Posted:
Dec 29, 2017 @ 6:59 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.

This is Travis Craft with an avalanche warning for Friday, December 29th, 2017.  This avalanche warning will expire at 6:00 am December 30, 2017.  The warning will either 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 27 F to 35 F in the region.  In the Bitterroot winds are 2 mph with gusts of 5 out of the SSW.  In the northern part of the advisory area, at Point Six, winds are reading 16 mph with gusts of 25 mph out of the W.  The forecast area received 5 to 8 inches of new snow in the last 24 hours.  The SWE totals range from .9 to 2.4 inches.

We are adding a significant new load to an already poor snowpack structure.  Travel in avalanche terrain is not recommended today.  This storm will continue through the weekend.

The primary concern is storm slabs.  Heavy new snow sitting on weak lighter snow.  These slabs will grow in size today.  There is potential for these slides to fail and step down to our deeper weak layers in the snowpack.

The second avalanche problem is persistent slabs.  These layers will be tested again with the addition of the new heavy snow.

I will issue the next advisory tomorrow December 30, 2017.

Ski and ride safe.

 

READ FULL ADVISORY  

Problem 1 - 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

    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.

New heavier wet snow on top of lighter dry snow. These slabs will continue to grow today and become more sensitive.  Travel in avalanche terrain is not recommended today.  Avoid runout zones.

Problem 2 - 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

    7-8

    3-4 (Very Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-6

    Likely/Very Likely

    The likelihood of an avalanche resulting from this problem.

With the addition of heavy wet snow these layers will become more reactive.  Avoid runout zones and travel in avalanche terrain is not recommended.

FORECAST & OUTLOOK

Travel in avalanche terrain is not recommended today. Avoid runout zones.  This storm is predicted to last into the weekend.  Avalanche danger will rise and  very dangerous avalanche conditions exist.

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.