February 27 Avalanche Advisory
Posted Friday February 27th, 2009 at 0600.
Hello backcountry riders and sliders! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for February 27th, 2009. 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. Expect avalanche conditions to change as weather conditions change. Our advisory area includes the Bitterroot Mountains from Lost Trail Pass North to near Lookout Pass, the Rattlesnake Mountains and the Southern Swan and Mission Mountains near Seeley Lake. Avalanche information about the St. Regis Basin can be found on the Idaho Panhandle Forest Avalanche Center website.
Weather and Snowpack Analysis
After a lengthy period of little or no snow, our mountains received abundant snowfall this week reminding us that it is still winter. Bitterroot Mountain SNOTEL sites recorded an average of 2 inches of snow water equivalent (SWE) while Stuart Peak and the North Fork Jocko SNOTEL recorded 1.5 inches of SWE. Saddle Mountain SNOTEL was the loser with only .7 inches, Hoodoo Basin SNOTEL piled up 3.3 inches of SWE. In other words, we were dumped on!
This snow came in warm and heavy which is good because it appears to have bonded quickly to the hard surface that formed during the sunny warm weather last weekend. All our observers reported dangerous avalanche conditions on some terrain features but not as bad as you would have expected given the set up we had. During and immediately after the initial storm on Tuesday and Wednesday conditions were very sketchy as the old snow was getting loaded quickly and heavily putting a great deal of stress on a couple of weak layers. We have been talking about buried crusts and faceted snow associated with those crusts for a few weeks now. These layers of concern are the sun crust that formed last weekend, the crust and facets that formed around Groundhog Day and the significant melt-freeze crust and facets that formed around Martin Luther King Day.
We received an outstanding report and some excellent photos of this very situation from John Lehrman in the Bitterroot Mountains near Downing Mountain. Check it out on John’s website backcountryfocus.com .
All our observers described very similar conditions regarding the new snow. It has bonded well to the old snow surface, it takes a lot of force to get it to fail, it does not fail cleanly nor with energy but when it fails it fails at the thin faceted crust layer that formed in early February and it is a big slab. You don’t want to get caught up in something this big. So the recommendation is to take it easy for a couple of days to give the new snow a chance to settle out and give these crusts some time to glue together.
Current Avalanche Danger
In the Bitterroot Mountains above 5000 feet on terrain steeper than 35°, the avalanche danger is CONSIDERABLE. Dangerous unstable slabs can be found on many steep slopes especially those that have been recently wind loaded. Human triggered avalanches are probable, natural avalanches possible.
At all other locations above 5000 feet in West Central Montana, the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. Below 5000 feet, the avalanche danger is LOW where natural avalanches are very unlikely and human triggered avalanches are unlikely. You can almost always find isolated pockets where there are dangerous avalanche conditions on some terrain features.
The Bitterroot Range received the most precipitation this week but keep in mind there may be pockets of considerable avalanche danger in the Rattlesnake and in the Southern Mission and Swan Ranges near Seeley Lake.
Weather Forecast and Avalanche Outlook
Snowfall has tapered off this morning and we can expect a few lingering showers over the mountains for the next few hours. Winter storm warnings have expired but winds may still be an issue at the highest elevations. A ridge of high pressure will build over our area Friday night into Saturday with another weather system expected to enter the area Sunday and Monday. Temperatures may drop into the single digits early Saturday under clear skies then again climb back into the 30 degree range for the weekend.
Expect conditions to improve unless we undergo rapid warming under clear skies or receive rain on this newest snow. High winds will also contribute to instability on steep leeward terrain.
The past few weeks we’ve enjoyed a very stable snowpack and conditions that allow us to travel into terrain that was impossible to get to during the New Year. Now that we’re getting the snow we dream about and can get to the big lines, we need to step or throttle it back, be a little extra cautious, take a little more time to fully assess stability conditions on the particular slope you want to jump into before committing yourself and others. Then when you decide to do it, do it one at a time. Always!
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 issued Monday March 2nd, 2009.