By Ann Saphir
NORMAN, Oklahoma (Reuters) - When a tornado touched down in Norman, Oklahoma, last Friday about two miles from the headquarters of the nation's storm prediction nerve center, local forecasters were able to give a detailed warning only three minutes before it hit.
No one was killed in Norman but six people died after a tornado struck Woodward, Oklahoma, early on Sunday while it was still dark and the town's storm siren failed to sound. An estimated 75 tornadoes tore through four states of "Tornado Alley" over the weekend.
It is the job of the 36 meteorologists at the national Storm Prediction Center in Norman, and hundreds more in 122 local offices around the country, to stay a step ahead of tornadoes.
"The reason I come to work every day is to give people information to help them not be killed or injured in a storm," said Rick Smith, a warning coordination meteorologist in the Norman local office, which warned of the tornado last Friday.
Despite the latest computer technology, forecasters give tornado warnings to specific locations on average only about 15 minutes ahead of time. Nationwide, 75 percent of those warnings never pan out and even those that do give no information on how severe the tornado is likely to be.
"The problem with tornadoes is, we are nowhere near being able to accurately predict tornado intensity," said Greg Carbin, one of the meteorologists who look at the national situation.
In a modern building that stands on the edge of the University of Oklahoma campus, meteorologists work at stations with eight computer screens each, and with several larger screens tracking national patterns on the walls.
Using sophisticated, computer-driven modeling, the center had warned three days before the weekend of a significant tornado outbreak in the Plains states from Texas through to Iowa - an unusually long lead time, according to a post mortem done by the National Weather Service.
But turning that broad prediction into useful warnings for specific neighborhoods at an exact time is even more difficult.
In the case of the Norman tornado, Smith said the local office issued a warning of severe thunderstorms that could produce tornadoes at 3:55 p.m. Friday. They followed four minutes later with a tornado warning - which means take cover. The tornado hit three minutes after that.
At one point some staff at the storm center hustled down to an auditorium below ground to take cover themselves.
"The tornado developed very quickly, and unfortunately that happens in quite a few situations - the radar doesn't detect signs of a developing tornado," Smith said.
Surrounded by computers and high technology, national meteorologist Carbin starts each forecasting shift with a low-tech box of colored pencils. He prints out the surface conditions for the continental United States from the computer. Then he marks it up with what is really happening.
In just a few weeks the center will ramp up a massive computer prediction project that is so intensive it must reserve time on supercomputers in four cities in addition to its own. One goal of this project is to determine how accurately computers can predict destructive storms without human help.
But in their regular eight-hour forecasting shifts, Carbin said, most of the meteorologists at the Norman facility also rely on manual analysis such as with the colored pencils.
The role of computers is critical and will become even more so as the weather service replaces its current radar network with much more data-intensive sensors that detect more detailed atmospheric conditions.
Until the day the computers are more precise, forecasters rely on judgment of a mix of data and human observation.
"We've probably reached some point in the science where we really can't go much farther than that," Carbin said. "If you wanted to increase lead time, you probably are also going to increase false alarm: they go hand in hand."
The National Weather Service is experimenting with a more graphic warning system in a project launched after a catastrophic tornado leveled Joplin, Missouri, last May, killing 161 people.
The pilot begun earlier this month in Kansas and Missouri issues stark warnings such as: "This storm is not survivable," or "Mass devastation is highly likely" to shock people into action.
Even if the so-called tiered warning pilot project is successful, the potential for false alarms is probably never going to go away, Carbin said.
"Here's the rub ... If you happen to miss a tornado event with no warning, and it goes through and damages a community and kills a few people, that is a bad thing. It is perceived as being much worse than having the warning out and having no tornado," said Carbin.
(Additional reporting by Greg McCune; Editing by Jackie Frank)