Today: Snow. High near 15°F. Wind chill values as low as -15°F. Chance of precipitation is 100%. Total daytime snow accumulation of 1 to 3 inches possible.

Tonight: A 40 percent chance of snow, mainly before 11:00 pm. Mostly cloudy, with a low around -14°F. Wind chill values as low as -30°F. New snow accumulation of less than a half inch possible.

Thursday: Mostly sunny, with a high near 14°F and a low around -15°F. Wind chill values as low as -25°F.

Friday: Mostly sunny, with a high near 18°F and a low around -7°F. South southwest wind between 8 and 11 mph.

Saturday: Mostly sunny, with a high near 19°F and a low around 2°F. West southwest wind between 7 and 11 mph.

Sunday / Christmas Day: Mostly sunny, with a high near 25°F and a low around 5°F.

Monday: A slight chance of snow. Partly sunny, with a high near 20°F and a low around 3°F.

Tuesday: A slight chance of snow. Mostly cloudy, with a high near 23°F.

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Mountain Weather

It started snowing in the mountains around midnight which will continue today. At 6:00 a.m. the Gallatin and Madison Ranges (Bozeman to West Yellowstone) are showing 5 to 6 inches of light powder with Cooke City getting 2 to 3 inches and the Bridger Range 1 to 2 inches. Mountain temperatures are in the teens Fahrenheit with northwest winds averaging 10-15 mph and gusts into the 20s. Snowfall will end later today with an additional six inches falling at the higher elevations.

Snowpack and Avalanche Discussion:

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

If the storm continues as predicted, the avalanche danger will rise. We have a weak snowpack and the new snow will likely be the first real test of its integrity.

Throughout southwest Montana, this new snow is falling onto either a thin (under 2 feet), weak snowpack made up almost entirely of facets, or a thicker snowpack (over 2 feet) with two weak layers: facets buried a few inches under the surface and another near the ground.

In the Bridger Range we've gotten reports of a thin snowpack consisting of facets. Skiers around the Throne and Flathead Pass found lots of weak snow that will quickly become unstable as it gets loaded.

In the Madison and southern Gallatin Ranges from the mountains around Big Sky to West Yellowstone, the snowpack is also thin and weak. I was in Beehive Basin north of Big Sky on Sunday, to the south on Buck Ridge yesterday and in the Taylor Fork and Bacon Rind areas last week. I dug many snowpits and found large-grained, unsupportable facets throughout, which explained why I was sinking to ground every time I stepped out of my skis or off my snowmobile.

In the mountains around Cooke City and the northern Gallatin Ranges, the snowpack is three feet deep in most areas. A layer of facets near the ground is slowly gaining strength, but should not be discounted. Another prominent weak layer in the snowpack is small-grained faceted snow or surface hoar found a few inches under the snow surface. Yesterday a skier triggered a small slide on a near surface weakness outside Cooke City.

The snowpack will not tolerate much of a snow load. Although the current (6:00 a.m.) 4 to 6 inches of 5% density snow is not enough to tip the scales, continued snowfall today will ripen the slopes for avalanches.

Right now the Avalanche Danger is rated MODERATE, but another 4 to 6 inches would ramp up the danger towards CONSIDERABLE and possibly even HIGH if we get an especially robust snowfall.

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.

PHOTOS, SNOWPITS, and VIDEOS

1. We've recently uploaded more photos and snowpits to our web site, more than what are linked in the advisory.

2. We're creating a series of "How To…" stability test videos. So far we've got clips on performing a CT and ECT. There are located under Stability Tests on the Resources page.

Information provided by Doug Chabot, Mark Staples, and Eric Knoff from the Gallatin National Forest Avalanche Center. For Photos and Videos, please visit the Avalanche Centers Website!

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.