February 28, 2011 Avalanche Advisory

The avalanche danger is MODERATE on all slopes steeper than 35 degrees above 5000 feet in most areas of west central Montana. Natural avalanches are unlikely. However, human triggered avalanches are possible.

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

The avalanche danger is Low below 5000 feet in west central Montana.

Good Morning. This is Tim Laroche at the West Central Montana Avalanche Center with your backcountry avalanche advisory for Monday, February 28th, 2011.

Weather and Snowpack Analysis

The weather has changed, and the next storm is upon us. The latest storm system is coming in warm with a fair amount of wind. New snow amounts this morning range from 1-10 inches. The northern Bitterroots from Hoodoo Basin to Lookout Pass receiving the higher amounts. Most other areas only receiving 1-3 inches so far this morning. Temperatures are in the low to mid twenties and the winds are out of the south and west at 5-10mph.

The recent wind events that have plagued west central Montana have created soft and hard wind slabs on all aspects at upper and mid elevations. The soft slabs fail on an old wind crust 6 inches to 1 foot in depth but are mostly manageable if you are not caught by surprise. The harder wind slabs are smooth, rounded, and hollow sounding. These features are more stubborn and should be avoided. They could break 1-2 feet deep and well above where you are skiing or riding. These will be harder to detect with new snow masking their appearance.

We are expecting 1-2 feet of new snow in west central Montana the next 2 days. New snow sluffs and soft slabs will be easier to trigger as we get more snow. The new snow is falling on a weak cold snow interface that won’t support much weight. This condition already exists in the northern Bitterroots on wind loaded slopes as they have already received 10 inches of new snow with .9 inches of snow water equivalent(SWE).

The cornice build up on exposed ridgelines is phenomenal now. These cornices continue to get loaded with every new storm. Maintain a safe distance when traveling near these features. They can break further back on the ridgeline than you might expect.

There are variable conditions out there right now due to the winds blowing from all different directions the past few days. We found snowpack heights to be similarly deep on all aspects above 7000 feet in the Rattlesnake Mountains. We also found the best skiing and riding conditions in treed and sheltered areas off exposed ridgelines.

Weather Forecast and Avalanche Outlook

A strong westerly flow aloft will maintain a steady diet of pacific moisture into west central Montana through Tuesday morning. Strong westerly winds will cause considerable blowing and drifting of snow across the higher terrain today and into tonight. Surface temperatures will warm gradually. We may see 1-2 feet of new snow by Wednesday, when another storm system arrives.

I expect avalanche conditions to rise steadily during the next 24 hours. A robust westerly storm with plenty of moisture and wind will push the avalanche danger to CONSIDERABLE and possibly HIGH on steep wind loaded slopes that receive a significant amount of new snow.

Steve will issue the next advisory on Friday, March 4th.

If you get out and have the time to send us some information about what you are seeing, please use our “public observations” link on our website. This information is invaluable to us and in turn comes back to you in the form of a better forecast.