Avalanche Advisory for December 28-30, 2007

Weekend Avalanche Advisory
Posted December 28, 2007 at 0700

Hello! This is Steve Karkanen at the West Central Montana Avalanche Center with the weekend avalanche advisory for December 28-30, 2007. The avalanche danger rating expires at midnight on December 28, 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.

Weather and Snowpack Analysis

We continue to receive a steady dose of moisture at the higher elevations in West Central Montana. Most SNOTEL sites accumulated around 2” of Snow Water equivalent (SWE) which translates to about 20” of new snow. Wind speeds remain in the 30’s at 8000′ so this new snow is getting moved around. The snowpack has had a few days to settle out and gain strength but the cooler temperatures are allowing weak layers to persist in many areas. The major weakness is associated with a buried surface hoar layer and faceted snow that formed after the warmer temperatures of early December.

Observers are reporting an overall strengthening of the snowpack but the described instabilities are persisting in all areas. Stability testing produces fast clean shear plane failures with a moderate amount of force. In some areas of the Bitterroot (Lolo Pass and Gash Peak) Rutschblock scores are very low and failing at the buried surface hoar layer (RB2 Q1). This is pretty sensitive so the recommendation to riders and skiers is to play on the lower angle slopes for a few more days.

We received several reports of both natural and human triggered avalanches that occurred last weekend in the Rattlesnake, on Wisherd Ridge, in the Crystal Theater near Lolo Pass and many naturals in all locations. No one was injured in any of these incidents and in all cases the individuals felt good about stability after digging a pit and performing stability testing.

Today’s Avalanche Danger

At all locations above 6000′ in the Bitterroot Range from Lost Trail Pass to Lookout Pass, the Rattlesnake Mountains and the Southern Missions and Swan Range near Seeley Lake, the avalanche danger is CONSIDERABLE on open terrain steeper than 35 degrees. Wind loaded areas will be especially sensitive during new snowfall. Natural avalanches are possible, human triggered avalanches are probable.

At other mountainous locations throughout the advisory area the avalanche danger is MODERATE. Natural avalanches are unlikely, human triggered avalanches are possible.

At elevations below 5000′ the avalanche danger is LOW.

Weather Forecast and Avalanche Outlook

An active weather pattern is setting up for the weekend. A strong Pacific system will move into the area late tonight and remain through the weekend. This system has the potential to bring heavy snowfall amounts to the higher mountains.

This is a tricky snowpack right now. Pit observations aren’t always showing any weakness yet avalanches are being triggered just a few yards from a pit site. The deeper snowpack near ridgelines and in wind loaded areas appear more stable than shallower snow just down slope. All our observers are reporting a strengthening condition but are still getting high quality shears when enough force is applied. Keep in mind there is a weak layer out there that is persistent and just waiting for the right trigger. Expect avalanche conditions to worsen if the anticipated weather system drops heavy snowfall amounts at the higher elevations. Be especially mindful during periods of heavy snowfall and wind.

The next avalanche advisory will be posted on January 4th, 2007.