ANCHORAGE — The recipe for flooding along Alaska’s river system is abundant snow and ice, cold April temperatures and a surge of May heat, but the National Weather Service says it’s too soon to say whether all four conditions will line up in 2013.
River ice remains thick, the snowpack is above average, and Alaska experienced below-normal April temperatures. But the most critical factor is the weather in the next few weeks, said hydrologist Scott Lindsey, and atmospheric models that predicted April’s cold are giving mixed signals for May.
“We don’t have nearly the certainly that we had about what’s going to happen next week,” he said Monday.
Flooding occurs when thick river ice stays intact while spring melt water fills river channels. The water lifts up ice, breaks it and moves it downriver, but if the ice volume is too much, it can settle at river chokepoints, creating a natural, temporary dam that sends water upriver into villages.
A monthly hydrological outlook the weather service released Sunday lists the threat of spring flooding as “moderate” for most of the state.
The outlook notes normal to above-normal snowpack in much of the state, meaning 90 to 130 percent of the usual amount along major rivers. Likewise, ice thickness remains between 100 and 125 percent of normal.
Temperatures in April and the first week of May were well below normal, especially in interior Alaska, where the Yukon River, America’s third longest, crosses east to west in an arc through the state’s midsection.
Flooding that moved buildings off their foundations and caused millions of dollars in damage at Eagle and other Yukon River communities four years ago was preceded by a cool April and then five days of record-high temperatures in the 70s, Lindsey said.
“The ice just wasn’t ready to move yet,” Lindsey said.
Breakup starts upstream in Canada and moves downriver, east to west. When breakup is imminent, Lindsey flies the river to see where the open water begins. He’s usually in the air by now, but April’s cold temperatures delayed his trip until at least Saturday.
Whitehorse, Yukon Territory, was in the 40s over the weekend and has areas of open water, but a Dawson City webcam shows the historic Klondike Gold Rush town locked in winter, Lindsey said. Ice on the Klondike River usually goes out a week before the Yukon at Dawson but remains frozen.
“It’s still solid ice, snow still on the hills and mountains,” Lindsey said. “It doesn’t look like breakup is happening soon.”
One atmospheric model forecasts warming this week, followed by cooling next week, he said.
“That would be ideal,” Lindsey said. “That would put the brakes on the big slug of snowmelt getting into the rivers and allow us to break up a little slower.”
From the airplane, Lindsey will look for signs of imminent flooding. Ice jams, he said, can back water into a village in as little as three hours. River forecasters in 2009 were able to predict flooding at Eagle was going to be bad because 50 miles of moving ice from tributaries was flowing toward a Yukon River jam just downstream of the community.
“That’s our purpose out there,” he said.