The Weather Service has struggled over the years with how much to let the public in on what it doesn’t exactly know. In April 1997, Grand Forks, N.D., was threatened by the flooding Red River, which bisects the city. Snowfall had been especially heavy in the Great Plains that winter, and the service, anticipating runoff as the snow melted, predicted that the Red would crest to 49 feet, close to the record. Because the levees in Grand Forks were built to handle a flood of 52 feet, a small miss in the forecast could prove catastrophic. The margin of error on the Weather Service’s forecast — based on how well its flood forecasts had done in the past — implied about a 35 percent chance of the levees’ being topped.
The waters, in fact, crested to 54 feet. It was well within the forecast’s margin of error, but enough to overcome the levees and spill more than two miles into the city. Cleanup costs ran into the billions of dollars, and more than 75 percent of the city’s homes were damaged or destroyed. Unlike a hurricane or an earthquake, the Grand Forks flood may have been preventable. The city’s flood walls could have been reinforced using sandbags. It might also have been possible to divert the overflow into depopulated areas. But the Weather Service had explicitly avoided communicating the uncertainty in its forecast to the public, emphasizing only the 49-foot prediction. The forecasters later told researchers that they were afraid the public might lose confidence in the forecast if they had conveyed any uncertainty.