Posted:
Apr 8, 2019 @ 5:45 am

Rain, wind, and heavy snow have increased the avalanche danger to CONSIDERABLE. Human triggered avalanches are likely. Warm temperatures and saturated snow with more moisture today are creating dangerous conditions that require conservative decisions and astute route-finding.

Good morning, this is Logan King with the West Central Montana Avalanche Center’s avalanche advisory for Monday, April 8th, 2019. This danger rating does not apply to operating ski areas, expires at midnight tonight and is the sole responsibility of the U.S. Forest Service.

Weather and Snowpack

Mountain temperatures this morning are in the low thirties. Winds have subsided overnight while rain and snow continues to fall across the region. Snow lines have oscillated up and down through the storm complicating things. Upper elevations have received 6-10 inches of snow as of this morning while mid elevations have 2-5 inches mixed with rain and lower elevations have seen entirely rain. All of which adds up to over an inch of SWE in the past 24 hours.

Rain lines were around 6,500 feet yesterday and may creep further upslope today after lowering slightly overnight. 36-hour SWE totals of up to almost 3 inches can be found in the Bitterroot Range with plenty more on the way. Widespread wet avalanches will be seen for the next few days. Below rain line, free water in the snow will decrease the strength of the snowpack and make it susceptible to human triggers and some natural failures. Above rain line, dense snow and strong winds are creating a heavy load of surface snow that is stressing the snowpack. Wet avalanches will be widespread and possible on all aspects and elevations with the largest slides possible at mid-elevations.

Strong wind accompanied the initial snowfall and will make wind drifted snow easy to trigger. Sundays winds moved a good amount of new snow and has created small windslabs at upper elevations that were easily triggered yesterday in the rattlesnake. Newly drifted snow may be sitting on crusts or at a wet snow interface and have poor bonding. Avoid steep wind loaded terrain above 6,000 feet where avalanches will easily be triggered by humans today.

Bottom Line: The heavy load of dense snow and rain have destabilized the snowpack and will continue to do so for the next few days. This is likely the catalysts that will kick off the first significant springtime avalanche cycle.

Avalanche and Weather Outlook

A very moist system continues to impact the region with plenty more to come (forecast). Snow and rain will continue today further enhancing the avalanche danger. However, snow levels look to drop Tuesday and transitions to more snow than rain. The avalanche danger will continue to increase as the storm progresses and avalanche problems may even change as precipitation transitions from rain to snow.

We will conclude our regular forecasts later this week as the storm cycle ends but will continue to post updates as necessary after that. Observations on our public observations page will continue to get posted and shared with the community so keep sending in what you find.

Ski and ride safe.

READ FULL ADVISORY  

Problem 1 - Wet Slab Avalanches

  • 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-4

    Possible/Likely

    The likelihood of an avalanche resulting from this problem.

Rain to about 6,500 feet and wet dense snow at higher elevations will result in an active snowpack.

Problem 2 - Loose Wet

  • 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-5

    Likely

    The likelihood of an avalanche resulting from this problem.

Wet loose activity is likely in steep terrain at all elevations today.

Problem 3 - Glide avalanches

  • TYPE

    glide-avalanche

    Glide Avalanches

    Release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. The are often proceeded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

    Predicting the release of Glide Avalanches is very challenging. Because Glide Avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide Avalanches.

  • 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.

Saturated snow and free water in the snowpack are increasing glide activity.

Problem 4 - 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-5

    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
    NW - Northwest

Wind drifted snow at upper elevations will fail easily today.

FORECAST & OUTLOOK

Low elevation rain with upper elevation snow will persist today as a moist system continues to impact the region.

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.