Never the same river twice: Forecasting how much the Red River will rise is tricky businessBy Steve Karnowski, AP
Friday, March 19, 2010
Many tools but no guarantees in forecasting floods
FARGO, N.D. — A year ago, weather forecasters changed their estimate late in the game of just how high the Red River would rise, stoking an 11th-hour sandbagging flurry in Fargo that proved unnecessary in the end because the new prediction was wrong.
Now, as the Red swells again toward an expected crest on Sunday, tens of thousands of Fargo residents are weighing the latest National Weather Service forecasts, well aware that predicting what happens on the river is anything but an exact science.
Forecasters analyze a numbing array of factors when making their predictions. Hydrologists use computer models that account for soil moisture, frost depth, snowpack, temperatures, rate of snowmelt and more. Then there are the unknowns like how much rain might spill into the river.
All of these play out over thousands of square miles of Red River Valley so flat that the flooding here can best be described as spilling a glass of water on a pool table. On Friday, the weather service changed its crest level prediction again, lowering it a half-foot to 19.5 feet above the flood stage on Sunday.
“I think they do a wonderful job, provided that they’re looking into their crystal ball with all the wisdom they have,” said Fargo resident Richard Thomas, 61.
Thomas — for now — is not too worried about flooding, with a home that sits 2 feet above Sunday’s projected crest. A year ago, he weathered the crest of nearly 23 feet above flood stage thanks to a special water-filled tube. He’s got it on standby if crest predictions go higher this year.
The record high water of 2009 helped forecasters by giving them new data on how the river behaves at those levels, said Greg Gust, warning coordination meteorologist at the weather service in Grand Forks. That makes the weather service more confident this year, he said.
“I wouldn’t say we’re relaxed,” Gust said. “We’re more relaxed, or less hectic than other years, and that’s simply because the planning process has enabled us to get some more things in place.”
Recent history in the Red River Valley has been painful for the weather service.
In 1997, forecasters knew there would be record flooding on the Red River in 80 miles north of Fargo in Grand Forks, but they didn’t realize just how bad it would be in time for the city to build its dikes high enough. The Red swelled to a record 26 feet above flood stage and the defenses failed, forcing most of the area’s 60,000 residents to evacuate.
Last year in Fargo, after forecasters belatedly increased their crest prediction to 25 feet above flood stage, the city raced to pile its sandbags higher. The estimate proved to be about 2 feet too high, though the dikes held when the Red topped off.
Meteorologists and disaster officials sometimes refer to major floods as “500-year” or “100-year” floods, but many argue that the terms should be dropped because they’re often misunderstood to mean such a flood will occur only once in that time period.
Fargo’s second big flood fight in as many years can be largely laid at the feet of El Nino, the phenomenon that has affected weather nationwide. In the Upper Midwest, an unusually early warm-up meant rapid snowmelt. And with the ground still frozen, that melted snow moved quickly into streams and rivers.
Donald Schwert, a geology professor and flood expert at North Dakota State University in Fargo, pointed out that the Red River Valley is unlike most others — it’s not even much of a valley — so traditional forecasting methods don’t work very well on it.
When a typical river floods, he explained, it spills out onto its flood plain and is then contained by its valley walls. But when the Red overflows, it spills onto an ancient lake bed and a vast, shallow flood develops.
“Here we have this immense surface that’s one of the flattest places on earth and doesn’t easily fit into existing hydrological models,” Schwert said. “Special models have to be developed and tested, which makes this a very difficult terrain to anticipate how floods will develop and operate.”
Gust said forecasters have been getting a better handle since 1997 on how to predict flooding. That disaster ripped out many river gauges, Gust said. Much of the replacement equipment is stronger and less vulnerable, and the data often flows via satellite instead of fragile phone lines. Forecasters also have developed better ways of determining the mathematical probabilities of flooding, Gust said.
Knowing those odds, helps people make better dollars-and-cents decisions. Gust pointed to Fargo’s decision a few weeks ago to try to stockpile 1 million filled sandbags ahead of time, which gave the city a head start when a stretch of unusually warm weather accelerated flooding projections.
Thanks to increases in computing power, Gust said, forecasters can run alternative scenarios fairly quickly, something they couldn’t do in 1997.
“Every year there’s a different wrinkle that shows up on the terrain,” he said.
Flood fighters have come to learn that they still need to accept some uncertainty. Fargo Mayor Dennis Walaker — who last year criticized the weather service for putting his residents on an emotional roller coaster of river forecasts — said as much this week when asked if the weather would be a wild card.
“It’s not an exact science,” Walaker said. “We understand the variables.”
In Thomas’ neighborhood, former rodeo cowboy Bryan “Snake” Johnson was helping his parents prepare their home, which had a swamped basement last year. Johnson calls the predictions “just a guessing game” with serious consequences. Forecasters, he believes, should be held accountable for river forecasts that go terribly awry.
Still, “I kinda give them a break,” he said. “It’s the nature of the beast of what happens around here. You can’t predict what happens.”
Karnowski reported from Minneapolis. Associated Press writer Dave Kolpack contributed to this story from Fargo.
On the Net:
North Dakota State University Fargo Flood Homepage www.ndsu.edu/fargoflood
(This version corrects the river’s expected crest to 19.5 feet above the flood stage.)
Tags: Fargo, Grand Forks, North America, North Dakota, United States, Weather Forecasts