Forecasters warned of “certain death,” a possible 25-foot surge of water that would wash across the Texas and Louisiana coast, wiping away towns in a white-capped, churning mess of debris.

What Hurricane Ike actually brought was a storm surge about half that, still causing widespread flooding and damage, but far less than the catastrophic predictions.

The experts were “dead on” with their latest forecasts that Ike would come ashore close to Galveston, even as the track shifted over several days, and that it would hit as a strong Category 2 storm, noted National Hurricane Center spokesman and meteorologist Dennis Feltgen.

Yet, storm surge size remains one of the most daunting calculations to make, say hurricane forecasters. Ike’s maximum surge was about 13.5 feet near the Texas-Louisiana border.

“To get a perfect storm surge forecast, you have to have a perfect forecast for the track, a perfect forecast for its intensity and a perfect forecast of its structure, and we don’t know how to do any of those perfectly,” said former National Hurricane Center Director Max Mayfield. Benton McGee, supervisory hydrologist at the U.S. Geological Survey’s storm surge center in Ruston, La., points to the “many variables.”

“There’s the size of the storm, which obviously in this case wasn’t a good indicator, the speed, the tide cycle, the orientation of the coast, how the hurricane comes in,” he said.

Computer models that produce storm surge estimates also aren’t perfect, especially with such a large, slow-moving storm as Ike—nearly 600 miles across, almost as large as the state of Texas.

“Models are based on data, so if you don’t have a lot of data, then the model is limited in what it can do,” McGee said. “We’re really just now putting out enough (surge) sensors and enough instruments in the field to really record exactly what’s happening.”

Storm surge is basically water that is gradually pushed onto shore by the force of the winds circulating in a hurricane. It’s similar to a bathtub filling with water while being violently stirred, then overflowing.

Typically, the stronger and bigger a storm and the shallower the coastal slope—as is the case with Texas and Louisiana—the bigger the surge will be. But nothing is certain in nature.

“The storm itself changed a little bit. I think it tightened up more, and more of the energy went into the center,” stealing some of the punch from the predicted surge, said Wilson Shaffer, chief of the National Weather Service’s evaluation division.