Posted:
Jan 23, 2021 @ 6:15 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 Travis Craft with the West Central Montana Avalanche Center advisory on Saturday, January 23rd, 2021. This advisory is sponsored by LB Snow. 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.

The snowpack is gaining strength throughout most of the region. Shallow snowpacks( 3 feet deep) are the weakest. You can find these shallow points in lower elevations and throughout the Rattlesnake. Yesterday, we saw small point releases in Lolo, St Mary’s, Seeley Lake area, and the Rattlesnake. The Jan 13 rain crust is the layer to investigate before committing to avalanche terrain. There are facets below the crust that are sometimes reactive in pit tests. The Rattlesnake has the weakest, shallowest snowpack. The weak structure in the Rattlesnake is creating Moderate avalanche danger.

Today, I would choose simple, low angle slopes and avoid slopes with shallow snowpacks in the Rattlesnake. In the rest of the advisory area, I would avoid wind-loaded terrain, choose simple slopes, and then work into more complicated terrain. The places to avoid on slopes are near cliff bands, rocks, and rollovers. These areas will have shallower snowpacks where it more likely to trigger an avalanche.

Bottom Line

Today, do your homework before committing to steep slopes. 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

    4-5

    2 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-2

    Unlikely/Possible

    The likelihood of an avalanche resulting from this problem.

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 becoming low likelihood, high consequence problems. The worst faceting is in the Rattlesnake.

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 it that can propagate. Located at the surface or 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 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

    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.

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.

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

VIDEO

Observations 01/23/2021

GALLERY

St. Marys Area-22-Jan
St. Marys Area-22-Jan
Jenny Bowl-22-Jan
Jenny Bowl-22-Jan
Marshall Mountain-22-Jan.jpg
Marshall Mountain-22-Jan.jpg
pbr2
pbr2
pbrpit2
pbrpit2
Point 6 windslab, 01_22_2021
Point 6 windslab, 01_22_2021
Rattlesnake facets, 01_22_2021
Rattlesnake facets, 01_22_2021

Observations 01/22/2021

FORECAST & OUTLOOK

  • Danger Trend

    same

    Same Danger

  • Area Forecast

    Light Snow

Light snow and winds out of the West 15 mph over the next couple of days. See the forecast.

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.