No Danger Rating

April 11, 2019

Spring Conditions

Posted:
Apr 11, 2019 @ 6:18 am

This is Logan King with the end of the season update for the West Central Montana Avalanche Center. It doesn’t feel like spring in the mountain as copious snow has fallen over the past week but it’s not uncommon for large snow events this time of year. Whether we are ready for it or not spring is here and it necessitates a different set of skills and approach to recreating in the mountains.

Yesterday, while touring in the Rattlesnake we found full on mixed spring conditions with a mix of dry and wet avalanche problems. The name of the game is now adjusting to conditions as they rapidly change throughout the day. Keep in mind that spring can bring dry avalanches in the form of storm slabs and wind slabs or more challenging hazards like wet avalanches and cornice failures.

At upper elevations or in dry snow watch for wind slabs failing on interfaces of old snow or crusts. Also, stay aware of storm slabs following storms that bring snow to the region. Carefully assess the new snow for how well it is bonding to the old snow surface. New or wind drifted snow on crusts can result in easy failures that have a lot of pop that rapidly accelerate on slick crusts.

Long days, warm temperatures, and sunshine will result in the loss of strength within the snowpack. Wet avalanche problems are a function of timing. Get in and get out before the snow loses enough strength to fail. When snow becomes wet and starts to form roller balls move to a different aspect or call it a day. Wet slabs are challenging to predict but are best identified by lack of overnight freezes and free water in the snowpack. Remember that the harder or deeper the freeze overnight the more time you will have the following day before conditions turn. Wet slabs and glide avalanches are to be expected this time of year, wet slabs are a challenge to predict and glides are nearly impossible. With glide avalanches being so unpredictable its best to avoid them altogether if you see glide cracks don’t travel below them and find different terrain free of cracks in the snow. Finally, remember cornices will continue to lose strength as well. Give cornices plenty of space as they can break back farther than anticipated.

There is plenty of moisture still on tap for the foreseeable future and we will continue to post updates as the season draws to a close. We will continue to post public observations as we receive them.

All of us at the WCMAC would like to express our appreciation for all of the amazing support that we receive from the community. It has been a great season and we wouldn’t be able to do what we do without the outstanding support from all of you. Thanks again for another great season.

Ski and Ride Safe!

READ FULL ADVISORY

Problem 1 - Wind Slabs

TYPE

Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically erodes 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.
SIZE

1-2 (Small-Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Possible
The likelihood of an avalanche resulting from this problem.
IMAGE

Wind slabs will fail in dry snow at interfaces and on crusts.

Problem 2 - Storm Slabs

TYPE

Storm Slabs
Release of a soft cohesive layer (a slab) of new snow which breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
SIZE

1-2 (Small-Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Possible
The likelihood of an avalanche resulting from this problem.

Following storms assess if the new snow is bonding to the old snow surface.

Problem 3 - Wet Slabs

TYPE

Wet Slabs
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.
SIZE

1-2 (Small-Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Possible
The likelihood of an avalanche resulting from this problem.

Carefully monitor the snowpack as it looses strength. Wet slides will occur in warm and saturated snow.

Problem 4 - Glide Avalanches

TYPE

Glide Avalanches
Release of the entire snow cover as a result of gliding over the ground. 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.

Predicting the release of Glide Avalanches is very challenging. Because Glide Avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide Avalanches.
SIZE

2 (Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Possible
The likelihood of an avalanche resulting from this problem.

Avoid terrain with glide cracks due to glide avalanches unpredictable nature.

Problem 5 - Cornice Failures

TYPE

Cornices / Cornice Fall
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 down-wind side. They range 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.
SIZE

2 (Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Possible
The likelihood of an avalanche resulting from this problem.

Every cornice will eventually fail and melt out, its a matter of when and how big they are when they break. Avoid traveling on or under cornices especially later in the day.

GALLERY

Shallow Skier triggered wind slab

Burgundy

Wind Drifted Snow

Large Roller ball 4/8/19

loose wet slides

Skier Triggered Wind slab

FORECAST & OUTLOOK

Danger Trend

Same Danger
Area Forecast

Spring Conditions

From here on out rain or snow will be a possibility until the snowpack melts out.

This information is the sole responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight but the information can help you make a more informed decision regarding travel in avalanche terrain for the next few days.

Our advisory area includes National Forest System lands in the Bitterroot Mountains from Lost Trail Pass north to Granite Pass, the Rattlesnake Mountains north of Missoula and the Southern Swan and Mission Mountains near Seeley Lake, MT. Avalanche information for the Lookout Pass/St. Regis Basin area is available from the Idaho Panhandle Avalanche Center.