The weather forecast for the launch of space shuttle Atlantis worsened slightly Friday, with storms expected in the area hours before Sunday's launch.

There was a 40 percent chance that weather at the Kennedy Space Center would prohibit a lift off at 4:30 p.m. (2030 GMT) Sunday, NASA's first launch opportunity, said Kathy Winters, shuttle weather officer.

Area storms likely will push westward before liftoff, Winters said. But NASA will not launch if there are storms within 23 miles of the shuttle landing runway, in case astronauts need to make an emergency landing.

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"We're probably go red through the countdown," said Winters, referring to a violation.

Stormy weather and a lightning strike at the launch pad forced technicians Friday to delay fueling the system that will power the space shuttle during the mission. The interruption was not expected to affect the countdown since the fueling had started earlier than planned. The lightning struck a wire on a tower used as lightning protection and did not appear to cause damage, said launch director Mike Leinbach.

"It's not unexpected that we get a lightning strike at the pad this time of the year," Leinbach said.

Forecasters were also watching Tropical Storm Ernesto, although it was not expected to affect Sunday's launch. Ernesto was projected to enter the Gulf of Mexico on Tuesday or Wednesday.

If Ernesto struck Houston and caused workers at Mission Control to evacuate, the Atlantis mission likely would be aborted midway, and the shuttle and its astronauts would return to Florida, NASA manager said.

Last September, during Hurricane Rita, Johnson Space Center in Houston was locked down, the power turned off and monitoring duties for the international space station were turned over to Russian flight controllers outside Moscow. There was no space shuttle mission going on at the time. The same thing happened in 2002 when another approaching storm threatened the space center.

During their 11-day mission, Atlantis' six astronauts will restart building the international space station before the cargo-carrying shuttles are retired in 2010. Construction has been delayed since the 2003 Columbia explosion, which killed seven astronauts.

Atlantis will carry a 17.5-ton addition for the space station, costing $372 million, from which two solar wings will be opened up. The solar arrays eventually will provide a quarter of the space station's power when it is finished.

NASA wasn't tracking any major technical issues with Atlantis, said Pete Nickolenko, a NASA test director.

"All our systems are in great shape," he said. "The countdown is right on schedule."