Posted:
Feb 10, 2017 @ 6:43 am

An Avalanche Warning is in effect for the west central Montana backcountry. The current avalanche danger is HIGH. Very dangerous avalanche conditions are present. Travel in avalanche terrain is not recommended today. Human triggered avalanches are very likely, and natural avalanches are likely. Avoid runout zones.

Good morning, this is Travis Craft with an avalanche warning for February 10, 2017. This avalanche warning will expire at 0600 am on February 11, 2017.  The warning will be extended or terminated at that time.

This danger rating does not apply to operating ski areas and is the sole responsibility of the U.S. Forest Service.

Weather and Snowpack

Mountain temperatures range from 30 F to 37 F in the region. Winds are 12 mph with gusts of 18 mph out of the SSW in the Bitterroot. Point Six, in the northern part of the advisory area, winds are calm. The region has picked up several new inches of heavy wet snow and rain to 6500 ft in the last 24 hours. The new snow had SWE’s ranging from .3 to 2.1 inches of water.

Yesterday Snowbowl ski patrol got impressive results with explosives, slabs failed 8 to 12 inches deep then stepped down into deeper layers in the snowpack. All routes observed shooting cracks and collapse noises.  Rain fell to 6500 feet yesterday.

The main avalanche problem today is persistent slabs.  The new load of snow and rain are stressing our weak layers in the snowpack.  Travel in avalanche terrain is not recommended.

The second avalanche concern is storm slabs.  We are still accumulating snow.  The new snow is heavy and wet and is on top of lighter dryer snow. The snowpack needs time to adjust to the new load.  Avoid runout zones.

The final avalanche problem will be wind slabs above 6500 ft.  These slabs were stubborn to ski cuts yesterday on control routes but released easily with explosives.  The addition of new snow on these slabs will make them more sensitive to triggers.  Travel in avalanche terrain is not recommended.

Below 6500 feet the primary concern will be wet slabs.  Rain and temperatures above freezing have made unstable conditions in lower elevation snowpacks. Roller balls are sure signs of instability.  Avoid traveling in runout zones.

Avalanche and Weather Outlook

Colder air moves into the area today and tonight which will drop the snow line.  The forecast is for continued precipitation.  With the predicted weather expect the avalanche danger to stay the same.

If you are out in the backcountry, please send us your observations, these are very helpful in producing the advisory.  Logan will issue the next advisory on February 11, 2017.

Ski and ride safe.

READ FULL ADVISORY  

Problem 1 - Persistent Slabs

  • TYPE

    persistent-slabs

    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

    6-7

    3 (Large-Very Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-7

    Very Likely

    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:

    N - North
    NE - Northeast
    E - East
    SE - Southeast
    S - South
    SW - Southwest
    W - West
    NW - Northwest

  • IMAGE

Weak layers are being put to the test with the new load of snow and rain.

Problem 2 - Storm Slabs

  • TYPE

    storm-slabs

    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

    4-5

    2 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-7

    Very Likely

    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:

    N - North
    NE - Northeast
    E - East
    SE - Southeast
    S - South
    SW - Southwest
    W - West
    NW - Northwest

Heavy snow on top of light snow.  The snowpack needs time to adjust to the new load.

Problem 3 - 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

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-4

    Possible/Likely

    The likelihood of an avalanche resulting from this problem.

Additional loading on wind slabs will increase the sensitivity of the slabs to triggers.

Problem 4 - Wet Slabs

  • 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-5

    Likely

    The likelihood of an avalanche resulting from this problem.

Wet slabs are the primary concern below 6500 feet.  These are very unpredictable.  Roller balls on slopes is a bullseye clue that the snow surface is saturated with water.  Avoid runout zones.

VIDEO

PST Persistent Slabs

FORECAST & OUTLOOK

Cooler temperatures dropping the snow line to below 4000 feet.  Additional accumulations snow are predicted for today and 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.