Posted:
Feb 24, 2015 @ 5:21 am

The avalanche danger is LOW at all mountain locations in west central Montana. Natural and human triggered avalanches are unlikely. Conditions are generally safe but there are many isolated places where it is possible to trigger an avalanche. Steep terrain loaded from strong east winds last weekend and heavily shaded terrain steeper than 35 degrees are places where you may find a pocket of unstable snow.

Good morning backcountry skiers and riders! This is Steve Karkanen with the West Central Montana Avalanche Center’s backcountry avalanche advisory for Tuesday February 24, 2015. This information is the responsibility of the Forest Service and does not apply to operating ski areas.

 

Weather and Snowpack

Mountain temperatures are in the teens and low twenties this morning with a 23mph W-NW wind gusting to 28mph on Point Six. There has been no precipitation the past 48 hours.

A fast moving system dropped a few inches of snow at most locations in the advisory area last weekend with the Rattlesnake, southern Missions, and southern Bitterroot picking up 10-14 inches. An east wind hammered high elevation ridges and peaks Saturday and Sunday with gusts recorded at 50mph on Point Six early Saturday.  Cornices started to form on the west to southwest sides of high ridges and fresh wind slabs formed further down slope and in cross loaded areas.

New wind slabs are the primary avalanche problem but these will be spotty and in unusual places since the areas that typically load (E-NE aspects) were scoured down to the old, and now very hard, snow surface.

Dry, loose snow or point release avalanches involving the new snow is the other avalanche problem to be aware of.

The new snow bonded to the old snow surface and is not exhibiting signs of instability. It is possible to trigger loose snow sloughs on very steep pitches (40+ degrees) but these should be manageable. When it does warm up again, this avalanche problem will morph into a wet, loose snow avalanche problem.

 

Weather and Avalanche Forecast

A few more days of high pressure and cold air is expected to be the main weather feature. Our  atmosphere is stable now with light winds at most elevations. The wind is expected to increase by mid-week as another Arctic air mass slides down the east side of the Continental Divide spilling cooler air and some precipitation into western valleys. While not much snow is expected for west central Montana, increased wind speeds may be an issue. Stay tuned.

The avalanche danger will remain LOW until we see a significant change in temperatures, new snow or high winds.

I will issue the next regular advisory on Friday, February 27, 2015.

 

READ FULL ADVISORY  

Problem 1 - Sensitive wind slabs

  • 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:

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

A fast moving system dropped a few inches of snow at most locations in the advisory area last weekend with the Rattlesnake, southern Missions, and southern Bitterroot picking up 10-14 inches. An east wind hammered high elevation ridges and peaks Saturday and Sunday with gusts recorded at 50mph on Point Six early Saturday.  Cornices started to form on the west to southwest sides of high ridges and fresh wind slabs formed further down slope and in cross loaded areas.

New wind slabs are the primary avalanche problem but these will be spotty and in unusual places since the areas that typically load (E-NE aspects) were scoured down to the old, and now very hard, snow surface.

Problem 2 - Loose snow avalanches

  • TYPE

    loose-dry

    Loose Dry

    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-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:

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

Last weekend’s new snow bonded well to the old snow surface and is not exhibiting signs of instability. It is possible to trigger loose snow sloughs on very steep pitches (40+ degrees) but these should be manageable. Always consider what the consequences will be if you do get caught up in one of these “manageable” slides.

When it does warm up again, this avalanche problem will morph into a wet, loose snow avalanche problem.

GALLERY

Tim touring Spruce Creek area
Tim touring Spruce Creek area
Looking east and south to the Bitterroot Mountains l
Looking east and south to the Bitterroot Mountains l
Rattlesnakes wind on 2_21_15 loading to the South
Rattlesnakes wind on 2_21_15 loading to the South
Rattlesnakes_2_23_15
Rattlesnakes_2_23_15
Spruce Creek area 2/23/15
Spruce Creek area 2/23/15

Photos from Monday’s tour, wind loading on Saturday and a profile from the Rattlesnake.

FORECAST & OUTLOOK

A few more days of high pressure and cold air is expected to be the main weather feature. We are in a stable atmosphere now with light winds at most elevations. The wind is expected to increase by mid-week as another air mass slides down the east side of the Continental Divide spilling cooler air and some precipitation into western valleys. While not much snow is expected for west central Montana, increased wind speeds may be an issue. Stay tuned.

The avalanche danger will remain LOW until we see a significant change in temperatures, new snow or high winds.

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.