slopes you travel in the morning may be very different by mid-day. Be leery of traveling below cornices during these warm periods. They can fail naturally and may act as a trigger for other avalanches.
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
As this warm stretch of weather continues we’re hearing reports of larger avalanches on more aspects. This trend should continue on Monday. It won’t take long for the sun and above freezing temperatures to overcome a weak overnight refreeze of the snow surface. Be on the lookout for signs of wet surface snow. New rollerballs, wet snow deeper than your ankle, and new fan-shaped avalanche debris all indicate loose wet avalanches are likely. As the wet snow surface gets deeper, these avalanches may entrain more snow and grow larger. Avoid traveling on our under any slope where you see signs of wet snow.
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: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Wet Slabs
So why are we including this avalanche problem now? We know the snowpack structure is still very cold and winter-like. After several days, we are beginning to see signs of water moving in the snowpack. Will this water reach an old snow layer and cause a slab avalanche? Hard to say. Wet slabs are notoriously hard to predict. It’s difficult to gather direct pertinent observations. The best course of action may simply be staying off of large open slopes greater than 30 degrees until we see how this all shakes out.
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 18th, 2019 11:09AM
