A year after violent tornadoes ripped through Oklahoma and Kansas, neighborhoods are being rebuilt, tornado shelters are popular items, and scientists and forecasters are reflecting on the lessons learned from 1999's deadliest outbreak. During the busiest tornado months of April, May and June, those lessons are relevant for many people in the United States.

On May 3, 1999, a total of 66 tornadoes touched down across Oklahoma and Kansas, killing 46 people, injuring 800 and causing $1.5 billion in damage. The most expensive single tornado in U.S. history occurred that day, an F-5 tornado that moved along a 38-mile path from near Chickasha through south Oklahoma City and the suburbs of Bridge Creek, Newcastle, Moore, Midwest City and Del City. With 8,000 buildings damaged, the Oklahoma City tornado caused about a billion dollars in damage.

The event proved the effectiveness of the National Weather Service's watch and warning program. The Storm Prediction Center highlighted the tornado threat in their outlook issued at 1 a.m. Data from the wind profiler network and other information prompted them to upgrade the threat to a moderate risk and highlight the chance of violent tornadoes in their outlook at 11:30 a.m. The first tornado watch was issued at 4:30 p.m. and the first tornado, a very brief, weak tornado, occurred just before 5 p.m.

Over the next 11 hours, with as many as four tornadoes from different storms on the ground at once, the NWS Norman Forecast Office issued 116 county warnings for tornadoes and severe thunderstorms. The average lead time for all the tornado warnings out of Norman was 18 minutes, which is seven minutes more than the national average of 11 minutes. For the significant tornadoes, those that were later rated as F2 or greater based on the damage they caused, the average lead time was 48 minutes. Some areas in the path of the most violent tornado received more than a one-hour notice before being hit.

NOAA storm researchers estimate that more than 600 people would have died in the absence of warnings.

The citizens of Oklahoma received an unprecedented level of severe storm warning services during the May 3 outbreak," said Dennis McCarthy, meteorologist-in-charge of the National Weather Service Norman Forecast Office. "The event clearly demonstrates the effectiveness of a fully integrated warning decision and dissemination system that includes NWS forecasters, live television and radio reports, emergency managers, storm spotters, ham radio operators."

NOAA Research developed the tools used to provide warnings: NEXRAD radar, Warning Decision Support System and Advanced Weather Interactive Processing System. Technology allowed forecasters to examine each and every storm, make their evaluation and issue warnings as necessary across the entire forecast area, not just the area around the Moore tornado, McCarthy said. Also, NOAA Research such as the VORTEX project has added to forecasters' understanding of storm structure and tornado development.

The tornado outbreak provided important safety lessons and reminders, McCarthy said. The very last step in the warning process – the actions people take to protect themselves once they hear the warnings – can often be the most critical. Many people in Oklahoma and Kansas survived the tornadoes with only minor injuries by seeking shelter underground, in an above ground engineered shelter such as a safe room, or in a permanent building on the lowest floor in an interior room such as a bathroom or closet.

However, some people took shelter in one of the worst places you can be during a tornado – under highway overpasses.

"Overpasses are not acceptable storm shelters during a tornado, and we encourage people to avoid this situation by paying attention to weather forecasts and planning ahead," McCarthy said.

Strong winds and flying debris caused three deaths and numerous injuries for people hiding under overpasses in Oklahoma on May 3. In addition, parked cars under one overpass blocked traffic, trapping people in the path of the violent tornado. Once the tornado passed, the blocked road also prevented emergency vehicles from gaining access to the affected areas, endangering more lives.

As homes are being rebuilt, many families are choosing to add a safe room or install an in-ground tornado shelter, McCarthy said. Taking action now may ensure their safety when future tornadoes come roaring through the Plains.