Moderate Danger

March 18, 2017

Warm and Rain

Posted:
Mar 18, 2017 @ 6:14 am

The current avalanche danger in the West Central Montana Backcountry is Moderate. Human triggered avalanches are possible in specific terrain. Evaluate snow and terrain carefully to identify features of concern.

Good morning, this is Logan King with the West Central Montana Avalanche Center’s avalanche advisory for March 18, 2017. This danger rating does not apply to operating ski areas, expires at midnight tonight and is the sole responsibility of the U.S. Forest Service.

Weather and Snowpack

Mountain temperatures this morning range from 35-40 degrees and did not drop below freezing last night. The advisory area started getting rain early this morning and that looks to continue through the day today. Snotels across the region are showing .1-.2 inches of SWE so far. Winds are out of the WSW at Point 6 at 18mph and gusting to 26mph. At Deer Mountain this morning wind are from the SSE at 14mph gusting to 17mph.

The primary avalanche concern today will be wet loose slides. With more free water being added to the snow surface and not freezing overnight the surface snow will be saturated and moving again today. Stay aware of indicators like roller balls to identify that the surface snow is wet and loosing strength. Be cautious of terrain traps as they significantly increase the consequences of getting caught in a loose wet avalanche.

The majority of the advisory area has had a rain on snow event already this season and with rain to 8,500 feet the potential for wet slabs exists. Wet slabs are very challenging to predict so take the time to carefully evaluate conditions and terrain to identify if wet slab problems are present. If the snow is saturated you are better off looking elsewhere to find better conditions and not risk the possibility of a wet slab.

The interface of cold and warm snow is still present in the snowpack at upper elevations along with a preserved graupel layer. These layers are about 2-4 feet deep and are still reactive in stability tests in the Rattlesnake but are not propagating as readily. If you trigger an avalanche in this layer it will be large and have the potential to be very destructive. Take the time to identify if these layers are preserved and reactive before recreating today.

Finally cornices will be a concern again today. We observed large cornices in the Rattlesnake yesterday that were suspect. Without freezing overnight and more rain to 8,500 feet cornices will continue to lose strength today. Steer clear of cornices, as they can fail without warning, and often break farther back than expected.

Avalanche and Weather Outlook

Today looks to bring more rain to 8,500 feet. Rain total are not expected to be as abundant with this warm and wet system and look to bring a maximum of about 1 inch of water. Rain level will drop this evening and into tonight to near valley levels by Sunday as cooler and drier weather returns. Avalanche danger will be on the rise today as rain continues to fall and the temperatures climb.

If you are out in the backcountry, please send us your observations, these are very helpful in producing the advisory. The next advisory will issue the next advisory on March 21, 2017.

Ski and ride safe.

READ FULL ADVISORY

Problem 1 - Loose Wet

TYPE

Loose Wet
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.
SIZE

< 1 (Small)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Possible/Likely
The likelihood of an avalanche resulting from this problem.

Rain and warm temperatures will result in rollerballs and moving surface snow.

Problem 2 - 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

Unlikely/Possible
The likelihood of an avalanche resulting from this problem.

Wet snow and more rain are creating the potential for wet avalanches.

Problem 3 - Persistent Slabs

TYPE

Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Persistent, Deep-Slab.
SIZE

3 (Large-Very Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Unlikely
The likelihood of an avalanche resulting from this problem.

The cold snow interface and graupel layers are preserved in the snowpack at upper elevations. These layers are not propagating as readily but are still producing clean shears in tests. if this layer fail it will result in a large avalanche.

Problem 4 - Cornices

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.

With temperatures staying above freezing and more rain, cornices will be loosing strength today.

VIDEO

GALLERY

Totality-17-Mar

Rattlesnake Ridgeline

Point release Rattlesnake

Wind affected surface snow

Loose Wet Debris

Missions

Rattlesnake

Rattlesnake Cornice

0317_2017Lost Trail Pass

Rattlesnake

FORECAST & OUTLOOK

Danger Trend

Increasing Danger
Area Forecast

Rain

Rain will persist through the day today before temperatures drop tonight.

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.