Jan 5, 2010 @ 12:00 am

January 5 Avalanche Advisory Update

Good afternoon this Dudley Improta with an avalanche hazard update for January 5, 2010. On all slopes above 6000′ and steeper than 30 degrees there is CONSIDERABLE avalanche danger.
Many areas in West Central Montana have received over an inch of snow water equivalent (wet heavy snow). This snow fell on a weak layer in the top 10 inches of the snow pack. There is also a weak layer at the bottom of the pack. This wet dense snow may easily overload these existing weak layers. There is increased danger if there is rain on the snow pack.
My advisory from Monday Jan. 4 is below and explains these layers in the snow.

From January 4:

Weather and Snowpack Analysis
I don’t want to sound like a broken record (cd or mp3) but we have an interesting scenario in West Central Montana right now. The region received anywhere from 5 to 10 inches of new snow since New Year’s Day. Before the new snow arrived surface hoar had formed on the top of the snowpack. The surface hoar had been destroyed by sun and wind in the Rattlesnakes but not in the Bitterroots. The avalanche near Lolo Pass reported in the “current danger” was the new snow running on the surface hoar. We had the same surface failures in stability tests in the Rattlesnakes but it appeared to be a density change in the snow (the snow came in cold and finished warm). What is worrisome is in stability tests the snow is still failing on the depth hoar formed from early season.
The top layer avalanching could be a problem if it went into a terrain trap (i.e. over a cliff or into a gulley) and the top layer avalanching could step down to the weak snow in the bottom of the pack. There is some skiing or riding to be had out there but you need to pay close attention to steep terrain.
The You Tube shot explains the variability in our pit tests. But the difference in the tests has been the amount of force applied; the same layer continues to collapse cleanly.
I noted the same variability with backcountry recreationists this weekend. I talked to some cagey veteran backcountry skiers who turned around after snow observations. I saw some folks ski a conservative, anchored line one at a time and I saw some riders hucking cliffs with their buddies standing immediately below. The activity from the last observation is not advisable.

Weather Forecast and Avalanche Outlook
Pacific Moisture may collide with cold Canadian air over the region on Monday and Tuesday. As of this advisory it’s not clear how quickly the Canadian air will surge into Western Montana. Moderate to heavy snowfall is predicted along the Continental Divide along with Northeast winds. Wednesday should bring drier and colder conditions.
Expect the surface instability to increase with new snow amounts. With more snow the surface slab will be thicker as well. If we get new snow, winds and the predicted colder air the avalanche hazard will not decrease.
Keep in mind that our winds generally come out of the Southwest. Northeast winds are predicted for this storm; which makes all steep slopes above 6000′ suspect.

Steve Karkanen will post the next advisory on 1/8/10.


This information is the sole responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight but the information can help you make a more informed decision regarding travel in avalanche terrain for the next few days.

Our advisory area includes National Forest System lands in the Bitterroot Mountains from Lost Trail Pass north to Granite Pass, the Rattlesnake Mountains north of Missoula and the Southern Swan and Mission Mountains near Seeley Lake, MT. Avalanche information for the Lookout Pass/St. Regis Basin area is available from the Idaho Panhandle Avalanche Center.