Yellowstone Seven Day Forecast on 18 January 2011
by the National Weather Service Riverton, WY

Today...Breezy. Cloudy with a 50 percent chance of snow. Highs 24°F to 30°F. Southwest winds 15 to 20 mph.

Tonight...Breezy...snow. Snow accumulation of 3 to 6 inches. Lows 11°F to 17°F. Southwest winds 15 to 20 mph until early morning. Chance of snow near 100 percent.

Wednesday...Mostly cloudy. Snow likely in the morning...then chance of snow in the afternoon. Snow accumulation of 1 to 2 inches. Total snow accumulation 4 to 8 inches. Highs 19°F to 25°F. North winds around 15 mph in the morning. Chance of snow 70 percent.

Wednesday Night...Colder. Partly cloudy with a 20 percent chance of snow. Lows -3°F to 3°F.

Thursday...Mostly cloudy. A 20 percent chance of snow in the afternoon. Highs 16°F to 22°F.

Thursday Night...Mostly cloudy with a 40 percent chance of snow. Lows 9°F to 15°F.

Friday...Not as cold. Mostly cloudy with a 30 percent chance of snow. Highs 24°F to 30°F.

Friday Night...Snow likely. Moderate snow accumulations. Lows 14°F to 20°F. Chance of snow 60 percent.

Saturday...Cloudy with a 50 percent chance of snow. Highs 24°F to 30°F.

Saturday Night...Colder. Mostly cloudy with a 20 percent chance of snow. Lows 6°F to 12°F.

Sunday...Mostly cloudy in the morning then becoming partly cloudy. A 20 percent chance of snow. Highs 22°F to 28°F.

Sunday Night...Mostly cloudy in the evening then becoming partly cloudy. A 20 percent chance of snow. Lows 4°F to 10°F.

Monday...Mostly cloudy with a 20 percent chance of snow. Highs 23°F to 29°F.

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Avalanche Information
from the Gallatin National Forest Avalanche Center - this report is by Doug Chabot

Mountain Weather

The mountains picked up more snow yesterday. Cooke City got the most with 6-8 inches while West Yellowstone got the least with 2 inches; 3-4 inches fell everywhere else. Ridgetop winds averaged 25-30 mph with gusts exceeding 40 mph. Temperatures have cooled into the teens from the low 30s yesterday morning. Clouds will increase today as winds remain steady at 20-30 mph out of the west to north. Another moist system will bring snowfall tonight blanketing us with 3-6 inches of new snow by morning.

Snowpack and Avalanche Discussion:

The mountains around Cooke City and the Washburn Range:

The mountains around Cooke City have been the clear winner with almost three feet (2.4 inches Snow Water Equivalent) of new snow in the last 72 hours. Wind drifts made backcountry travel difficult yesterday. The Gallatin National Forest Snow Rangers couldn't even break trail on their sleds to Daisy Pass because of the deep snow. On Sunday, skiers triggered a wind slab near Lulu Pass.

With even more snow and strong west to northwest winds, I'm betting that there are many natural avalanches on wind-loaded slopes today which have a HIGH Avalanche Danger. All other slopes have a CONSIDERABLE Avalanche Danger.

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

Since Saturday evening about 6-8 inches has fallen in the northern mountains with 12-14 inches accumulating down south. Over the last six weeks we've been very concerned with a buried surface hoar layer near Big Sky, but we've been tracking it closely and watched it gain strength. I dug four pits on Sunday looking for it, and although I could still barely see it in the pit wall, it would not fracture in any of my stability tests. If it does exist, it'll be on small, isolated slopes. The snowpack is no longer like punished child standing alone in a corner separated from everyone else; it's finally behaving.

There are two stability issues to watch out for. The first, and most obvious, is wind-loaded slopes which could be easily triggered. The second, more subtle issue is slopes with a thin snowpack. We are finding that snows less than three feet deep have gotten weaker and more unstable as facets near the ground have lost strength and cohesion. We have found this in Beehive Basin and in the Bridger Range on a few west facing slopes as well as Argentina Bowl near Saddle Peak. Skiers reported finding this up Hyalite and also in the southern Madison Range. A thin snowpack is often comprised of sugary, weak facets and will not support new snow very well.

For today, the Avalanche Danger on all wind-loaded terrain is CONSIDERABLE. All other slopes have a 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 ~