Posted:
Jan 26, 2021 @ 6:46 am

The avalanche danger is MODERATE in the Rattlesnake and LOW in the central and southern Bitterroot, and Seeley Lake zone. It is possible to trigger an avalanche in the Rattlesnake. There are isolated avalanche problems in the other zones. Low does not mean no avalanche danger.

Good Morning. This is Jeff Carty with the West Central Montana Avalanche Center advisory on Tuesday, January 26th, 2021. This advisory is sponsored by the Rocky Mountaineers. This advisory does not apply to operating ski areas and expires at midnight tonight. The USDA Forest Service is solely responsible for its content.

Weather and Snowpack

Mountain temperatures range from 13 degrees to 22 degrees F this morning. No new snow overnight. Winds are light and southerly.

Low hazard is defined as: Generally safe avalanche danger, watch for unstable snow on isolated terrain features.

This is what we have throughout the majority of our forecast area. Overall it is unlikely to trigger an avalanche. However, due to spatial variability diligent hazard assessment is required despite the low rating. Suspect areas are shallow snowpacks, generally under 4 feet in depth, where persistent weak layers exist. These can be found at mid elevations and on wind scoured slopes. Upper elevation leeward slopes may contain wind slab. Cornices have been growing and are a hazard. Terrain traps will increase the consequences of these problems.

The exception to the low rating is the Rattlesnake, where there is moderate hazard. This range has received less snow than the rest of the forecast area and as a result the structure has been slowly deteriorating due to the growth of facets and depth hoar.  These layers are stubborn and deep. While it is not likely to trigger these layers, it is possible. Given the depth and stiffness of the slab, the resulting avalanche could be very destructive. 

Layers such as this can be tricky to assess. As persistent weak layers get buried deeper, they become unpredictable. They will likely not give warning signs such as cracking or whumphing, can avalanche despite being skied repeatedly, and as they get deeper can produce false stable results on extended column tests. The propagation saw test is a good test for deep faceted layers if you are not seeing propagation in the ECT. The best approach is to avoid slopes over 30º. 

Bottom Line

Generally stable conditions exist, except in the Rattlesnake where it is possible to trigger an avalanche. Due to spatial variability, isolated instabilities are present throughout the forecast area. Avoid slopes with weak sugary snow. Do multiple pits investigating the stripes in your pit wall. Avoid cornices and wind loaded terrain. Choose simple terrain that does not expose you to terrain traps. Avoid likely trigger points on slopes.

Travel one at a time in avalanche terrain, carry a beacon, shovel, and probe. Remember to reassess conditions throughout the day and stay alert for signs of instability. Dig a pit. Look for red flags.

Upcoming Education Events:

Please visit our education page for an up to date list of regional educational events and course offerings. Below are a few select events and opportunities to check out.

  • January 27th, 6-7:30 PM MST | FREE Online 1.5-hr Avalanche Awareness Session | Missoulaavalanche.org event | Delivered by A3 Pro instructors | Get more details and register HERE

Public Observations

Thank you to everyone who has taken the time to send in a public observation. Please keep sharing what you find and see while out in the backcountry. This online forum is a great resource to glean information about current conditions.

Remember, you can submit your observations through the observation page anonymously. When submitted anonymously, the forecasters review the observation and utilize it when generating the forecast. The information does not appear on the public observation page.

Ski and ride safe.

 

 

READ FULL ADVISORY  

Problem 1 - 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

    5-6

    2-3 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    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

Faceted layers can be found throughout the forecast area. Shallower snowpacks have worse structure than deeper areas and are more suspect. In some places, persistent slabs are up to 40 inches thick and have become low likelihood, high consequence problems. The worst faceting is in the Rattlesnake where depth hoar has developed at the base of the snowpack, especially on shaded, northerly aspects.

The problem facets tend to be concentrated around crusts. Look for these and soft, sugary snow above and below as you assess the snowpack.

The rain crust from 01/13/2021 has facets below but has been gaining strength. Located from the surface to 1 foot deep in the snowpack depending on location. It can be found up to 8500 ft in some areas.

The extended column test (ECT) and the propagation saw test (PST) are best for assessing the stability of these layers.

Problem 2 - Wind slab and cornice fall

  • 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-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:

    NE - Northeast
    E - East
    SE - Southeast

Winds are light and minimal snow transport has occurred. However, evidence of wind slab was found yesterday and cornices continue to be a hazard.

Upper elevations, just below ridgeline are where these are most likely found.

Watch for pillows and drift like formations. Shooting cracks are a warning sign.

Carefully assess wind loaded start zones and cross-loaded slopes greater than 35º.

Stay out from underneath cornices, and well away from the top.

VIDEO

GALLERY

Camas Lake, 01/25/2021
Camas Lake, 01/25/2021
Wind slab, Bitterroots, 01/25/2021
Wind slab, Bitterroots, 01/25/2021
S Fork Spruce-25-Jan.jpg
S Fork Spruce-25-Jan.jpg
Rattlesnake facets, 01_22_2021
Rattlesnake facets, 01_22_2021
Sweeney cornice, 01_20_2021
Sweeney cornice, 01_20_2021
Extended column test poster (1)
Extended column test poster (1)
Fractures in depth hoar (2)
Fractures in depth hoar (2)

FORECAST & OUTLOOK

High pressure will slowly shift eastward today into Wednesday as the trough over the West Coast attempts to move further inland. Moisture will also attempt to move into the Northern Rockies as it does so, however divergent flow aloft will split the moisture, keeping most of it out of west central Montana. If anywhere picks up light accumulations, it will be the southern Bitteroot range. As the center of the low associated with this trough remains centered in the Desert Southwest, we expect additional, disorganized, and generally weak pulses of moisture to be ejected into our region over the coming days. Generally light winds are likely through mid-week.

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.