Yellowstone Seven Day Forecast on 16 January 2011 by the NWS Riverton, WY

Today...Breezy...snow. Snow accumulation of 2 to 5 inches. Highs 31°F to 37°F. Southwest winds 15 to 25 mph. Chance of snow near 100 percent.

Tonight...Breezy. Snow with rain likely in the evening...then snow after midnight. Areas of blowing snow through the night. Snow accumulation of 1 to 4 inches. Lows 24°F to 30°F. Southwest winds 15 to 25 mph. Chance of precipitation 80 percent.

Monday / Martin Luther King Jr Day...Breezy. Areas of blowing snow in the morning. Snow. Snow accumulation of 2 to 3 inches. Highs 29°F to 35°F. West winds 15 to 20 mph. Chance of snow 80 percent.

Monday Night...Breezy...colder. Snow likely. Snow accumulation of 1 to 3 inches. Total snow accumulation 6 to 16 inches. Lows 17°F to 23°F. West winds 15 to 20 mph. Chance of snow 60 percent.

Tuesday...Cloudy with a 50 percent chance of snow. Highs 25°F to 31°F. Southwest winds around 15 mph in the afternoon.

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

Wednesday...Mostly cloudy with a 40 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...Patchy fog in the morning. Mostly cloudy with a 20 percent chance of snow. Highs 19°F to 25°F.

Thursday Night...Mostly cloudy with a 20 percent chance of snow. Patchy fog after midnight. Lows 6°F to 12°F.

Friday...Patchy fog in the morning. Mostly cloudy with a 30 percent chance of snow. Highs 23°F to 29°F.

Friday Night...Mostly cloudy with a 30 percent chance of snow. Lows 7°F to 13°F.

Saturday...Mostly cloudy in the morning then becoming partly cloudy. A 30 percent chance of snow. Highs 23°F to 29°F.

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Avalanche Information - from the Gallatin National Forest Avalanche Center this report is by Eric Knoff

Mountain Weather:

Warm, wet weather has parked it over southwest Montana producing valley rain and mountain snow. Since yesterday morning, 5-6 inches of heavy snow has fallen in the mountains around Cooke City totaling a half inch of water. The mountains around West Yellowstone have picked up 3-4 inches while the mountains around Big Ski and Bozeman have picked up 1-2 inches. Currently, temperatures are in the mid to high 20s Fahrenheit and winds are blowing out of the Westsouthwest at 15-30 mph with gust in Hyalite reaching 45 mph (Hyalite Weather Station). Today, more snow is expected with an additional 3-6 inches falling in the mountains around West Yellowstone and Cooke City, while 2-3 inches will accumulate in the mountains around Big Sky and Bozeman. Winds will stay strong out of the Westsouthwest and mountain temperatures will climb into the 30s Fahrenheit. An unsettled weather pattern will continue through tomorrow.

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:

The southern Ranges have received a healthy shot of snow over the past few days. This latest round of snow has been accompanied by strong WSW winds, which have developed thick, cohesive slabs on leeward slopes. Yesterday, an observer in Cooke City noted deep snow on Henderson Bench as well as substantial wind loading in the Rasta Chutes new Lulu Pass.

Although a well structured snowpack has produced a generally stable foundation for this new load, a few issues need to be addressed. First, warm weather has developed a dense layer of snow near the surface that now rests upon a less dense layer 10-12 inches down. Mark and I found this to be our main stability concern in Bacon Rind yesterday. This upside down type of instability doesn't last long and shouldn't produce avalanches that fracture over huge areas, however, this has the potential to be a problematic recipe as the snowpack tries to adjust, especially in areas that have received heavy deposits of wind drifted snow. Second, new snow and wind throughout the day will continue to add stress to the snowpack. Backcountry travelers should pay close attention to changing conditions and avoid steep slopes and avalanche run out zones if the snow continues to pile up.

Today, human triggered avalanches are likely on all wind loaded slopes where the Avalanche Danger is rated CONSIDERABLE. Slopes steeper than 35 degrees without a wind load have a MODERATE Avalanche Danger, and slopes less than 35 degrees without a wind load have a LOW Avalanche Danger.

The northern Madison Range:

A buried layer of surface hoar 2-3 feet deep continues to separate the northern Madison Range apart from the rest of our advisory area. This pesky layer has become less active over time, but continues to carry the potential for producing avalanches. Frequent snowpack evaluations are the best way to determine the strength and distribution of this layer. This is not always straight forward since this surface hoar layer is intermittent.

For this reason, extra caution on slopes steeper than 35 and/or wind loaded slopes is warranted in this area.

In addition to the buried surface hoar factor, strong Westsouthwest winds over the past few days have scoured some slopes and loaded others. Although warm temperatures have helped stabilize wind loaded slopes, the possibility of triggering a wind slab is not out of the question. As more snow and wind arrive today with the next system, the odds of triggering a wind slab avalanche will increase.

For today, human triggered avalanches are possible in the northern Madison range and the Avalanche Danger is rated MODERATE.

The Bridger Range and northern Gallatin Range:

The northern ranges have received only an inch or two of new snow over the past few days. This lack of precipitation has kept the avalanche danger from rapidly increasing. One component that has affected these areas is strong WSW winds. Yesterday, a skier north of Bridger Bowl reported stiff, punchy wind slabs along the ridgelines, which produced tricky skiing conditions. In some areas these thick, cohesive slabs rest over weak faceted snow that formed during periods of cold weather. These slabs have the potential to fail under the weight of a skier or rider creating heightened avalanche conditions in wind loaded terrain.

For today, a MODERATE Avalanche Danger exists on all wind loaded slopes or slopes steeper than 35 degrees. A LOW Avalanche Danger exist 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.

If you have any snowpack or avalanche observations, drop them a line at mtavalanche@gmail.com or call: 406-587-6984.

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

~ Mammoth Weather Forecast ~