March 6 Avalanche Advisory
Posted Monday March 6th, 2009 at 0630.
Hello! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for March 6th, 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. 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 National Forest Avalanche Center website.
Weather and Snowpack Analysis
Since our last report Monday, mountain weather conditions have gone from sunny and warm Monday to cloudy, rainy and warm mid-week to more winter like Thursday. Temperatures associated with a cold front moving through the area dropped allowing all the heavy precipitation embedded within the clouds to fall out as snow which has been heavy at times in some locations. SNOTEL sites report several inches of new snow since Thursday with the Lookout Pass area receiving the most at 17 inches, Lolo Pass 10 inches, Saddle Mountain 8 inches and Stuart Peak 4 inches. Anemometers on Point Six were registering wind speeds in the 40 mile per hour range from the West-Southwest yesterday, but are showing mostly light winds at 5-8 mph this morning. Temperatures are in the teens at most SNOTEL sites.
Observers Thursday all reported that the newest snow is bonding well to the hard snow surface and stability testing indicates a mostly stable condition in all areas with a few notable exceptions. The biggest exception of course is the fact that we all looked at the snow Thursday just as the winter storm entered the area. In areas that received 8 to 10 inches of new snow and wind overnight, I expect stability conditions to be elevated especially on steep wind loaded terrain above 7000 feet. The best assessment of avalanche conditions will be your own so you really need to be paying attention this weekend. High winds, heavy new snow, collapse or cracking of the snowpack, rapid warming and recent avalanche activity are the biggest clues of an unstable condition so be looking for any of these while you’re out looking to put fresh tracks on a steep slope.
The other concern are isolated pockets of weak faceted snow that formed several weeks ago that are now deeply buried. The warm weather during the past week helped settle and stabilize these layers on the more exposed slopes but we’re still noting failures with energy and ability to propagate fracture on East aspects at mid-slope. These areas appear to be most prevalent in sheltered pockets low on the slope. It takes a lot of force to produce failure in these locations and they are not widespread but certainly worth checking out in areas that haven’t seen much traffic.
Current Avalanche Danger
At all advisory area locations above 5000 feet on wind loaded terrain steeper than 35 degrees, the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. On all other slopes above and 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.
Weather Forecast and Avalanche Outlook
The Missoula Office of the National Weather Service is forecasting a vigorous arctic front to move South near the I-90 corridor this morning. East to Northeast winds of 30 to 40 mph are associated with this frontal boundary but are expected to taper off by Friday afternoon. A shortwave ridge of high pressure will move over the area late Friday bringing partial clearing and a very cold night. Another weather system moves into the area Saturday through mid-week with cooler temperatures and precipitation the bulk of which is expected early Sunday. After this frontal passage, snowfall will become convective with most of the precipitation expected over higher terrain.
As mentioned earlier, the above avalanche danger is based on conditions seen Thursday during the early stage of the storm currently over our area. Expect avalanche conditions to worsen especially on wind loaded terrain during this storm passage. It doesn’t look like there is a lot of snow expected at this time but the wind will determine where avalanches can be triggered and will be the biggest factor to consider this weekend.
We’ve been receiving many excellent reports this winter from many different locations within our advisory area. We try to cover approximately 1800 square miles of terrain in West Central Montana so these reports are of great value in that they help fill the gaps for us. Great job and thanks for sending them in!
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 5 level avalanche danger scale is being revised for next winter. In an effort to ensure a successful revision, please take the time to complete the following online survey:
Avalanche danger rating survey
The next avalanche advisory will be issued Monday, March 9th, 2009.
