Posted:
Feb 25, 2019 @ 5:51 am

An avalanche warning is in effect for backcountry terrain in the Bitterroot mountains. The current avalanche danger is HIGH from Granite Pass south to Lost Trail Pass. Very dangerous avalanche conditions exist. Travel in and around avalanche terrain is not recommended. For the rest of the forecast area, the avalanche danger is considerable.

This is Logan King with an avalanche warning for Monday, February 25th, 2019. This avalanche warning is valid for 24 hours. The avalanche warning will either be extended or terminated at 0600 on February 26th.

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

Weather and Snowpack

The Bitterroot mountains have accumulated 6-16 inches of new snow with southerly winds in the teens gusting to the twenties. The northern portion of the forecast area has received 2-6 inches of snow with strong Easterly winds of 10 mph with gusts into the twenties.

A hefty load of new snow has created dangerous avalanche conditions for the Bitterroot mountains. Weak layers of snow that are now buried under the new slab will be reactive with the increased load. Human triggered avalanches are likely and will be large in many areas. Avoid traveling in or near avalanche terrain today.

For the Southern Swans, Southern Missions and the Rattlesnake mountains a Considerable avalanche danger exists. Human triggered avalanches are likely. Look for red flag warnings like cracking, collapsing, whumphing and natural activity to identify areas of heightened concern. Careful snowpack evaluation, cautious route-finding and conservative decision making are essential for backcountry travel in these mountain ranges today.

Avalanche and Weather Outlook

Avalanche danger will continue to trend upwards as accumulating snow persists through the day and into tomorrow. More snow is expected to total between 7-14 inches for the northern part of the forecast area while the Bitterroot will continue to take the brunt of the storm with an additional 15-20 inches of snow expected by tomorrow. (Forecast)

Please feel free to share any observations on our Public Observations Page.

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

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-7

    Very Likely

    The likelihood of an avalanche resulting from this problem.

A significant load of new snow has fallen and will need a couple of days to settle and bond.

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

    5-6

    2-3 (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.

Freshly buried surface hoar and older surface hoar will be under significant stress from the new load of snow and will be touchy.

Problem 3 - Wind Drifted Snow

  • 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

    5-6

    2-3 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-4

    Possible/Likely

    The likelihood of an avalanche resulting from this problem.

With plenty of snow for transport and active winds, wind slabs will be problematic in specific terrain.

FORECAST & OUTLOOK

  • Danger Trend

    increasing

    Increasing Danger

  • Area Forecast

    Snow

Continued snow today with highest concentration located in the Bitterroot.

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.