December 27, 2010 Avalanche Advisory

Happy Holidays skiers and riders! This is Steve Karkanen with the West Central Montana Avalanche Center’s backcountry avalanche advisory for December 27, 2010. We’d like to thank Vann’s new On Store for their generous donation of Flip and GO Pro video cameras to help us better illustrate the avalanche conditions we describe in our advisories. The video we shot last week and yesterday (12/26) was from a Flip Video camera.

Current Avalanche Danger

On slopes steeper than 35 degrees, avalanche danger on all aspects and elevations above 5000 feet is currently MODERATEAdditional heavy snow or high winds will rapidly increase the avalanche danger in most areas. Pay careful attention to what the weather brings us early this week especially if the next storm comes in with heavy precipitation and high wind. The Bitterroot Mountains are already seeing 4-6 inches of snow this morning.

Below 5000 feet the avalanche danger is LOW.

Weather and Snowpack Analysis

Since last Thursday, mountain temperatures warmed into the 30’s at many SNOTEL sites with nighttime temperatures dropping back into the teens and low 20’s under clear skies. I would have expected to see significant surface hoar growth with those conditions however our airmass was dry with humidity readings in the 30’s and 40’s until Sunday when a weather system began moving through the area. Also, Saturday was windy, sunny and warm which would have destroyed any surface hoar in many locations.

We did find a very steep temperature gradient in the upper 10cm of the snowpack which has caused faceted crystal growth to occur just beneath the snow surface and zipper crust. This near-surface faceting has potential to be the next weak layer we need to keep in mind once it starts snowing again.

During our stability testing in several pits the buried surface hoar that formed in early December is still an obvious concern and was giving us very mixed results yesterday. The video we shot of a compression test then an extended column test tells the story best. The main concern is the tendency for the now stiff slab to propagate across the slope with energy. The spotty, variable nature of this persistent weakness is still a problem so we strongly recommend looking for it before committing to anything steep.

Most of our compression and extended column tests were scoring into the 20’s but with clean energetic shears when the column fails. A pit on a SE aspect at 7600′ scored CTH21 Q1 and an ECTP11 Q1. This is why we keep talking about the buried surface hoar. It may be a problem in some areas.

Weather Forecast and Avalanche Outlook

An active weather pattern with significant backcountry snowfall is in store for our area this week.

Moisture and energy associated with an upper level trough is beginning to impact the area today. A reinforcing jet streak will produce enhanced up slope precipitation in the higher elevations of the Bitterroot and Clearwater Mountains. Ridge top winds will increase from the west as the pressure gradient tightens over the area.

Snow potential remains high Tuesday and Wednesday with high ridge top winds from the west and southwest. Several inches of snow are expected with this system with potential for heavy amounts of snowfall Wednesday.

Expect avalanche conditions to worsen as this storm progresses.

The next avalanche advisory will be posted on New Years Eve unless conditions change significantly.