Yellowstone Seven Day Forecast on 17 January 2011
by the National Weather Service Riverton, WY

Today...Breezy...snow. Snow accumulation of 1 to 3 inches. Highs 31°F to 37°F. West winds 15 to 20 mph. Chance of snow near 100 percent.

Tonight...Breezy. Snow likely in the evening...then chance of snow after midnight. Snow accumulation of 1 to 2 inches. Total snow accumulation 2 to 8 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 24°F to 30°F. Southwest winds around 15 mph.

Tuesday Night...Breezy. Snow likely. Snow accumulation of 2 to 3 inches. Lows 11°F to 17°F. Southwest winds 15 to 20 mph. Chance of snow 70 percent.

Wednesday...Mostly cloudy with a 40 percent chance of snow. Highs 18°F to 24°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 showers. Highs 17°F to 23°F.

Thursday Night...Mostly cloudy with a 40 percent chance of snow showers. Lows 5°F to 13°F.

Friday...Mostly cloudy with a 40 percent chance of snow showers. Highs 23°F to 29°F.

Friday Night...Mostly cloudy with a 50 percent chance of snow showers. Lows 12°F to 18°F.

Saturday...Mostly cloudy in the morning then becoming partly cloudy. A 40 percent chance of snow showers. Highs 24°F to 30°F.

Saturday Night...Mostly cloudy with a 30 percent chance of snow showers. Lows 11°F to 17°F.

Sunday...Mostly cloudy with a 20 percent chance of snow showers. Highs 25°F to 31°F.

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

Mountain Weather

Over the past 24 hours the mountains around Cooke City have picked up over a foot of snow totaling 1.4 inches of water (Fisher Creek Snotel). The mountains around West Yellowstone have received 6-8 inches over the past 24 hours while the mountains around Bozeman and Big Sky have squeezed out 2-3 inches. Temperatures this morning are in the mid 20s to low 30s Fahrenheit and winds are cranking out of the Westsouthwest at 20-30 mph. Winds have been picking up over the past few hours with gusts in Hyalite topping 70 mph; gusts near Big Sky are breaking 50 mph. More snow and wind are forecasted today with an additional 2-3 inches falling in the south and 1-2 inches in the north. Mountain temperatures will remain warm with highs climbing into the upper 30s fahrenheit. An unsettled weather pattern will continue through tonight, with cooler temperatures arriving tomorrow.

Snowpack and Avalanche Discussion:

The mountains around Cooke City and the Washburn Range:

Over the past 48 hours, 1.5-2 feet of new snow has accumulated in the mountains around Cooke City - totaling 1.8 inches of water (Snow Water Equivalent). Not only has the snow been cranking, but so have the winds. This morning winds have been averaging 30 mph with gusts up to 50 mph at 10,000 feet near Lulu Pass (weather station). This combination of snow and wind has created Very Dangerous Avalanche Conditions.

Yesterday, a group of skiers triggered a wind slab avalanche outside of Cooke City that ran the entire slope. Today I expect wind loaded slopes to avalanche naturally. Under these conditions, avoiding steep wind drifted slopes and avalanche run out zones is highly advised.

With more snow and wind forecasted throughout the day, avalanche conditions will remain elevated.

Today, a HIGH Avalanche Danger exists on all wind loaded slopes; slopes without a wind load have CONSIDERABLE Avalanche Danger.

The southern Madison and southern Gallatin Ranges, the Lionhead area near West Yellowstone:

The mountains around West Yellowstone have picked up a foot of snow over the past two days totaling 1 inch of water. Although the snowpack prior to this storm lacked persistent weak layers, this new load has been applied rapidly, adding a large amount of stress to the pack.

When snow of this quantity arrives in a short period of time, persistent weak layers are not required for avalanches to occur. Failures within in the storm snow are common during or immediately after a storm of this size. Yesterday, skiers in the southern Madison range triggered numerous soft slab avalanches and witnessed one natural slide that broke up to a foot deep and propagated over 200 feet across. This is an obvious indication the snowpack is under an enormous amount of stress.

Although new snow avalanches are today's primary concern, triggering a slide on deeper layers is possible. Riders should be wary of shallow areas where the snowpack is less than a meter deep. These areas are likely weaker and could act as trigger points for avalanche activity.

Today, a fast and heavy load has made human triggered avalanches likely on all slopes and the Avalanche Danger is rated CONSIDERABLE.

The northern Gallatin and northern Madison Ranges and the Bridger Range:

The northern ranges have been the forecasted underdog in this storm and the predictions are proving true. With only 2-4 inches of snow falling in the mountains around Bozeman and Big Sky - the generally stable snowpack in these areas has had an easy time supporting this new load.

While the snowpack in the northern ranges remains relatively quiet, a few problem areas do exist. Our main concern is the strong winds that accompanied this storm. Constant winds have easily transported the new snow, forming soft slabs on leeward slopes favored by westerly winds. These slabs will likely fail under the weight of a skier or rider.

A secondary concern is weak faceted snow near the ground. This layer is not widespread, but can be found in areas where the snowpack is shallow. Doug found this layer in Beehive Basin yesterday while on the hunt for buried surface hoar. Fortunately Doug did not find the surface hoar to be reactive in stability testes, but was disappointed by the weakness of this faceted layer near the ground.

Today, strong winds and new snow have made human triggered avalanches likely on wind loaded slopes where the Avalanche Danger is rated CONSIDERABLE. All slopes that have not received a wind load have MODERATE Avalanche Danger.

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 ~