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National Weather Service tracks ice, snow depths in Alaska to forecast breakup severity

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Posted: Thursday, January 7, 2010 2:36 am | Updated: 1:03 pm, Wed Dec 26, 2012.

FAIRBANKS - Trudging onto frozen Chena Lake on Tuesday afternoon, Kelly Allen and Ed Plumb looked like a pair of ice fishermen. Plumb was carrying a gas-powered ice auger at his side while Allen had a scoop shovel slung over her shoulder.

But it wasn’t fish under the ice that Allen and Plumb were seeking. They were more interested in the snow on top of the ice, as well as the ice itself.

“We measure the ice thickness, the snow depth and the snow water equivalent,” said Plumb, a hydrologist for the National Weather Service in Fairbanks.

With that, Allen stuck an open-ended, plastic snow sampling tube into snow, which had been packed down by snowmachines. The snowpack was 2 inches deep. Allen gently slid a putty knife underneath the tube to trap the snow inside. Allen then turned the tube over and hung it from a scale that measures the water equivalent in the snow based on weight.

“Point six inches,” she said, eyeing the scale.

After shoveling off a patch of ice, Plumb picked up the auger and held it in place while Allen yanked on the starting cord. After a few pulls the engine fired to life. With Plumb holding one side and Allen the other, the auger bored a hole into the ice, pushing up a pile of ice shavings that Plumb and Allen pushed aside with their feet.

It took about 30 seconds for the auger to cut through the ice, which was evidenced when water began gushing out of the top, prompting Plumb and Allen to pull the auger from the hole.

Using a homemade measuring stick with a lip attached to the bottom, Allen shoved the stick into the water until she felt the bottom of the ice with the lip. The ice measured 25 inches.

“That’s 13 inches thicker than last month,” Plumb announced, checking his notes.

Their task complete, Allen and Plumb covered the hole with ice chips and snow and moved on to their next stop, the Chena River at Nordale Road.

Every month from November to April, the National Weather Service takes ice and snow measurements on 15 lakes, rivers and ponds around Fairbanks. In addition to tracking local ice and snow depths during the course of the winter for recreational users, pilots and others interested in ice, the measurements help hydrologists and meteorologists around the state gauge what kind of breakup it will be when the snow and ice melts.

The weather service has observers in towns and villages along most of the major waterways in the Interior and around Alaska who take similar measurements, which are then sent to the Alaska-Pacific River Forecast Center in Anchorage.

Ice thickness, snow depth and water density in the snow are three of the four main factors hydrologists like Plumb use to forecast breakup severity.

“This is just a piece of the puzzle,” Plumb said of the measurements he and Allen were taking on Tuesday. “The real driver is what the temperature is in the spring.”

Last spring, for example, was “the perfect storm for spring breakup in Alaska,” Plumb said.

“The ice was thicker than normal, we had a higher than normal snowpack so there was more water to melt and go into the rivers and we had record warm temperatures,” he said. “Last year all the pieces came together.”

The result was the most destructive breakup on the Yukon River in decades, Plumb said.

Ice thickness varies depending on how much snow is on top of the ice and whether or not you’re talking about a river or a lake, Plumb said. Snow acts as insulation, which slows the growth of ice, even in extreme cold weather. If snow gets packed down by vehicles or snowmachines like it was on Chena Lake, it has less insulating effect because it has less air in it.

“Just like a down sleeping bag,” Plumb said.

This month, the thickest ice — 30 inches — was in the Tanana River at Chena Pump Road. The thinnest ice — 14 inches — was at Olnes Pond.

Ice thickness increased between 4 and 12 inches during the past month, Plumb said. The smallest ice growth occurred at spots with an undisturbed, more insulating snowpack, such as Smith Lake at the University of Alaska Fairbanks and the Chena River at Nordale Road.

In spots where the wind has blown away or significantly reduced the snowpack or it has been packed down by vehicles or snowmachines, such as Quartz Lake and Chena Lake, the ice growth was greater, Plumb said.

Ice growth typically slows down as winter progresses because the snow on top of the ice gets deeper, though that hasn’t necessarily happened this year with only 19.7 inches of snow as of Wednesday, which is almost two feet below normal.

So far this winter, ice thicknesses are pretty close to average for the past 10 years, despite a lower-than-normal snowfall, Plumb said. In some spots it’s a few inches thicker and in other spots it’s a few inches thinner. On Chena Lake, the ice thickness this year is exactly the same as last year at 25 inches.

“There are no clear signatures of ice being a lot thicker or thinner than normal,” Plumb said.

Contact outdoors editor Tim Mowry at

459-7587.

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