Dec 25, 2009 @ 12:00 am

Christmas Day Avalanche Advisory

Merry Christmas! This is Steve Karkanen at the West Central Montana Avalanche Center with backcountry avalanche information for Christmas, 2009.

We have been experiencing technical problems with our server this morning so we are posting later than usual. Sorry about the delay.

Current Avalanche Danger

The avalanche danger above 7000′ on terrain steeper than 35 degrees where snow has covered anchors has improved but not enough to let your guard down. CONSIDERABLE avalanche danger exists in these locations.

Moderate temperatures helped settle the snowpack in many locations and on some specific aspects helped strengthen the weak interface between the newest snow and the weak sugary base of facets we have been concerned about.

At lower elevations, 5000 to 7000 feet, and in areas where the snow has not yet buried anchors the avalanche danger is MODERATE.

Below 5000 feet, the avalanche danger is LOW.

Weather and Snowpack Analysis

Very little snow accumulated since our last report Monday but what we did get was enough to tip the balance on many slopes. We received reports that east to north aspects avalanched on their own and we saw evidence of this on a tour in the Rattlesnake Thursday. These avalanches probably ran during the warm wet weather Sunday and Monday.

Mountain temperatures again plummeted Wednesday night and Thursday but are expected to return to a more normal pattern by the weekend. These cool temperatures will allow the facets we have been talking about to persist and lose some of the strength built up during the warm spell.

The next problem developing with this current weather condition is surface hoar.

Everybody is seeing it, it is not a problem now but if it is buried before it is destroyed by the wind or sun, we will be describing another weak layer backcountry travelers need to consider before crossing or jumping onto slopes steeper than 30 degrees.

We were experiencing moderate range stability test scores in the Rattlesnake as well as in the Bitterroots from Lost Trail to Lookout Pass.

A fair amount of time has passed and our snowpack has slowly been gaining strength. Stability test scores are showing this but the problem is when the test area fails; it fails with a lot of energy, is fast, clean and easily propagates.

Compression test scores throughout the area were failing in the high teens and 20’s for a moderate stability result but all with high quality, quick and smooth planar shears. On a west to southwest aspect at 7800′ an extended column test failed on isolation (ECTPV) and compression test scores were consistent CT5 all with high quality shears. A southeast aspect with similar depth appeared to be the most stable slope with CT scores at 23. As we moved to more east facing slopes the snow was weaker with CT scores of 19, ECTP19 and a Rutschblock score of 4; failed on one hard jump.

All these failures are at the interface of the slab and the faceted snow near the ground that formed in early December.

Many high elevation sites look very tempting right now and the next big snow will be just too good to pass up on an opportunity too get those first tracks in. There is an incredible amount of variability in our snowpack right now as we saw yesterday. The one thing you can count on is that above 7000 feet, there is now a slab sitting on about 50cm of super weak faceted snow. Yes, it is gaining strength, but slowly, and maybe only enough strength for one person to get away with it.

This YouTube video shows why we are concerned about the future of our snowpack in the higher, steeper ground in the Bitterroots, Swan and Mission Mountain ranges.

Weather Forecast and Avalanche Outlook

The Missoula Office of the National Weather Service is forecasting that our ridge pattern will shift inland this weekend with drier air moving into the area. We can expect to see strong surface inversions for the next few days. The next good opportunity for moisture appears to arrive by mid-week when the high-pressure ridge breaks down Tuesday under an approaching Pacific weather system.

Expect avalanche conditions to continue to slowly strengthen but quickly deteriorate if we receive heavy new snowfall. It is going to take several weeks for the weakness at the base of the upper elevation snowpack to gain strength.

The next avalanche advisory will be posted on December 28.

Transceiver Park Information

There are now 3 Beacon Basin avalanche transceiver parks in operation in western Montana. They are located at Lost Trail Powder Mountain, Lolo Pass Visitor Center and Montana Snowbowl. All three will be available for use for the entire season during operating hours at the ski areas and Lolo Pass Visitor Center.

The Beacon Basin at Montana Snowbowl requires purchase of a lift ticket to access the site located at the backcountry access point to Point Six and is available everyday. If the unit is not there, check in with the ski patrol in the Warming Hut at the top of the Lavelle lift and they will give you the control unit with directions to the site.

Easy, free and a fun way to practice using that fancy transceiver you have that will freak you out when you are in a real avalanche situation where your best friend is buried.

If you get out and see avalanche activity or want to send us quick snow observations, send us a note at [email protected] or call us at 406-530-9766. 530-9SNO.


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.