Dec 20, 2010 @ 6:06 am

December 20, 2010 Avalanche Advisory

Good morning, this is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for December 20, 2010.

Current Avalanche Danger

On all slopes above 5000 feet and steeper than 30 degrees there is a MODERATE avalanche hazard. On remaining terrain within the advisory area the avalanche hazard is LOW.

Weather and Snowpack Analysis

We toured Wisherd Ridge just Northeast of Missoula yesterday and dug pits on three exposures. The surface hoar layer that Steve discussed Friday appears to have strengthened in this area. Reports from observers near Lolo Pass and Lost Trail Pass confirm that the surface hoar layer is less reactive there also; but we all feel we should look for it before recreating on steep slopes; therefore the Moderate hazard rating.  I could still get a clean propagation on a NW slope at 7000’, albeit with a lot of force (ECTP 25 Q 1). So, the potential is there to release a 50 – 60 cm (around 2 feet) slab by hitting a weak spot on steep terrain. This layer is the primary concern in our region right now. The surface hoar layer is distributed unevenly on different exposures at higher elevations throughout the region. With another 3-6 inches of new snow on it this morning (maybe more on lee aspects) it just makes sense to look for it with a quick pit.

Two other notes – Steve and I observed some folks riding right beside each other in a steep bowl yesterday. You should always go one at a time. Also, right now, there is some beautiful powder skiing and riding out there; so it would be very easy to jump into a steep run or bowl without checking for the surface hoar which is now about 50 – 60 cm (around 2 feet) from the surface.

Weather Forecast and Avalanche Outlook

Snow and wind will continue to impact Western Montana through the morning hours. New snow accumulations of 3-6 inches will be commonplace across the region. The snow should lighten up by this evening. A drier pattern is expected by Tuesday with another wave of moisture approaching from the Southwest Wednesday.

The existing surface hoar weakness is showing signs of strengthening and bonding; but with the new load the potential is there for a slab avalanche. So at the the risk  of repeating myself; check  for this layer in the top 2 feet of the snowpack before  committing to a steep slope above 5000 feet.

The next advisory will be posted Christmas Eve, 2010.

If you get out and see avalanche activity or want to send us quick snow observations please use our public observations form on the home page.


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.