Posted:
Dec 5, 2020 @ 6:50 am

The avalanche danger in the west central Montana backcountry is moderate in upper elevation terrain and LOW elsewhere. 

Good morning, this is Jeff Carty with the West Central Montana Avalanche Center’s avalanche advisory for December 5, 2020. This advisory does not apply to operating ski areas, expires at midnight tonight, and is the sole responsibility of the U.S. Forest Service.

The high pressure is still with us, and it looks like it will be holding on till Wednesday. The inversion has intensified and higher peaks did not freeze last night with temps ranging from 31 º to 38º above 7000′ while the valley bottom was 15º. Winds have been calm with brief gusts. Temperatures up to 50º are forecast for the highest peaks today. 

The extended above freezing temperatures and strong solar warming be loosening bonds within the snowpack today. Loose wet avalanches are likely and wet slab avalanches are possible.

Yesterday, the extent of the surface hoar was not nearly as extensive as we expected. With above freezing temperatures, it did not grow at high elevations last night. As the night time temps dip back below freezing expect surface hoar development but mostly isolated to northern aspects and lower elevations. While it is not a stability issue currently, surface hoar is a persistent weak layer that can cause stability issues for a long time once it is buried. 

We found near-surface facets in the upper 4″ yesterday in the Rattlesnake, but the lower snowpack was well bonded without failures in stability tests at 7500′ on a northeast aspect. The areas of shallow snowpack such as windswept slopes and below 7000′ are of more concern.  These areas are developing facets more extensively and may be a stability issue with further snow load.

In the Rattlesnake yesterday wind slab from Monday was well bonded. However, there may still be pockets of reactive wind slabs in steep wind-loaded start zones. 

The bottom line:

Elevated overnight temperatures and solar radiation will be creating wet snow problems. Wet loose avalanches are likely and wet slab is possible. Southern aspects will be most affected. Wind slabs may still be present, practice caution in wind-loaded start zones greater than 35º.  Practice safe travel protocols. Travel one at a time in avalanche terrain, carry a beacon, shovel, and probe, and stay alert for signs of instability. Dig a pit. Look for red flags. 

The avalanche center forecasts for a 1420 square mile area stretching from Lost Trail to Seeley Lake, we can’t see it all weekly. Your snowpack and conditions observations help fill in the gaps and produce a more accurate forecast, please submit them here.

Ski and ride safe

 

 

READ FULL ADVISORY  

Problem 1 - Wet Loose

  • TYPE

    loose-wet

    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

    2-3

    1 (Small)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    Possible

    The likelihood of an avalanche resulting from this problem.

  • ADDED DANGER

    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    SE - Southeast
    S - South
    SW - Southwest
    W - West

Warm temperatures, an inversion, and solar radiation are warming southerly slopes and creating conditions for loose wet avalanches.

Significant early season hazards increase the likelihood of trauma from any avalanche involvement.

As slopes warm move to shaded aspects, and avoid steep southerly slopes. Beware of overhead hazard.

 

Problem 2 - Wind slab

  • TYPE

    wind-slabs

    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

    2-3

    1 (Small)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-2

    Unlikely/Possible

    The likelihood of an avalanche resulting from this problem.

  • ADDED DANGER

    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    NE - Northeast
    E - East
    SE - Southeast

The wind slab deposited Monday may still possible to trigger in isolated start zones.

Slopes greater than 35º, above 7500′ are most suspect.

Watch for stiff hollow sounding snow, wind pillows, shooting cracks, and practice caution in steep start zones.

Problem 3 - Wet Slab

  • TYPE

    wet-slabs

    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

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    Possible

    The likelihood of an avalanche resulting from this problem.

  • ADDED DANGER

    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    E - East
    SE - Southeast
    S - South
    SW - Southwest
    W - West

Above freezing overnight temps above 7000′ and strong solar warming make wet slab avalanches possible today.

Avoid steep sun-warmed slopes greater than 30º today.

Be aware of overhead hazards such as steep slopes with rock that can concentrate solar warming.

Wet slabs can be unpredictable, sticking to cold and dry areas is a good game plan for the mountains today.

GALLERY

IMG_20201Sheep Mt. Views, 12:4:2020
IMG_20201Sheep Mt. Views, 12:4:2020
In the pit, 12:04:2020
In the pit, 12:04:2020
Sheep Mountain-04-Dec
Sheep Mountain-04-Dec
The rattlesnake from Sheep Mt. 12:04:2020
The rattlesnake from Sheep Mt. 12:04:2020
Pin wheels
Pin wheels

FORECAST & OUTLOOK

High pressure will reach peak strength today,but remain dominant over West Central Montana through the weekend and into early next week. Temperatures in the terrain have remained above freezing overnight and will be in the 30s and 40s today under a blue bird sky. In fact, the highest peaks above 8000 feet may briefly clip 50 degrees today! Some breezy winds of 10 to 15 mph may be possible on the peaks early next week. The next chance for precipitation comes by Wednesday with much better chances for mountain snow by next weekend.

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.