Avalanche Information - from the Gallatin National Forest Avalanche Center

Mountain Weather:

The final storm of 2010 deposited an additional 7-8 inches of snow in most areas except near West Yellowstone where an additional 10-12 inches fell since yesterday morning. Although light snowfall will continue today, only an inch or two should accumulate. This morning temperatures had dropped to the single digits F near West Yellowstone and around -5°F everywhere else. Winds were blowing 5-15 mph from the North and East. Today temperatures should drop further to almost -10°F with cold East and Northeast winds blowing 10-15 mph.

Snowpack and Avalanche Discussion:

The northern Madison Range:

The mountains around Big Sky have received over 1 foot of snow during the last 48 hours, and ski patrols on Lone Mountain have easily triggered slab avalanches on wind loaded slopes. At Moonlight Basin, the ski patrol triggered one avalanche that broke on a layer of surface hoar buried 2 feet deep. With plentiful new snow and sufficient wind (blowing Southwest during the first half of the storm then switching to the North and East), avalanches occurring in the new snow are likely. Many slopes contain a layer of surface hoar buried 2 feet deep that will also be a problem. Around Big Sky the key will be to find low angle slopes lacking both wind deposited snow and the buried surface hoar layer. For today, the avalanche danger is HIGH on all slopes that are wind-loaded. All other slopes have a CONSIDERABLE danger.

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

The mountains near West Yellowstone have been the big winner during this storm receiving about 20 inches of snow in the last 48 hours while other areas received about 10-12 inches. Generally the snowpack is strong and avalanches will mostly be confined to the new snow. Yesterday Doug and I skied near Hebgen Lake and found no weak layers in the snowpack similar to our findings near Lionhead, Teepee Basin, and the Taylor Fork area. We observed minor cracking in the new snow but felt safe skiing mellow slopes without wind deposited snow. Near Cooke City a skier observed a small avalanche on a Northeast facing slope and reported a skier triggered avalanche on an East facing slope near Lulu pass on Tuesday. This avalanche was about 100 feet wide and 1 foot deep.

Although the snowpack is generally strong, some areas may have avalanches breaking into deeper layers of snow. Near Cooke City, a layer of small facets exists on all slopes about 3 feet deep though this layer hasn’t been very sensitive. Near the Bacon Rind drainage and near Flathead Pass, buried surface hoar has been found, and on the west side of the Bridger Range on South and West facing slopes, the snowpack is thin and weak. Today avalanches are most likely on wind loaded slopes which have a HIGH avalanche danger. Non wind loaded slopes that are steeper than 35 degrees have a CONSIDERABLE avalanche danger. Until the snowpack has time to adjust to the weight of this new snow, search for slopes without wind blown snow that are less than 35 degrees where the avalanche danger is rated MODERATE.

The northern Gallatin Range:

Unfortunately the northern Gallatin Range, including the Hyalite Canyon area, has only received about 6 inches during this storm. In simple terms, less snow means less avalanches and the snowpack was mostly strong and stable prior to this storm. The old snow surface was highly variable and the new snow needs some time to bond to it. Additionally, isolated areas contain some weak snow that will be stressed by the weight of wind deposited snow. For these reasons today, the avalanche danger is rated CONSIDERABLE on wind loaded slopes and MODERATE on all others.

Gallatin National Forest Avalanche Center ~ Mammoth Weather Forecast

Yellowstone Seven Day Forecast on 30 December 2010 by the NWS Riverton, WY

Today: Cloudy. Snow in the morning...then chance of snow in the afternoon. Snow accumulation up to 1 inch. Total snow accumulation up to 3 inches. Highs 5°F to 11°F. North winds around 15 mph in the afternoon. Chance of snow 90 percent.

Tonight: Cloudy. Chance of snow in the evening...then slight chance of snow after midnight. Lows -8°F to -14°F. North winds around 15 mph early in the evening. Chance of snow 40 percent.

Friday: Mostly cloudy in the morning then becoming partly cloudy. A 20 percent chance of snow. Highs 0°F to 6°F. Lowest wind chill readings -20°F to -30°F in the morning.

New Years Eve / Friday Night: Mostly cloudy in the evening then becoming partly cloudy. Lows -13°F to -19°F.

New Years Day: Partly cloudy. Highs 2°F to 8°F. Lowest wind chill readings -22°F to -32°F in the morning.

Saturday Night: Partly cloudy in the evening then becoming mostly cloudy. Areas of freezing fog after midnight. Lows -3°F to -11°F.

Sunday: Not as cold. Areas of freezing fog in the morning. Partly cloudy with a 20 percent chance of snow. Highs 11°F to 17°F.

Sunday Night: Mostly cloudy with a 20 percent chance of snow. Lows -4°F to 4°F.

Monday: Mostly cloudy with a 20 percent chance of snow. Highs 16°F to 22°F.

Monday Night: Mostly cloudy with a 20 percent chance of snow. Lows -1°F to 7°F.

Tuesday: Mostly cloudy in the morning then becoming partly cloudy. A 20 percent chance of snow. Highs 16°F to 22°F.

Tuesday Night: Partly cloudy in the evening then becoming mostly cloudy. A 20 percent chance of snow. Lows 3°F to 9°F.

Wednesday: Mostly cloudy with a 20 percent chance of snow. Highs 19°F to 25°F.

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