Posted:
Mar 5, 2020 @ 6:49 am

The avalanche danger is Considerablein wind loaded terrain and moderate in all other terrain the west central Montana backcountry. 

Good morning, this is Jeff Carty with the West Central Montana avalanche advisory for March 5, 2020. Today’s advisory is sponsored by The Trail Head. This advisory 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

Over the past 48 hours westerly winds have blown up to:

  • 54mph with gusts to 77 mph at Point 6
  • 80 mph at Lolo Pass

There were sustained strong winds for over 24 hours throughout the forecast area. Do to local terrain affects multiple aspects are impacted by these winds. Winds are currently light and forecast to be light with strong gusts throughout the day.

Mountain temperatures have ranged from:

  • 15º to 33º above 6000’ and are forecast to rise as high as 49º today. 

Snow accumulation in the past 48hr is roughly:

  • 10”  in the Seeley-Swan
  • 5”  in the Rattlesnake
  • 8″ at Lolo Pass
  • 8” new in the central Bitterroot
  • 3″ in the southern Bitterroot

Our primary concern is wind slab. These have grown to over three feet thick in some areas, are very sensitive and likely to be triggered by a skier or rider.

Yesterday at Lolo, Travis was getting 10’ long shooting cracks while touring. Snowbowl was closed due to wind, expect extensive wind slab in the Rattlesnake. Wind transport and loading have been extensive and considerable throughout the forecast area (video). Due to the strength of the wind, slabs may be lower on slopes than normally encountered, and deposited on multiple aspects. Steep wind loaded terrain >33º should be approached with caution. 

In other areas such as the Gash and the first bowl at Wishard Ridge, wind slabs were not an issue, the snowpack has bonded well despite wind transport. There is a lot of variability throughout the forecast area but the extent of the wind effect means you should approach any avalanche slope with caution and evaluate the touchiness of windslab before preseeding.

Where there is not wind loading the snowpack is, overall, well bonded and stable. There are, however, exceptions.

Buried surface hoar can be found in isolated areas. It is present and reactive on sections of Mt. Fuji (video) and maybe lurking on other sheltered north slopes. Dig a pit, perform tests and look for it before committing to steep terrain.

There are near-surface facets growing underneath the 02/28 crust. While it is not currently an issue, it may become a problem layer with a load of new snow on it.

In the southern Bitterroot, depth hoar at the bottom of the snowpack is still a concern. Choose a shallow spot to dig. If you find weak sugary snow at the bottom of your pit, choose a different slope where this structure is not present.

Warm, above seasonal temperatures today and Friday will increase the risk of wet loose avalanches, especially on sun warmed aspects. The extent of warming will loosen bonds within the snowpack and may make layers that have been unreactive more sensitive, increasing the risk of slab avalanches. The hazard from warming will increase throughout the day with rising temperatures.

Bottom Line

Strong winds have built dangerous wind slabs at a variety of elevations, avoid steep wind loaded terrain. Very isolated pockets of a buried surface hoar and crust/facet layer exist. Perform a stability test to evaluate these layers. Depth hoar in the southern Bitterroot is a low likelihood, high consequence problem and still deserves caution. Wet loose avalanches are an increasing risk throughout the day as temperatures rise. Travel one at a time in avalanche terrain, carry a beacon, shovel, and probe, and stay alert for signs of instability. Look for red flags. Give cornices a wide berth, they will be more likely to fall with the warming.

Education

Your observations are extremely helpful! If you get out, please take a minute to fill out the observation form on our website (missoulaavalanche.org), or shoot us a quick email at [email protected]

We offer a variety of avalanche courses throughout the winter. Go to our course offerings page on our website. Check out the list and get yourself enrolled in one of our many courses this winter!

Ski and ride safe.

 

READ FULL ADVISORY  

Problem 1 - Wind Slab

  • 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

    4-5

    2 (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.

  • 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
    NW - Northwest

Winds to 80 mph out of the west have built dangerous wind slab over three feet deep that could bury injury or kill a person

It is likely to trigger a windslab today.

High winds have deposited windslab further down slopes than normally encountered and have wrapped around terrain features to deposit windslab on a variety of aspects.

Be aware of cross loading as you travel in the mountains today.

Be on the lookout for dense pillows of hollow sounding snow and shooting cracks.

Avoid steep, 33º+, wind loaded terrain.

High winds have been adding to cornices, which were already huge, give these a wide berth.

The likelihood of cornice fall is increasing with warm temperatures today.

Problem 2 - Persistent weak layers

  • 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

    3-4

    1-2 (Small-Large)

    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:

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

There are isolated weak layers in our snowpack. Dig a pit about 4 feet deep and look for stripes of snow. Perform a pit test and see how reactive these layers are.

In the southern Bitterroot, dig in a shallow spot less than 5 feet, and if you find weak sugary snow at the bottom of the pit, choose a different slope.

Problem 3 - 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-4

    Possible/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

Freezing levels are forecast to rise as high as 8000′ today.

Loose wet avalanches are possible, especially on sun-warmed aspects.

At lower elevations, <6000′ they are possible on all aspects.

While low risk on their own, wet loose avalanches become more of an issue if they push you into hazards or terrain traps.

Wet loose avalanches are a sign of weakening bonds in the snowpack and are a red flag.

Existing layers in the snowpack that have been unreactive may become more touchy, especially as we move into Friday with above freezing temperatures forecast for tonight in many areas.

 

VIDEO

FORECAST & OUTLOOK

High clouds with sunshine and above average temperatures can be expected today and Friday. Temperatures in the mountains will be much warmer tonight and Friday night, in the 30s for many locations. There will be some fluctuation in mid-level cloudiness on Friday as higher level moisture begins to move in ahead of a Pacific system…but it should remain dry. A Pacific cold front brings a chance for snow above 5,000 feet by Saturday afternoon…down to 4,000 feet by Saturday evening. It will be cooler Sunday with run-of-the-mill snow showers in the mountains and sunny periods. Increasing clouds with light snow possible Monday.

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.