President’s Day 2010 Avalanche Advisory

Good morning, this is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for President’s Day, 2010.

Current Avalanche Danger
On slopes above 6000 feet and steeper than 35 degrees there is a CONSIDERABLE avalanche hazard. On other steep terrain above 5000 feet there is a MODERATE avalanche hazard. On remaining terrain within the advisory area the avalanche hazard is LOW. Normal caution is always advised; you should be increasingly cautious during CONSIDERABLE hazard.

Weather and Snowpack Analysis
We received reports and photos of slab avalanches from across the advisory area. The slab is propagating on the late January surface hoar and is anywhere from 20 inches (Mission Mountains) to 10 inches (Rattlesnakes) deep.
Tim Laroche and I traveled in the Rattlesnakes yesterday to check out a report of a skier-triggered slide. We checked out that slide and observed three other slab avalanches that appeared to have run on Saturday (2/13/10) as well. All of these three avalanches were in a steep East-facing bowl; at least one of them appeared to be skier-triggered. As to the first avalanche we were looking for the YouTube video will give you most of the story. I did a quick stability test in the crown which showed the surface snow was still very weak and failing on the late January surface hoar. And, as Tim reports in the video, we ski cut some of the remaining slab and got it to go way too easily.

httpv://www.youtube.com/watch?v=_AoN_-6ZG9M

Although the slab in the video is only 8 to 12 inches (20 to 25cm) thick, if it propagates like this one did, 150 feet wide; it could be a problem in a gulley, over cliffs or into trees. The slabs reported from the Bitterroots and the Missions were larger and could have easily buried someone.
We received reports of similar activity from Wisherd Ridge near Missoula, from two popular backcountry areas in the Bitterroots and the Southern Missions. All these slabs were on East, Northeast and North slopes. All reported slabs were near ridgetops which have accumulated wind deposited snow.
So this tells me, even without a major storm or big weather change, a few inches here, a few inches there, add some wind loading, put this on the surface hoar from late January, and the recipe produces slab avalanches.
We also received numerous reports of whumphing and shooting cracks in the snow from the Rattlesnakes, the Southern Missions and the Bitterroots on Saturday. Tim and I noted whumphing and cracking on out tour Sunday also.
I have posted photos of slides in the Bitterroots, Missions and Rattlesnakes on our Photo Gallery mslaavalanche2.wpengine.com/gallery.php?albumid=5416369076257222705 .
There are some pictures of slides near Gash Point in the Bitterroots at picasaweb.google.com/MontanaMountaineers/GashSlides20100213?authkey=Gv1sRgCND5zNn-k5WQWg# .

Weather Forecast and Avalanche Outlook
A shortwave system should pass through until midday today. Light snow accumulations are possible. Another system will approach the region on Tuesday morning. Snow amounts are predicted to be light. My radar is up pretty high right now as far as steep slopes are concerned. Any new amount of snow on top of this existing surface hoar layer will increase the sensitivity. For the time being, steep slopes on all aspects above 6000 feet are suspect .

Avalanche Hazard Ratings

The next advisory will be posted on February 19, 2010.
THANKS to all who sent in reports or photos this weekend!

Missoulaavlanche.org is currently running a survey this season to improve avalanche advisories and education. To access the survey visit www.surveymonkey.com/s/PC6Z5BH . Please take a few minutes to help us out. At the end you can enter a drawing to win a BCA Tracker Beacon, a Voile Avalanche Shovel or a missoulaavalanche.org t-shirt

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).