March 20 Avalanche Advisory
Posted Friday March 20th, 2009 at 0600.
Good morning! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for March 20th, 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 but you can use the information we provide below to help you make more informed decisions regarding travel in avalanche terrain for the next few days. 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 NF Avalanche Center website.
Weather and Snowpack Analysis
Today is the first day of spring and if you ventured into the mountains yesterday you would believe it. It was sunny in the morning and stayed warm throughout the day. Cloud cover moved in by mid-afternoon which will act like a blanket to keep air temperatures warm through the night. Nearly all SNOTEL sites remained above freezing under cloudy skies and only dropped below freezing for a short time last night. These warm temperatures have caused melt water to percolate several inches into the snowpack to a faceted layer associated with a sun crust that formed in late February. This was the weakest feature in the snowpack yesterday in the Rattlesnake and we were able to trigger a 14 inch deep slab on a 40 degree Northeast facing slope with a ski cut. We also received a report of a large slab avalanche that was remotely triggered by snowmachines in Bowl 5 near Sheep Mountain Wednesday.
Observers in the Bitterroot Mountains near Lost Trail Pass and in the Northern part of the range south of Taft, MT report mostly stable snow conditions but noted that warm temperatures will increase instability in these areas as well. All locations were seeing roller ball activity including many North shaded aspects which is a good indicator of instability. The Bitterroot Range received significant snowfall earlier in the week so I would be very suspicious of any steep wind loaded terrain there during the warm weather. The avalanche activity described above is a good example of what can happen when temperatures rise quickly.
Current Avalanche Danger
At all advisory area locations above 6000 feet, during periods when temperatures are above freezing and on terrain steeper than 35 degrees, 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 are possible. On all other slopes above 6000 feet the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. Below 6000 feet the avalanche danger is LOW, 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 upper level air flow will be from the Southwest Friday and Saturday resulting in a warmer, moist and unstable atmosphere with shower activity increasing during the afternoon and evening hours. Rain can be expected up to 7000 feet into Saturday. A trough of low pressure is expected to pass over the Northern Rockies Sunday bringing cooler air from the Northwest.
Expect avalanche danger to remain elevated during this period of warmer temperatures. If the higher elevations receive any rain, avalanche conditions will quickly become very dangerous and you don’t want to find yourself on or below open terrain steeper than 30 degrees. Once temperatures start to cool down again, the snowpack will be glued together like plywood and will ski about as well.
We have received many excellent reports from backcountry snowmobilers and skiers this winter. These informal observations are very valuable to us in that they help us produce a more accurate avalanche advisory. The information you provide may save a life. Many thanks to everyone who sent observations or a quick email to us this winter!
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 Friday, March 27th, 2009. We will no longer (for this season) issue regular avalanche advisories on Mondays. We will post information as needed based on weather and snowpack conditions.