The upslope phenomenon | SummitDaily.com
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The upslope phenomenon

BOB BERWYN
summit daily news
AP Photo
AP | AP

SUMMIT COUNTY – Last week’s snowstorm that passed parts of Colorado with several feet of snow was a classic upslope storm that delivered significantly more precipitation along the Front Range than in the High Country.

Upslope conditions develop when there’s a low-pressure center east and south of the Continental Divide. As the winds rotate counter-clockwise around the low, they push moist air up against the east slope of the Rockies.

As the air climbs, it expands and cools. Since cold air can’t hold as much water as warm air, the moisture condenses and falls as rain or snow.

During many upslope storms, the maximum snowfall is recorded at relatively low elevations, around Idaho Springs, Genesee, Evergreen, and even as low as Golden, Castle Rock and Golden.

In meteorological terms, this process is called “orographic lifting.” Most snowfall in Colorado results from this phenomenon, Because of their orientation, the mountains in Summit County get the most snow when they’re hit by a moist flow out of the northwest.

Often, when a juicy storm blasts the San Juans out of the southwest, Summit County stays dry until the storm moves east and the flow switches to come more from the north or northwest.

Because of the complex, rugged topography, snowfall totals can vary tremendously within just a few miles. Often, the variations are related to the orientation of the slopes.

For example, Vail, Beaver Creek, Copper Mountain and Steamboat ski areas can get a lot of snow with a flow of moist air from the west. That’s why Copper will sometimes pick up more than a foot, while Keystone may only record a few inches.

Since news outlets like major television stations cover the entire state, they don’t always differentiate between the heavier snowfall low on the Front Range and drier conditions in the High Country.

One of the best sources for localized weather information is the website of the Colorado Avalanche Information Center (http://avalanche.state.co.us/). By necessity, the forecasters with the center try to pinpoint their forecasts to predict avalanche hazards and taking into account local wind patterns and other factors.

Statistics compiled by the National Weather Service and automated data from remote SNOTEL sites tell the story of the most recent storm.

Pinecliffe, along the Front Range, reported 43.8 inches of snow, and Evergreen picked up three feet.

By contrast, the SNOTEL site at 10,500 feet elevation at Copper Mountain reported six inches of snowpack (Oct. 29). At the Loveland Basin SNOTEL site (11,400 feet), there were 15 inches of snow on the ground. The Vail Mountain SNOTEL site (10,300 feet) recorded an eight-inch snowpack and the Grizzly Peak site (11,100 feet), near Arapahoe Basin, recorded an 11-inch snowpack. The Hoosier Pass site, at 11,400 feet, had the most snow in Summit County, with 14 inches on the ground on Oct. 29.

The early snow has also resulted in a string of early season avalanche incidents. According to the Colorado Avalanche Information Center, six people have been caught in slides, beginning with a pair of climbers Oct. 5 in Rocky Mountain National Park.

A 40-foot wide, 6-inch deep slab broke loose and took the climbers for a short ride down a steep, rocky pitch, the avalanche center reported.

A week later, three skiers near Independence Pass remotely triggered an avalanche in a steep, north-facing gully. That slide ran about 1,200 vertical feet.

On Oct. 17, a skier triggered a medium-size slide, about 2 feet deep. The avalanche took with it all the new snow from the October storms, right down to the surface of the permanent snowfield on the Tyndall Glacier in Rocky Mountain National Park.

At Jones Pass, another popular early season backcountry ski spot, a skier was caught in the middle of a slab and ended up with his legs buried in debris. Another skier suffered bumps and bruises when he was caught Oct. 25 in an avalanche on Flattop Mountain, in Rocky Mountain National Park. The latter slide broke 4 feet deep, showing how early season snowfall can quickly lead to mid-winter avalanche conditions.

Also on Oct. 25, a skier triggered a slab on a hard ice crust at 12,000 feet on an east-facing slope at Loveland Pass. The skier tumbled about 150 vertical feet, but there wasn’t enough snow to bury the skier. According to the avy center, the slide’s crown (where it broke away from the surrounding snow) was about 20 inches deep.

The avalanche center will begin forecasting for highway corridors on Nov. 1 and for backcountry recreation later in the month.

The local hotline is at (970) 668-0600.

Copper Mountain, 10,500 feet – 6 inches

Loveland Basin, 11,400 feet – 15 inches

Grizzly Peak, 11,100 feet – 11 inches

Vail Mountain, 10,300 feet – 8 inches

Hoosier Pass, 11,400 feet – 14 inches

Avalanche and weather information:

http://avalanche.state.co.us/

More information on upslope storms:

http://www.weather.com/encyclopedia/winter/upslope.html


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