New Years Eve Avalanche Warning

This is Steve Karkanen at the West Central Montana Avalanche Center with an avalanche conditions update for New Years Eve and New Years Day. The avalanche danger rating expires at midnight on December 31, 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 is in effect for the mountains of West Central Montana above 6000′. This warning includes the Bitterroot Range from Lost Trail Pass North to Lookout Pass, the Rattlesnake Mountains (including Sheep Mountain) and the Southern Swan and Mission Mountains near Seeley Lake. 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. All SNOTEL sites are showing 1” to 2” of SWE or more than 12”-20” of new snow since Friday morning. This is over 40” in many areas in the past few days. 8000′ wind speeds have been in the 30-45 MPH range from the W-SW for the duration of this current storm system. These winds have loaded E_NE aspects heavily and they are especially sensitive right now. Other aspects may have been cross-loaded. This warning expires at midnight on New Years Eve and you can expect the avalanche danger to remain HIGH into New Years Day. 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 (from 12/28)

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.

On Thursday, observers were 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 (updated 12/30/07)

At all locations above 6000′ in the Bitterroot Range from Lost Trail Pass to Lookout Pass, the Rattlesnake Mountains (including Sheep Mountain) and the Southern Missions and Swan Range near Seeley Lake, the avalanche danger is HIGH on open terrain steeper than 30 degrees. Wind loaded areas are especially sensitive now. Natural and human triggered avalanches are likely.

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

SNOW AND GUSTY WINDS ARE EXPECTED THROUGH TONIGHT AS BRISK MOIST NORTHWEST UPPER LEVEL FLOW CONTINUES OVER THE NORTHERN ROCKIES. CONDITIONS WILL BECOME DRIER BEGINNING MONDAY AS HIGH PRESSURE BUILDS OVER THE AREA. SOUTHERLY FLOW BEGINS TO PUMP WARMER AIR OVER THE REGION BY MONDAY AFTERNOON…BUILDING VALLEY INVERSIONS INITIALLY…THEN BRINGING WARMER TEMPERATURES TO BOTH MOUNTAINS AND VALLEYS BY WEDNESDAY. (From NOAA Missoula Office National Wx Service).

We are now at the tail end of this current storm system. While most avalanches happen during and immediately after a storm, many avalanche accidents happen on the first sunny day after a storm. It’s looking like we may have a bit of a reprieve in the stormy weather for a day or two and the lure of this very deep snow can easily overpower our better judgment.

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.