Avalanche Information - from the Gallatin National Forest Avalanche Center

Mountain Weather:

Snow is falling this morning and will continue into tonight. At 6:00 a.m. 8-10 inches fell around West Yellowstone; 5-7 inches around Cooke City and the Big Sky area; and 2-4 inches in the Bridger Range and Hyalite. Winds have increased out of the southwest at 20-40 mph with mountain temperatures in the teens. Snowfall will continue today and taper off tonight as temperatures dive to near zero by tomorrow morning. I expect another 3-6 inches to fall in the north and 6-10 inches in the southern ranges. Southwest winds will remain moderate throughout the storm.

The northern Madison Range:

The mountains around Big Sky have already gotten 5-7 inches and winds are loading leeward slopes. All this new snow is loading our most unstable snowpack. The primary avalanche concern today is this windblown snow. On many slopes a buried layer of surface hoar will be failing and avalanching from this rapid load. Even on slopes where this layer is absent, fresh wind slabs will likely be breaking naturally. Yesterday Mark took the sleds and rode up Buck Ridge to MacAtee Basin. One to one-and-a-half feet under the snow he found the surface hoar layer harder to trigger than a week ago (a sign of strengthening), but with today’s snow load this information takes a back seat. For today, the avalanche danger is HIGH on all slopes that are wind-loaded. All other slopes have a CONSIDERABLE danger.

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

The southern mountains, especially West Yellowstone, is getting center punched by this storm. Eight to ten inches has fallen this morning, plus a conservative guess of ten more inches today will bump up the avalanche danger. Winds are blowing strong out of the southwest and I anticipate natural avalanche activity on many wind-loaded slopes. Generally the snowpack is strong, but this snowstorm will test all the layers. Around Cooke City a weakness of small facets is found a few feet under the surface. On steep slopes these may avalanche. With isolated areas of weaker snow (i.e. Bacon Rind) the possibility of triggering slides increases with slope angle. I’ll be skiing with an inclinometer in my pocket today, hyper aware of the terrain above me. Avalanches are almost guaranteed on wind-loaded slopes which I’m rating HIGH today. Slopes fortunate enough to be spared the wind will have a CONSIDERABLE danger if they are steeper than 35 degrees and MODERATE on lower angled terrain. Remember, a considerable danger means it’s likely to trigger a slide, and moderate means it’s possible.

The Bridger Range and the northern Gallatin Range:

The Bridger and northern Gallatin Ranges have finally gotten freshened up with new snow after a spell of neglect. Winds are clocking 15-20 mph out of the southwest; plenty strong to wind-load leeward slopes. The snowpack in these mountains has good stability, but a there are a few isolated slopes with thinner and weaker snow, plus spots in the northern Bridgers with buried surface hoar. Even though these ranges have the most stable snow in our advisory area, with today’s snowfall backcountry travel will require careful evaluation. The greatest potential of triggering a slide will be on wind-loaded slopes which have a CONSIDERABLE danger. All other terrain is rated MODERATE.

Gallatin National Forest Avalanche Center ~ Mammoth Weather Forecast

Yellowstone National Park Seven Day Weather Forecast on 29 December 2010 by the National Weather Service Riverton, WY

Today: Breezy. Areas of blowing snow in the morning. Snow. Snow accumulation of 2 to 5 inches. Highs 22°F to 28°F. Southwest winds 15 to 20 mph. Chance of snow near 100 percent.

Tonight: Snow. Snow accumulation of 3 to 5 inches. Lows -4°F to 2°F. Southwest winds around 15 mph early in the evening. Chance of snow near 100 percent.

Thursday: Colder. Snow in the morning...then chance of snow in the afternoon. Snow accumulation of 1 to 3 inches. Total snow accumulation 7 to 16 inches. Highs 3°F to 9°F. North winds around 15 mph in the afternoon. Chance of snow 90 percent. Wind chill readings -11°F to -21°F.

Thursday Night: Colder. Cloudy with a 40 percent chance of snow. Lows -12°F to -18°F. Northeast winds around 15 mph in the evening.

Friday: Mostly cloudy in the morning then becoming partly cloudy. A 20 percent chance of snow. Highs -1°F to 5°F. Lowest wind chill readings -24°F to -34°F in the morning.

Friday Night: Mostly cloudy in the evening then becoming partly cloudy. Lows -14°F below to -24°F.

New Years Day: Partly cloudy. Highs 4°F to 10°F.

Saturday Night: Partly cloudy in the evening then becoming mostly cloudy. Areas of freezing fog after midnight. Lows -2°F to -12°F.

Sunday: Not as cold. Areas of freezing fog in the morning. Partly cloudy with a 20 percent chance of snow. Highs 12°F to 18°F.

Sunday Night: Mostly cloudy with a 20 percent chance of snow. Lows -2°F below to 6°F.

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

Monday Night: Mostly cloudy with a 30 percent chance of snow. Lows 0°F to 8°F.

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

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