Friday, November 23, 2007

To snow or not to snow?

Where does our white stuff come from (or not)?

Express Staff Writer

The winter snowpack in the northern Wood River Valley is often deep, but does fluctuate with the unpredictable behavior of Mother Nature. Photo by Willy Cook

Sliding south out of the Gulf of Alaska, a spinning storm sucks moisture from the cold waters of the Pacific Ocean before creeping into the Northwest. Seattle and Portland get fog and drizzle, but the bottom falls out as clouds roll up and over the Cascade Range, where heavy snow falls fast and deep.

In Sun Valley, darkening clouds, swirling winds and the certain smell of snow turn sullen powder junkies into smiling freaks as the promise of a massive dump builds. But it doesn't always happen.

Following an anxious night of bottomless dreams, the day dawns calm with only a thin tapestry of white dusting the rooftops. The sun even looks like it could make an unwelcome appearance.

Sound familiar?

How, with the water-logged Cascades to the west, does Sun Valley's Bald Mountain receive on average of only 200 inches of snow a year?

"It's all orographics," explained Doug Abromeit, director of the U.S. Forest Service National Avalanche Center, based in Ketchum.

Basically, ranges to the west, like the Cascades and the Salmon River Mountains, absorb storms moving south and east off the Pacific, literally stealing the Wood River Valley's snow.

But that doesn't mean Sun Valley is left high and dry. Big snowstorms do bury Baldy—they just need the right push.

The Rocky Mountains are composed of dozens of ranges and countless peaks and valleys, basins and plateaus. The enormously complex and varying topography can affect the flow of moisture, acting as weather barriers or funnels.

The Cascades and Sierra Nevada typically get abundant moisture, but isolated areas within those ranges are particularly susceptible to big snows. Mount Baker, in the northern Cascades, and Mammoth Mountain, in the southern Sierra, are both surrounded by topography that fosters heavy snow.

In the 1998-1999 season, Baker set a world record for the most snowfall in one season—1,140 inches. In the 2004-2005 season, Mammoth received more than 600 inches of snow.

"When you look at the topography, you can see the way the storms get channeled in," said Jeff Dozier, a professor at the University of California-Santa Barbara who specializes in snow hydrology. "Mammoth gets a lot more snow than 50 miles to the north or south."

The same can be said for the Sun Valley area, only in reverse—topography to our west acts as a weather barrier. Like Abromeit said, it's all about orographics.

Orographic lift refers to air being forced up and over mountains or other high points. As the air is forced aloft it begins to cool, and if it's moist, precipitation begins to fall. As the air reaches the leeward side of the mountains it sinks, warms and loses moisture. That's what happens as storms cross the Cascades and other smaller ranges west of the Wood River Valley, including the Smoky Mountains.

"The Cascades get the first brunt of the moisture," said Ron Abramovich, snow and water supply specialist with the National Resource Conservation Service in Boise. "As the clouds move over the mountains, they have to drop their precipitation." By the time Pacific Northwest storms reach Sun Valley, they can be tapped out.

Exactly how much snow Sun Valley gets in an average year is apparently debatable. Some say amounts are as low as 160 inches, others say as high as 220. Abromeit said it's closer to the former.

About 25 miles northwest of Sun Valley, the Vienna Mine receives an average of 360 inches of snow a year—about 200 inches more per year than Baldy, according to Abramovich. Meanwhile, 10 miles to the northeast of the Vienna Mine, Galena Summit receives an average of 210 inches of snow a year. The sites are almost equal in elevation at 8,780 feet and 8,960 feet—Vienna being the higher of the two. But Vienna is farther west and deeper into the Smoky Mountains. Bald Mountain is on the eastern edge of the Smokies and Galena Summit is on the northeastern tip.

While our snowmelt ends up in the Pacific Northwest, our snowstorms form elsewhere. Baldy and the Wood River Valley are surrounded by mountains to the west (the Smokies), the north (the Boulders), and the east (the Pioneers). But there's an opening to the south. From the mouth of the Wood River Valley to Reno lies a massive expanse of prairie and high desert-land that doesn't block the flow of moisture.

That's why in the Sun Valley area, the best snowfall occurs with a southwest flow, said Vernon Preston, a meteorologist with the National Weather Service in Pocatello. Storms that strike the coast of California and churn northward toward Idaho riding strong southwest winds often pound Sun Valley with heavy snow.

Every year, as the summer air grows crisp and the leaves golden, amateur prognosticators in ski towns across the country issue their forecasts for the coming winter. Some use the height of grasses and weeds, like mullen. Others point to the plumpness of the wooly bear caterpillar. The taller the mullen and the fatter the caterpillars, the bigger the winter, or so they believe.

While some rely on nature's signs to predict the coming winter, and some on official Weather Service projections, others rest their faith on cycles. For years ski bums have sworn by the seven-year cycle, which refers to a peak in dry and wet weather about every seven years.

Looking at annual flows on the Big Wood River dating back to 1917, distinct cycles can be seen. The early-to-mid 1980s were generally wet, while the late 1980s and early 1990s were pretty dry. Wet conditions returned in the mid-1990s with several years of high stream flows. Beginning in 2000, a dry period re-emerged.

However, the winter of 2005-2006 was one of the best on record. Snowfall on Baldy measured 241 inches that year.

Even during low snow years in Sun Valley, Abromeit said, the skiing can be excellent.

"The key is that the snow stays good around here forever because we don't have these huge warming spikes and we typically get cold temperatures," said Abromeit, who moved from snowy Alta, Utah, to Sun Valley in 1995, a year after one of the driest winters on record in southern Idaho.

He said that even when Sun Valley gets hit with heavy, wet snow—common in a southwest flow—cold weather often wraps in behind the storms, sucking moisture out of the snowpack.

"Essentially the snow dries out, it gets lighter, and the skiing gets better as time goes on," he said. Plus, the snowmaking system on Sun Valley is one of the best in the country.

Some years, such as in 1997, the storms just line up and pound Sun Valley all winter. During those years, conditions are phenomenal everywhere, not just on Baldy, Abromeit said. Mid-valley areas, while prone to big avalanches, can turn into backcountry hubs.

"Sometimes," Abromeit said, "the skiing is just outrageous near Hailey."

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