Dec 13, 2009 @ 12:00 am

December 13 Information update

This is Steve Karkanen at the West Central Montana Avalanche Center with an early season avalanche safety information statement.

We are a bit edgy here at the avalanche center with the arrival of our first major winter storm. A lengthy period of cold windy weather at the higher elevations turned the base of our future snowpack into a weak faceted mess.

A winter storm warning has been issued for western Montana with snowfall amounts estimated to be 12 inches to 17 inches above 6000 feet by Monday.

This storm will easily overload an already weak snowpack even at the lower end of the precipitation estimates. Any slope steeper than 30 degrees that receives more than 6 inches of new snow should be suspect until proven otherwise. Wind-loaded terrain will be especially sensitive during this storm cycle so pay close attention to any indicator of instability. This will be a dangerous time to ski or ride on recently loaded slopes steeper than 30 degrees on any aspect. Please keep informed about this storm as it develops.

Average snow depths at the SNOTEL sites in our area are only 20 inches and average temperatures have been well below 20 degrees since the first of the month with a six-day period when temperatures remained below zero with high winds mostly from the east and northeast during this arctic blast.

This cold period with little or no snow accumulation worsened an already weak layer at the ground and grew faceted snow throughout the snowpack. An additional layer of concern is a warm wx or sun crust that formed sometime around Thanksgiving. Some locations have two of these crusts with weak faceted snow above and below these layers.

In addition to the cold temperatures, east winds redistributed snow from the usual leeward east to northeast aspects to west to southwest aspects so expect weak basal snow on all aspects when the first major winter storm arrives.

That storm arrived just as the Montana Grizzlies beat Appalachian State today, 24-17, a great way to bring on the snow!

We have received several excellent reports this past month all of which describe mostly thin early season conditions. The northern Bitterroot Mountains near Hoodoo Pass have adequate snow for the hardcore but even here, the goods have been limited to remote and sheltered wind loaded terrain.

The major red flags or indicators of an unstable snowpack condition are:

Recent avalanche activity– this is the clearest indicator of an unstable snowpack that many people overlook.

Heavy new snowfall or rain– most avalanches occur during or immediately after a storm.

Collapse noises or a whumpfing sound of the snow pack – if you experience this on flat terrain assume unstable conditions on a steeper slope.

Shooting cracks or fracture propagations running out from you as you travel, typically associated with collapse noises.

High winds – any amount of snow with wind can quickly raise the avalanche danger on leeward terrain.

Warming temperatures – a storm that starts out cold and turns warm puts heavy snow on cooler weaker layers.

If you see any of these indicators, the snowpack is unstable.

If you do get out and see any activity or just want to let us know what you see out there, send us a note at [email protected]. We really appreciate observations from you, it helps us provide better information in our advisories.

We begin issuing regular Friday avalanche advisories on December 18 and regular Monday advisories as soon as the snow begins to accumulate.


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.