Avalanche Warning Update

Posted 1:15pm Sunday Dec. 23
Avalanche Warning Update

Good afternoon, this is Dudley Improta with the West Central Montana Avalanche Center. The conditions that Steve described below in the Friday advisory still exist. We have confirmed two skier triggered avalanches in the mountains north and east of Missoula that occured Saturday Dec. 22. The avalanches were big enough to bury or injure someone. We don’t have detailed information on either of the incidents reported to us at this time. Cold weather has allowed the weak layers to persist in the snowpack.
The avalanche danger is HIGH in the Bitterroot Range from West of Hamilton North to Lookout Pass and in the Rattlesnake Mountains (including Sheep Mountain). Natural and human triggered avalanches are likely.
This danger rating expires tonight (Sunday Dec. 23) but
these conditions may take several days to stabilize.
Steve will post the regular advisory Friday morning Dec. 28. Have a safe and happy holiday!

Weekend Avalanche Advisory
Posted December 21, 2007 at 0700

Hello! This is Steve Karkanen at the West Central Montana Avalanche Center with the weekend avalanche advisory for December 21-23, 2007. The avalanche danger rating expires at midnight on December 21, 2007. This report is based on field observations and data collected on Thursday and describes conditions seen at that time. This advisory is the sole responsibility of the Forest Service and does not apply to operating ski areas.

Avalanche Warning

An Avalanche Warning has been issued for the mountains of West Central Montana. This warning includes the Bitterroot Range from West of Hamilton North to Lookout Pass and the Rattlesnake Mountains (including Sheep Mountain). The avalanche danger is HIGH: Natural and human triggered avalanches are likely. In the past 48 hours, heavy snow has been deposited onto a weak snowpack structure. Wind loaded slopes are especially sensitive right now. Natural and human triggered avalanches have been reported to the Center and we expect this condition to continue for the next several hours. This warning expires at midnight Friday night and you can expect the avalanche danger to remain CONSIDERABLE for a few more days. Avoid traveling on or beneath slopes steeper than 30 degrees, carry an avalanche transceiver, probe pole and shovel and never expose more than 1 person at a time to avalanche terrain.

Weather and Snowpack Analysis

Most of the advisory area received abundant snowfall and high winds this past week with the Northern Bitterroot receiving the heaviest amount of moisture. Snow water equivalent at Lookout Pass SNOTEL was 3.9”, Hoodoo at 4.8”, Lolo Pass 3.2”, Twin Lakes 2.6” and Saddle Mountain at .9” SWE. This translates to well over 2.5 to 3.5 feet of new snow in the Northern Mountains. This heavy snow was deposited on a weak layer of facets or a hard crust that formed in late November and was exhibiting all the classic signs of instability Thursday. We received several reports of natural and human triggered avalanches in the Bitterroot and Rattlesnake and widespread collapsing and whoomping of the snowpack in all locations except the very Southern portion of the Bitterroot near Lost Trail Pass and in the areas near Seeley Lake where the snowpack remains in a mostly stable condition.

As noted from SNOTEL data, snowfall amounts taper off as you go south and east. Most of our observers were reporting easy failures at the interface of the older snow surface and the accumulation from the past 2 weeks, now more than 2′ deep. At the higher and more open sites, there is buried surface hoar at this interface and it is very sensitive to weight. This condition appears to be widespread above 7000′. Observation sites near Lookout Pass, Lolo Pass and in the Rattlesnake were noting easy failures at this level (about mid-pack). We have not seen conditions like this for quite some time now so prudence dictates that we go with an avalanche warning for these areas for the next few hours.

In the very Southern Bitterroot Mountains near Lost Trail Pass and in the Southern Swans near Seeley Lake, these areas did not receive the heavy snow and observers here indicate mostly stable conditions.

Today’s Avalanche Danger

The avalanche danger is HIGH in the Bitterroot Range from West of Hamilton North to Lookout Pass and in the Rattlesnake Mountains (including Sheep Mountain). Natural and human triggered avalanches are likely. Avoid wind loaded slopes steeper than 30 degrees.

Near Lost Trail Pass the avalanche danger is MODERATE increasing to CONSIDERABLE as you travel north into areas that received heavy snowfall above 6500′ this week.

The avalanche danger is MODERATE in the Southern Swan Mountains near Seeley Lake.

At elevations below 5000′ the avalanche danger is LOW.

LOW: Natural avalanches very unlikely. Human triggered avalanches unlikely.

MODERATE: Natural avalanches unlikely. Human triggered avalanches possible.

CONSIDERABLE: Natural avalanches possible. Human triggered avalanches probable.

HIGH: Natural and human triggered avalanches likely.

EXTREME: Widespread natural or human triggered avalanches are certain.

Weather Forecast and Avalanche Outlook

The weather system that brought the snow is exiting the area today. There is enough moisture in this unstable air mass to allow a few more inches of snow to get squeezed out by this evening when a ridge of high pressure begins building. By Saturday evening this short lived break will give way to more mountain showers and high winds for the next several days.

Snow stability conditions are pretty sketchy right now but will improve with time. The transition from an unstable condition to a stable condition won’t happen overnight and you must consider that we now have a persistent weak layer (buried surface hoar) in many areas. We are at the tail end of an active avalanche cycle so use extra caution when recreating in the backcountry this weekend. This is not a good time to jump into a big open steep slope. Collapse noise and recent avalanche activity are huge clues we to often ignore. The snow was screaming instability at us yesterday so give it a chance to adjust. We received an interesting report from skiers near Downing Mountain (west of Hamilton) who triggered 2 slab avalanches after digging a pit and finding stable conditions. They triggered the slides on wind loaded terrain some distance from their pit. Good travel protocol and communication between partners made the difference for this group. This is a good example of just how variable snow conditions can be and to be careful when making decisions based on one set of data.

The next avalanche advisory will be posted on December 28, 2007.