Yellowstone Seven Day Forecast on 11 January 2011 by the NWS Riverton, WY

Today: Mostly cloudy. Patchy fog in the morning. Occasional flurries in the morning. Highs 8°F to 14°F. Southwest winds around 15 mph. Lowest wind chill readings -20°F to -30°F in the morning.

Tonight: Mostly cloudy. A 20 percent chance of snow after midnight. Lows -4°F to 2°F. Southwest winds around 15 mph.

Wednesday: Breezy. Not as cold. Cloudy. Chance of snow in the morning...then snow likely in the afternoon. Snow accumulation around 1 inch. Highs 18°F to 24°F. Southwest winds 15 to 20 mph. Gusts up to 35 mph. Chance of snow 70 percent.

Wednesday Night: Breezy. Not as cold. Snow likely. Snow accumulation of 1 to 2 inches. Lows 12°F to 18°F. Southwest winds 15 to 20 mph with gusts to around 35 mph. Chance of snow 60 percent.

Thursday: Not as cold. Snow. Snow accumulation of 3 to 6 inches. Highs 26°F to 32°F. Southwest winds around 15 mph. Chance of snow near 100 percent.

Thursday Night: Snow in the evening...then snow likely after midnight. Moderate snow accumulations. Lows 17°F to 23°F. Chance of snow near 100 percent.

Friday: Cloudy with a 50 percent chance of snow. Highs 27°F to 33°F.

Friday Night: Snow likely. Light snow accumulations. Lows 19°F to 25°F. Chance of snow 60 percent.

Saturday: Snow likely. Light snow accumulations. Highs 26°F to 32°F. Chance of snow 60 percent.

Saturday Night: Cloudy with a 50 percent chance of snow. Lows 14°F to 20°F.

Sunday: Snow likely. Highs 20°F to 26°F. Chance of snow 70 percent.

Sunday Night: Snow likely. Lows 8°F to 14°F. Chance of snow 60 percent.

Monday / Martin Luther King Jr Day: Cloudy with a 50 percent chance of snow. Highs 20°F to 26°F.

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Avalanche Information - from the Gallatin National Forest Avalanche Center this report is by Doug Chabot

Mountain Weather:

Under partly cloudy skies mountain temperatures are a few degrees below zero with light westerly winds averaging 10-15 mph. Today temperatures will break the zero degree mark and warm into the upper single digits; not exactly balmy, but a warming trend none-the-less. Clouds and wind speeds will increase this afternoon although no snow is expected for the next 24 hours.

Snowpack and Avalanche Discussion:

The northern Madison Range:

The mountains around Big Sky have a layer of surface hoar buried 2-3 feet deep. It's strengthening…slowly. On some slopes it already collapsed or cannot be found in snow pits, while on others it's ripe to avalanche. Last Thursday, skiers found a pocket of surface hoar the hard way: they triggered a slide. It was buried by a wind drift only a short distance away from where it was absent in their snowpit. It caught them by surprise and carried a snowboarder down the hill without injury. This scenario is waiting to play out again. We know the weak layer may be tricky to find, especially as it gets buried deeper in the snowpack. That makes it all the more crucial to look for. Additionally, winds at the higher elevations created slabs which the Moonlight Basin Ski Patrol was able to trigger with ski cuts. For today, wind-loaded slopes steeper than 35 degrees have a CONSIDERABLE Avalanche Danger. All other slopes have a MODERATE Avalanche Danger.

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

Yesterday, Eric, Randy Elliott, and I trekked out to investigate Sunday's avalanche below the south summit of Saddle Peak named Argentina Bowl. This southeast facing, 35-40 degree slope was triggered by a skier as he traversed the top of the bowl. The slide ranged in depth from 1-3 feet deep, was 75-100 feet wide and ran 800-1,000 feet vertical. It was triggered in thin snow where facets grew and were capped 10 days ago by a very dense, knife hardness wind slab. Even though the surface area of the slide was not very big, the volume of debris was impressive and had the toughness of cinder blocks making survivability unlikely. An avalung, airbag, or helmet would not have made much of a difference; you would have been beaten senseless. There are likely other spots near the ridgeline where thin, rocky sections have pockets of facets that will fracture into stronger, deeper snow. These zones are not to be trifled with. Since the slabs are denser than normal, pushing your pole into them is impossible—a quick test to see if you're standing on one.

On Sunday, Eric skied the west side of the Bridger Range and found stable snow except on southwest aspects that had facets buried two feet deep, breaking easily. Skiers in the southern Madison Range also found isolated facets two feet under the surface breaking clean on one slope out of their three tested. Variability of wind slabs along ridgelines and thinner, faceted slopes demand extra investigation. Given the heightened avalanche awareness needed to discern stable slopes, for today, the Avalanche Danger is rated MODERATE on all wind loaded terrain. Any slope steeper than 35 degrees will also have a MODERATE Avalanche Danger, while less steep terrain has a LOW Avalanche Danger.

I will issue the next advisory tomorrow morning at 7:30 a.m. If you have any snowpack or avalanche observations, drop us a line at mtavalanche@gmail.com or call us at: 406-587-6984.

Beacon Park in Bozeman

Feeling rusty with your avalanche transceiver? The new beacon park at Beal Park in Bozeman is up and running. It's got four transmitters and the park is open 9:00 a.m. to 8:00 p.m. every day. The Friends of the Avalanche Center and the City of Bozeman worked together to make this possible.

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

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

Gallatin National Forest Avalanche Center ~ Mammoth Weather Forecast