February 18, 2011 Avalanche Advisory

On all slopes above 5000 feet, the avalanche danger is MODERATE. Natural avalanches are unlikely, human-triggered avalanches are possible. There are heightened avalanche conditions on some terrain features.

In the northern Bitterroots from Hoodoo Pass to Lookout Pass the avalanche danger is CONSIDERABLE on wind-loaded terrain steeper than 35 degrees, natural avalanches are possible; human-triggered avalanches are likely. The avalanche danger is MODERATE elsewhere.

Good morning! This is Steve Karkanen with the West Central Montana Avalanche Center’s backcountry avalanche advisory for Friday, February 18, 2011. Today’s advisory is sponsored by the Trail Head, where you can buy discounted avalanche safety gear during the annual Gamblers sale event underway now.

Areas of concern are steep slopes recently wind-loaded where sensitive wind-slabs have formed.

Below 5000 feet the avalanche danger is LOW. Natural and human-triggered avalanches are unlikely. Be aware of unstable snow in isolated steep areas that have been influenced by wind.

Weather and Snowpack Analysis

A strong winter storm blasted through the I-90 corridor Tuesday and Wednesday with several inches of snow and very high SW-W winds. The heaviest snowfall was in the area around Lookout Pass, which reported well over 25 inches of snow since Monday. The Rattlesnake, southern Missions and Swans picked up 14-18 inches with similar amounts at the higher elevations in the Bitterroot. Lost Trail Pass received just a couple inches of new snow.

The storm started out warm and turned much cooler Wednesday night leaving a right-side up snowpack. This warmer snow also bonded fairly well to the sun crust that formed during the warm clear weather on Monday. 35-40 mph winds scoured westerly facing aspects and left wind slabs scattered along ridgetops in many areas.

Wind direction changed to N-NE on Wednesday night Thursday morning and scoured north facing aspects. East aspects were not as affected by all this wind and skied remarkably well.

Cornices are huge and extremely dangerous now and should be avoided. A snowmobiler avoided serious injury last week when a large cornice dropped out from under him while riding a ridge near Cedar Log Lake on the MT-ID Stateline.

Wind-slabs and cornices are the biggest watch-outs now. The newest snow fails easily in stability testing but it isn’t deep enough to be unmanageable. The change in wind direction from the NE scoured slopes where we were expecting to see bigger slabs. They are out there and some of them are fairly large. Play it safe and give them a wide berth as you don’t want to get caught on one if it happens to break under you.

Weather Forecast and Avalanche Outlook

The atmosphere will remain cool, moist and unstable as the upper air becomes southwest. Snow showers are expected to increase in number and intensity during the afternoon.

On Saturday, a deepening trough of low pressure will move inland over the Great Basin with a secondary trough extending north through the northern Rockies. A more robust surge of cold dry Canadian air will move into western Montana with northeast winds. Chances of precipitation will continue across west central Montana.

Expect avalanche conditions to remain similar until the next change in weather.

I will issue the next advisory on Presidents’ Day, February 21, 2011.