High Danger

February 5, 2021

Avalanche Warning

Posted:
Feb 5, 2021 @ 7:32 pm

An avalanche warning has been issued for the southern Mission, southern Swan, Rattlesnake, and southern and central Bitterroot mountains. The avalanche danger for the west central Montana backcountry is HIGH. The avalanche hazard is increasing with continued snowfall, rising temperatures, and wind. Human triggered avalanches are certain. Avalanches may run long distances. Travel in avalanche terrain is not recommended. This avalanche warning is valid for 24 hours. The avalanche warning will either be extended or terminated at 7:00 pm on February 06, 2021.

Weather and Snowpack

Strong winds and new snow are creating dangerous avalanche conditions. Today in the southern Missions, we found 20 new inches of snow and saw active transport of snow to leeward aspects. New snow is testing the weak layers in the snowpack. We were able to remotely trigger slides at Gash Point in the central Bitterroot on multiple aspects on buried weak layers. We had shooting cracks and propagations at Lolo Pass. These are all signs to stay away avalanche terrain.

Avoid traveling in avalanche terrain today. Avoid being under run out zones. Expect the avalanche danger to continue to be elevated as more snow and wind continue till Monday.

The Bottom Line

Travel in avalanche terrain is not recommended. Avoid runout zones. You can trigger an avalanche remotely from the side, below or above you.

This warning will be terminated or extended tomorrow at 7:00 pm.

Ski and ride safe.

READ FULL ADVISORY

Problem 1 - Storm slabs

TYPE

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

2-3 (Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Very Likely
The likelihood of an avalanche resulting from this problem.

10 to 20 inches of new snow throughout the advisory area. This is a significant amount of new snow and will take time to bond.

Problem 2 - Wind Drifted Snow

TYPE

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 (Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Very Likely
The likelihood of an avalanche resulting from this problem.

New snow is being loaded onto leeward slopes. Winds are shifting and are cross loading slopes.

Problem 3 - Persistent Weak Layers

TYPE

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 (Large-Very Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Possible/Likely
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 specific areas, 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.

Pay attention to shooting cracks and collapsing in the snowpack. If you get off your machine or skis and punch through to the ground, it is a sure sign of a poor snowpack structure.

The January 13 crust facet combo is now 2 to 4 feet deep and reawakening.

The new snow will be testing these weak layers in the snowpack.

It is very possible to trigger an avalanche remotely from the side, below and above.

GALLERY

Lake-Elsina-05-Feb

gash

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gash

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Deep Snowpack Brushy Fork

Rattlesnake-windslab-02_05_20211

Rattlesnake-shooting-crack-02_05_20211

Observations 02/05/2021

FORECAST & OUTLOOK

Danger Trend

Same Danger
Area Forecast

Snow

Snow will pick back up tonight and continue through Monday. The addition of new snow will keep the avalanche danger elevated. 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.