Avalanche Advisory published on April 10, 2017 @ 6:00 am
Issued by Erich Peitzsch - Flathead National Forest

Whitefish Range
Swan Range
Flathead Range and southern Glacier National Park

How to read the advisory

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Above 6000 ft.

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5000-6000 ft.

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3500-5000 ft.

Spring Avalanche Statement -Spring in northwest Montana brings a mixed bag of weather. Snow, rain, sun, and everything in between can occur all in one day. This can greatly affect avalanche conditions. It is essential to pay attention to changing conditions as these changes can happen rapidly and stability can also deteriorate fairly quickly. Avalanches still happen in the spring and it is important to not let your guard down.

advisory discussion

Thanks again to all who supported the Flathead Avalanche Center this year! We greatly appreciate all of your observations, time in the field, and sincere motivation to help save lives through avalanche education and advisories. We wish to thank the Friends of the Flathead Avalanche Center and Glacier National Park for their in-kind and financial support. 

Here are some of the more common avalanche problems you may encounter this spring as well as associated travel and terrain management advice for each:


WET LOOSE AVALANCHE

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. Typically, this involves starting early and heading back early. 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 SLAB AVALANCHE

Wet slabs are still a possibility this spring, particularly on shaded aspects at upper elevations where the transition to spring lags.  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. 

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).

 


GLIDE AVALANCHE

In our travels over the past couple of weeks, we noticed glide cracks forming on many slopes. 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

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.

 

  
STORM SLAB       WIND SLAB
Large storms accompanied by strong winds 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.

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 everyone still playing in the mountains (submit snowpack or avalanche observations on the website or email fac.admin@flatheadavalanche.org). For now, thanks for all of the observations, feedback, and general support of the Flathead Avalanche Center this year. We greatly appreciate having the support of this amazing community in northwest Montana. Thanks!

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Disclaimer

This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires at midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.