Yellowstone Seven Day Forecast on 09 January 2011 by the NWS Riverton, WY

Today: Cloudy with a 50 percent chance of snow. Highs 8°F to 14°F.

Tonight: Cloudy with a 30 percent chance of snow. Lows -6°F to -12°F.

Monday: Mostly cloudy with a 20 percent chance of snow. Highs 0°F to 6°F. Lowest wind chill readings -17°F to -27°F in the morning.

Monday Night: Cloudy with a 20 percent chance of snow. Lows -10°F to -18°F.

Tuesday: Mostly cloudy with a 20 percent chance of snow. Highs 3°F to 9°F. Lowest wind chill readings -25°F to -35°F in the morning.

Tesday Night: Partly cloudy in the evening then becoming mostly cloudy. A 20 percent chance of snow. Lows -2°F to -10°F.

Wednesday: Not as cold. Mostly cloudy with a 40 percent chance of snow. Highs 15°F to 21°F. Lowest wind chill readings -19°F to -29° in the morning.

Wednesday Night: Breezy. Not as cold. Cloudy with a 50 percent chance of snow. Lows 10°F to 16°F.

Thursday: Not as cold. Cloudy with a 50 percent chance of snow. Highs 25°F to 31°F.

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

Friday: Cloudy with a 50 percent chance of snow. Highs 22°F to 28°F.

Friday Night: Colder. Cloudy with a 50 percent chance of snow. Lows 7°F to 13°F.

Saturday: Cloudy with a 50 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 Eric Knoff

Mountain Weather:

Since yesterday morning, 4-6 inches of new snow has accumulated in the Bridger Range, northern Gallatin Range and mountains around Cooke City. The rest of our advisory area picked up 2-3 inches. As the snow moved out the cold temperatures moved in. Currently, mountain temperatures are in the single digits above or below zero and winds are blowing out of the west at 5-15 mph. Today, conditions will remain cold and calm with temperatures struggling to reach the double digits. Winds will stay light out of the west blowing at 5-15 mph under mostly cloudy skies. Snow showers will likely taper this morning with no real accumulations expected over the next 24 hours.

Snowpack and Avalanche Discussion:

The northern Madison Range:

Over the past 24 hours, 2-3 inches of low density snow has fallen in the mountains around Big Sky. This new snow alone is likely not enough to push the buried surface hoar layer in this area to its breaking point, but with the added stress of a wind load, things could get interesting.

On Thursday, a snowboarder in Beehive Basin triggered and was caught in a slide. This avalanche failed on buried surface hoar already stressed due to a heavy wind load. Although this layer has gained strength over time, it continues to demonstrate the ability to propagate fractures and produce avalanches. The surface hoar layer responsible for the slide on Thursday is spatially variable, meaning it is found on some slopes but not on all - even on slopes with similar aspects and elevations. With this sort of variability, careful snowpack evaluation is necessary before riding in avalanche terrain.

Strong westerly winds have accompanied the delivery of the new snow, forming sensitive soft slabs on leeward slopes. These fresh slabs now rest over a variety of snow surfaces, one being a thin rain crust that formed during the warm, wet weather on Thursday and Friday. The Moonlight Basin Ski Patrol reported finding this crust over 9,000 feet and mentioned the new snow is having problems bonding to this layer. Although this layer is thin and not found in all areas, it has the potential to be problematic, mainly in steep wind loaded terrain.

Today, human triggered avalanches are likely on wind loaded slopes where the Avalanche Danger is rated CONSIDERABLE. Slopes without a wind load 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:

New snow and wind have loaded leeward slopes creating dangerous avalanche conditions in wind loaded terrain. Yesterday, the Bridger Range and northern Gallatin Range received strong winds and 4-6 inches of new snow in a short period of time, developing sensitive soft slabs that will easily fail under the weight of a skier or rider. The Bridger Bowl Ski Patrol reported the new snow as being manageable if you were prepared, but did mention it produced enough force to knock a person over or carry someone into rocks or trees if caught off guard. Doug also found wind loading to be an issue in the steep gullies up Hyalite. In this type of steep, narrow terrain, it does not take a large avalanche to create a serious situation.

Cooke City also picked up 6+ inches out of this latest storm. An observer in the area reported strong winds forming drifts near the ridgelines. He also found a thin rain crust up to elevations of 10,000 feet on westerly exposed aspects. This curst does not appear to be a problem in the area, but will be something that needs to be considered as we get more snow.

Slopes that have received recent wind loading are today's primary avalanche concern. Recognizing and avoiding areas of wind drifted snow is the best way to avoid being caught in an avalanche. Avalanche activity will likely stay confined to the new snow since the snowpack is generally stable in most areas.

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

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

Feeling rusty with your avalanche transceiver? The new beacon park at Beal Park in Bozeman is up and running. It's got 4 transmitters and the park is open 9:00 am to 8:00 pm every day. The Friends of the Avalanche Center and the city of Bozeman worked together to make this service 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