Yellowstone Seven Day Forecast on 15 January 2011 by the NWS Riverton, WY

Rest Of Today...Breezy. Snow likely late in the morning...then snow in the afternoon. New snow accumulation around 1 inch. Highs 28°F to 34°F. Southwest winds 15 to 20 mph. Chance of snow 90 percent.

Tonight...Snow likely. Snow accumulation of 1 to 2 inches. Lows 19°F to 25°F. Southwest winds around 15 mph. Chance of snow 70 percent.

Sunday...Breezy...snow. Snow accumulation of 3 to 5 inches. Highs 30°F to 36°F. Southwest winds 15 to 25 mph. Chance of snow 90 percent.

Sunday Night...Breezy. Snow likely. Snow accumulation of 2 to 4 inches. Lows 24°F to 30°F. Southwest winds 15 to 20 mph. Chance of snow 70 percent.

Monday / Martin Luther King Jr Day...Snow likely. Snow accumulation of 2 to 3 inches. Total snow accumulation 9 to 16 inches. Highs 27°F to 33°F. West winds around 15 mph. Chance of snow 70 percent.

Monday Night...Colder. Cloudy with a 50 percent chance of snow. Lows 16°F to 22°F.

Tuesday...Cloudy with a 50 percent chance of snow. Highs 25°F to 31°F.

Tuesday Night...Cloudy with a 50 percent chance of snow. Lows 13°F to 19°F.

Wednesday...Mostly cloudy with a 30 percent chance of snow. Highs 19°F to 25°F.

Wednesday Night...Colder. Partly cloudy with a 20 percent chance of snow. Lows 2°F to 8°F.

Thursday...Mostly cloudy with a 20 percent chance of snow. Highs 18°F to 24°F.

Thursday Night...Mostly cloudy with a 20 percent chance of snow. Lows 5°F to 11°F.

Friday...Mostly cloudy with a 20 percent chance of snow. Highs 21°F to 27°F.

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Avalanche Information - from the Gallatin National Forest Avalanche Center this report is by Mark Staples

Mountain Weather:

Warm and windy weather continues with temperatures ranging from the low 20s Fahrenheit to the high 20s Fahrenheit and winds blowing 10-30 mph from the West. Since yesterday no snow fell except near West Yellowstone which received a trace and Cooke City which received 2 inches. Today's weather shouldn't change much with high temperatures in the high 20s to near 30 degrees Fahrenheit and winds blowing 15-35 mph from the West. Only 1-2 inches of snow should fall today and tonight mostly near West Yellowstone and Cooke City, but more significant snowfall should start tomorrow morning with even warmer temps and stronger winds.

Snowpack and Avalanche Discussion:

The southern Madison and southern Gallatin Ranges, the Lionhead area near West Yellowstone, the mountains around Cooke City and the Washburn Range:

Yesterday my partner and I rode in the vertigo inducing fog on Lionhead with Gallatin Snow Rangers where the weather felt very coastal (ie – warm and wet). This weather affects the snowpack in two ways. First, warm temperatures have a long term stabilizing effect forming a strong snowpack which will easily support loads from future storms. Second, warm temperatures have created upside down conditions within the new snow, the primary concern today. Dense snow is resting on lower density snow about 10-12 inches deep. In one place in the southern Madison Range a skier found faceting in this lower density snow. This upside down type of instability doesn't last long and shouldn't produce avalanches that fracture over huge areas.

For today, the Avalanche Danger is rated MODERATE on all wind loaded slopes, MODERATE on all slopes steeper than 35 degrees, and LOW on less steep slopes without a wind load.

The northern Madison Range:

The mountains near Big Sky contain an annoying pest called surface hoar. It is buried 2-3 feet deep. Although it is becoming less of problem every day, it remains a concern and its strength should assessed in stability tests. It exists on some slopes but not on others. In many areas above treeline, strong winds have provided natural grooming, but in other areas strong winds have caused widespread drifting. Warm temperatures helped stabilize these drifts, but their presence as well as the presence of buried surface hoar on many slopes makes human triggered avalanches possible.

Today the Avalanche Danger is rated MODERATE.

The Bridger Range and northern Gallatin Range:

With no new snow and no widespread weak layers, things are pretty quiet in the mountains near Bozeman. The main avalanche concerns are isolated areas near ridgelines where the snowpack was thin during recent cold weather. In these places cold weather created weak faceted snow now capped by stiff wind slabs on specific terrain features. Despite warm weather, this faceted snow remains weak and can produce human triggered avalanches.

For today, the Avalanche Danger is rated MODERATE on all wind loaded slopes, MODERATE on all slopes steeper than 35 degrees, and LOW on less steep slopes without a wind load.

Information provided by Doug Chabot, Mark Staples, and Eric Knoff from the Gallatin National Forest Avalanche Center.

For detailed Avalanche Terms utilized here, please see the Avalanche Glossary.

~ Mammoth Weather Forecast ~