Avalanche Warning December 29, 2008
Good morning! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for December 29th, 2008. 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.
An avalanche warning is in effect for the mountains of West Central Montana. The avalanche danger is HIGH, natural and human triggered avalanches are likely. This warning is for mountainous locations above 5000′ and includes the Bitterroot Range, the Rattlesnake Mountains (including Sheep Mountain in the Lower Blackfoot) and the Southern Missions and Swan Range near Seeley Lake. Backcountry travel is not recommended. Avoid travel on or underneath open slopes steeper than 30°.
Weather and Snowpack Analysis
The faceted weak snow structure above and below the rain crust that formed in early December is now a serious problem in all areas where the snow is deep enough to cover anchors. A strong winter storm entered our area this weekend with heavy snowfall and high wind speeds. This system came in at much warmer mountain temperatures than we’ve seen in about 3 weeks giving us avalanche conditions that have not been seen here in years.
SNOTEL sites are reporting 1.5” to nearly 3” of SWE (snow water equivalent) in the Bitterroot Mountains and 1.5” to 2” SWE in the Rattlesnake and Southern Missions the past 48 hours. This translates to 15” to 30” of new snow in some locations. Temperatures are much warmer with this storm with temperatures approaching 30° at many mountain locations Sunday. Rain was reported to about 5500′ in many areas and wind speeds were in the 80mph range on Point Six. This heavy dense snow was deposited onto very cold light snow from the past 2 weeks of cold weather and many natural avalanches ran during the day Saturday. In areas that received rain, avalanches were noted at lower elevations along road cuts or on any unanchored slope. These generally went to the ground. At higher elevations the heavy new snow has brought most slopes to the brink as the weaker faceted snow near the ground is now unable to support the weight. We’ve received excellent reports from backcountry travelers in the Bitterroot who witnessed large avalanches run near St Mary’s Peak and Gash Point. Check out the photo of the avalanche near St Mary’s that broke through the ice of a small lake in the runout zone in our photo gallery.
Stability testing in the Rattlesnake at 7600′ was producing easy failures to the facets above the rain crust we’ve been describing. This is 20cm from the ground and depending on the location and how the wind has loaded the particular slope, the slabs that failed were anywhere from 2′ to 5′ deep.
One of the more disconcerting things we’re seeing is the length fractures have been able to propagate. We’re not talking about a few feet; we’re seeing slopes fracture for hundreds of yards.
Current Avalanche Danger
On all aspects above 5000′ the avalanche danger is now HIGH on all open slopes steeper than 30°. Natural and human triggered avalanches are likely. Unstable conditions exist on any open slope especially those that have been wind loaded or where anchoring vegetation such as stumps or boulders are covered by snow. Travel in avalanche terrain is not recommended.
Below 5000′, the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. The weak features we’ve been describing (weak faceted snow on a rain crust) have come into play in areas where the snow is deep enough to cover anchors. Unstable slabs can be found in isolated areas that have been wind loaded. These are typically on the leeward side of ridges or other terrain features, especially in open areas and can be found on all aspects.
SPECIAL NOTE: The foothills around Missoula are beginning to accumulate respectable snow depths. While these areas are outside of our avalanche advisory area, there is terrain steep enough to slide and avalanches have happened under the right conditions. In January of 1993, a young man died in an avalanche on Mt. Jumbo within sight of East Missoula. It’s worth paying attention to during unusual winter conditions and we have unusual conditions this year.
Weather Forecast and Avalanche Outlook
The Missoula Office of the National Weather Service is forecasting that a strong Pacific Jet Stream will continue to bring moisture into Western Montana. There is potential for continued significant snowfall at the higher elevations Monday and again Wednesday. “The nature of these Pacific storm systems will likely result in more dense snow than that of a few weeks ago”.
That says it all. Heavy dense snow piled onto a very weak snowpack. You bet, all this new snow is a good thing. We need it, it’s coming in a way that will give us good riding and travel conditions later and it isn’t 20° below zero anymore. But it is coming with a price and that price is dangerous avalanche conditions.
This is simply not a good time to travel in avalanche terrain. The snow needs time to strengthen. Avoid being on or under open slopes steeper than 30°. Does that mean you have to stay home? NO! There are plenty of places to play that are safe. There’s something about making first tracks that reaches into our exploratory spirit even on the flats so use this time to explore new trails, get used to your new equipment and practice using your new (or old) avalanche transceiver. The reward for satisfying your desire for taking risks on steep ground in these conditions may end up being the worst nightmare you, your family and friends can imagine. Don’t buy into the concept of it being a good thing to die while doing something you love. If you love it that much, do everything possible to make it happen again, and right now that means waiting for a safer day.
Have a super fun and safe New Year!
Your weather and snow observations are very important to us. Information about avalanche activity or weather conditions you see out there may save a life. If you have any information you’d like to share or have questions about anything related to snow safety, please contact us at [email protected]
The next avalanche advisory will be posted Friday January 2nd, 2009.