Avalanche Advisory for December 14, 2007

Weekend Avalanche Advisory
Posted December 14, 2007 at 0700

Hello! This is Steve Karkanen at the West Central Montana Avalanche Center with this weekend’s avalanche advisory. 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. The avalanche danger rating expires at midnight on December 14, 2007.

Today’s advisory is dedicated in memory of Ben Richards of Missoula, who died in an avalanche near Big Sky last March. Enjoy your turns Ben, you’ve earned them.

Weather and Snowpack Analysis

Since our last update on December 7th, West Central Montana stayed cool with temperatures dropping into the single digits early in the week and only a few inches of snowfall on Monday and early Thursday. Our observation sites picked up anywhere from 6-10 inches of new snow in the past 7 days. The most important weather factor this week has been the wind. Wind speeds at 8000′ have been in the steady 20-30 mph range all week coming from the East last Friday and Saturday then turning more westerly on Saturday (12/8). These high winds moved a lot of snow onto the leeward slopes. Most locations have low total snow depths but the higher elevation lee ridgeline slopes have as much as 200cm (80”) in isolated areas. It is currently snowing at most mountain locations with wind speeds into the 30 mph range at 8000′.

Southern Bitterroot Mountains from Lost Trail Pass to Lolo Peak:
We received many excellent reports from the Southern Bitterroot where skiers have been keeping an eye on faceted snow near the ground. The area experienced an active avalanche cycle on December 3 when more than 2′ of heavy dense snow was deposited onto a thin, weak snowpack. Many of our observers did find faceted snow near the ground but this layer has gained quite a bit of strength over the past 2 weeks. It is still something to be aware of in areas where the snow is shallow.

Lolo Pass Area:
The area around Lolo Pass doesn’t yet have enough snow to cover anchors and where the snow was deep enough stability tests indicated a mostly stable condition. Higher elevation sites will be similar to the Southern Bitterroot.

Northern Bitterroot near Lookout Pass:
In the St. Regis Basin observers found faceted snow near the ground and under a rain crust at mid-pack that formed during a rain event last week. Stability testing in 73cm of total snow produced failures at 47cm with moderate force. Rutschblock testing on a 35 degree slope was producing quality 2 failures at the rain crust when skis were weighted (RB3, Q2). While not a big slab, this will need to be watched carefully as the area receives more snow. The overall snowpack here is thin and colder temperatures will allow this weakness to persist. Wind loaded slopes will be the most sensitive with new snow and wind.

Rattlesnake Mountains:
The most sensitive feature in the Rattlesnake was the interface of the newest snow on a firm wind slab with surface hoar crystals. This is of low consequence until we receive several inches of new snow. Faceted snow near the ground has gained strength. Stability testing indicates a mostly stable condition with the newest snow sloughing easily on the older hard surface and the weaker faceted snow failing after application of moderate to hard force. RB5/Q2-3, CTM17/Q2, ECTN all at 20cm. Watch out for rocks.

Seeley Lake Area-Southern Swan and Mission Mountains:
Observers at Seeley Lake had difficulty getting into the higher terrain and reported low snowpack depths. The snow at 6500′ was 70cm (28”) with new snow on a hard rain crust at 60cm. The snow underlying this crust is faceted to the oldest snow on the ground and it was easy to break through it. The colder temperatures are allowing this weakness to persist. Again, not much snow to be a problem yet at the lower elevations but this could be a problem layer if we receive heavy snow.

Today’s Avalanche Danger

In the Rattlesnake, Southern Mission, Southern Swan and Bitterroot Mountains above 7000′, the avalanche danger is MODERATE on open wind-loaded East to North aspects steeper than 35 degrees. At all other elevations and aspects the avalanche danger is LOW. Many locations have a thin snowpack where vegetation and other natural anchors have yet to be covered. It is possible to find pockets of instability in areas where the snow is thin and where recently wind loaded

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 Missoula Office of the National Weather Service has issued a winter storm watch for Friday night through Saturday afternoon. A Pacific storm system will bring a chance of significant snow and high winds across the area before a weak ridge of high pressure builds over the Northern Rockies on Sunday. Models indicate that another strong system will move into the area Monday. Expect an active weather pattern for the next few days.

With that in mind, expect avalanche conditions to worsen during periods of heavy snow and wind. Be aware of the weaknesses described above, especially places where surface hoar was buried and areas where the snow remains shallow and weak near the ground.

We need more snow so we’ll take it any way we can get it. If you want to report on conditions or see avalanche activity please send an email to [email protected] We really like those reports! Thanks!

The next avalanche advisory will be posted on December 21, 2007.