SPRING AVALANCHE INFORMATION
Spring in northwest Montana brings a mixed bag of weather. We can see snow, rain, sun and everything in between all in one day. This can greatly affect avalanche conditions. Thus, it is essential to pay attention to changing conditions as these changes can happen rapidly and stability can also deteriorate fairly quickly. Avalanches (and sometimes big ones) still happen in the spring and it is important to not let your guard down. There are a few things to keep in mind while out riding or skiing and enjoying the longer and sunnier days this spring.
This is probably the most common type of avalanche during the spring in northwest Montana. Wet loose avalanches are a 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. 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. Some of these avalanches may seem harmless, but can be deadly in high consequence terrain like above cliffs or a terrain trap. An important point to remember is that 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 slushy. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Wet slabs could be a very real possibility this spring given our current snow structure. We have numerous layers within the snowpack that could produce wet slab avalanches if the right weather conditions develop this spring. Wet slab avalanches are a 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 slabs can be very destructive. Hopefully, the weak layers behave this spring and don’t turn into nasty gremlins when hit with free water this spring.
Avoid terrain where and when you suspect wet slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty of this type of wet snow avalanches. It is important to pay attention to rapid warming especially with light or no refreeze overnight or rain on snow events. Free water in the snowpack acts a lot like the water in your sink drain pipes. Once you turn off the water there is a bit of a lag and water still moves through the drain. Imagine the snowpack as the drain with some layers of snow impeding water from moving directly down to the ground like a turn in the drain pipes. If a weak layer exists anywhere along the way and enough water affects the bonds of that weak layer then wet slab avalanches could occur. Thus, wet slab avalanches can even occur after prolonged sunny days or rain events (when the water shuts off).
Earlier in the winter glide cracks and avalanches made an unwanted appearance then went away for a while. In our travels over the past couple of weeks, we noticed glide cracks beginning to rear their ugly heads again. Glide avalanches are, unfortunately, a rather common spring occurrence here in northwest Montana, and are a release of the entire snow cover as a result of gliding over the ground. Adding insult to injury this type of avalanche is rather poorly understood and it is difficult to predict when they might fail (or even if the glide crack might fail at all). Thus, it is best to avoid slopes where glide cracks exist.
Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage. Safe travel relies on identifying and avoiding those slopes. Glide cracks and recent glide avalanches are a good indicator of future glide avalanche activity.
Cornice fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice fall can entrain loose surface snow or trigger slab avalanches. Cornices are susceptible to failing particularly on warm, sunny days.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridge line areas with cornices. They often overhang the ridge edge and can be triggered remotely. Avoid areas underneath cornices. Even small cornice fall can trigger a larger avalanche and large cornice fall can easily crush a human. Periods of substantial temperature warming are times to be particularly aware.
Large storms can occur every spring. It is not uncommon to have spring storms drop 2-3 feet of new snow at upper elevations. So, just like in the winter it is best to give the snowpack ample time to adjust to the new load. Often in the spring this new snow will fall on melt-freeze crusts (sun or rain crusts). This crust provides a great bed surface for the new snow to slide on. Storm slabs are a release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm slab problems typically last between a few hours and few days. It is also important to pay attention to wind slabs that form both during a storm and during high wind events.
You can reduce your risk from storm slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
We dealt with some pretty tricky avalanche conditions this winter, and one of them was a nasty persistent slab with a crust/facet layer that formed in late January/early February. This persistent slab as well as weak layers at the ground/snow interface still lurk. Deep persistent slabs are a release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep persistent slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. Deep persistent slabs are destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
On that note, enjoy the spring as it can be a great time of year to get out and enjoy the mountains, but keep your avalanche eyeballs open at all times. We will be working on the Flathead Avalanche Center Annual Report and will post it on the website as soon as it is complete. Please send in your observations as you are out and about this spring as they are still valuable to us (firstname.lastname@example.org or call 406.261.9873). For now, thanks for all of the observations, feedback, and general support of the Flathead Avalanche Center this year.