Considerable Danger

January 1, 2018

Considerable

Posted:
Jan 1, 2018 @ 6:51 am

The current avalanche danger is CONSIDERABLE in the West central Montana backcountry. Careful snowpack evaluation, cautious route-finding, and conservative decision-making are essential to recreate today. Natural avalanches are possible and human triggered avalanches are likely across the region.

Good morning and happy New Year, this is Travis Craft with the West Central Montana Avalanche Center’s avalanche advisory for January 01, 2018. 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 range from 9 F to 21 F in the region. In the Bitterroot winds are 7 mph with gusts of 12 out of the SSW. In the northern part of the advisory area, at Point Six, winds are reading 24 mph with gusts of 28 mph out of the WNW. The forecast area received 0 to 1 inch of new snow in the last 24 hours.

We are getting reports of skier triggered slides from yesterday. These slides failed on the facets above the Thanksgiving day crust. Skier or rider triggered avalanches are likely.

The primary avalanche problem today is wind slabs. Leeward terrain will have large wind slabs. Look for rounded pillows of snow near ridgelines and recognize signs of instability such as cracking in the surface snow. These slabs will be sensitive to human triggers.

The second avalanche problem is persistent slabs. The layer of facets above the Thanksgiving crust has been stressed. This layer has been shown to fail in the last two natural avalanche cycles. This means careful snowpack evaluation, cautious route-finding, and conservative decision-making are essential to recreate today. Look for clues from the snowpack shooting cracks and localized collapsing. Dig a pit on low angle terrain in a safe spot out of runout zones to see how the layers are adjusting to the new load.

The final avalanche problem is storm slabs. These slabs are becoming less reactive as the snowpack adjusts to the weight of the new snow. Look for shooting cracks and try small test slopes with low consequence to see how the new snow is bonding.

Avalanche and Weather Outlook

Our next chance for snow will be in the middle of the week. With the forecast look for the avalanche danger to slowly decrease as the snowpack adjusts to the new snow.

If you are out in the backcountry, please send us your observation, these are very helpful in producing the advisory. I will issue a weather update tomorrow January 01, 2018.

Ski and ride safe.

READ FULL ADVISORY

Problem 1 - Wind Slabs

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

Likely
The likelihood of an avalanche resulting from this problem.

Leeward terrain will have large wind slabs. Look for rounded pillows of snow near ridgelines and recognize signs of instability such as cracking in the surface snow.

Problem 2 - Persistent Slabs

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
The likelihood of an avalanche resulting from this problem.

The layer of facets above the Thanksgiving crust has been stressed. This layer has been shown to fail in the last two natural avalanche cycles. This means careful snowpack evaluation, cautious route-finding, and conservative decision-making are essential to recreate today. Look for clues from the snowpack shooting cracks and localized collapsing. Dig a pit on low angle terrain in a safe spot out of runout zones to see how the layers are adjusting to the new load.

Problem 3 - 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

1-2 (Small-Large)
The potential size of avalanche resulting from this problem.
LIKELIHOOD

Possible/Likely
The likelihood of an avalanche resulting from this problem.

These slabs are becoming less reactive as the snowpack adjusts to the weight of the new snow. Look for shooting cracks and try small test slopes with low consequence to see how the new snow is bonding.

FORECAST & OUTLOOK

Danger Trend

Same Danger
Area Forecast

Sunny

Next chance for snow is mid week. Here is a link to the discussion.

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.