Chris O'Meara Associated Press

A line of thunderstorms moves over Kennedy Space Center in Cape Canaveral, Fla., on Sunday, when NASA delayed Endeavour's liftoff.

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Afternoon thunderstorms thwarted NASA's third launch attempt of space shuttle Endeavour on Sunday, just minutes before the final “go” decision.

If the orbiter doesn't blast off today as scheduled, the flight is on hold for two more weeks.

Last month, hydrogen leaks on a 160-foot external tank forced NASA to scrub two earlier launch attempts. Saturday's launch was canceled before NASA fueled the orbiter, so it didn't reach the status of a launch attempt.

On Sunday, all systems appeared to support a launch until early evening thunderstorms rolled near the shuttle landing site at Kennedy Space Center in Florida.

“Weather unfortunately did not cooperate with us today. We had some colliding sea breezes,” said Pete Nickolenko, launch director at the Kennedy Space Center.

Liftoff set for 5:51 p.m.

NASA protocol requires a “no-go” for a launch if weather is unacceptable at the launch or landing site. If one or more engines shut down early, siphoning necessary energy to reach alternate landing sites in Spain and France, the orbiter must be able to swing back to land at Kennedy. So if bad weather shrouds the landing site, NASA calls it off.

The launch is now scheduled for 5:51 p.m. CDT today. If unsuccessful, NASA will have to hold off until July 27 because a Russian supply ship is slated to travel to the international space station in the interim.

Endeavour's six-man, one-woman crew heard the news while suited up and strapped in aboard the orbiter.

Endeavour Commander Mark Polansky, 53, and mission specialist Tom Marshburn, 49, smiled and waved at a NASA camera as they boarded the shuttle.

The last of the crew to board, flight engineer Julie Payette, 45, of the Canadian Space Agency, smiled, said in sign language “I love you all” and then blew a kiss to her family. She is married with two children.

Saturday's launch was scrubbed before NASA approved fueling the shuttle, when the mission management team wanted to evaluate possible effects of lightning strikes near the launch pad on Friday.

Expecting a large crowd

Endeavour is prepared for a 16-day flight in which the crew is slated to deliver a “porch” for the international space station's $1 billion Japanese Kibo complex. The word Kibo means “hope” in Japanese.

The third and last piece to be added to the complex will hold science experiments exposed to the harsh environment of space.

The crew will conduct five spacewalks to install the equipment, set up storage and conduct various maintenance duties.

In addition to Polansky, Marshburn and Payette, Endeavour's crew members are: pilot Doug Hurley, 42; mission specialists Christopher Cassidy, 39, and Dave Wolf, 52; and flight engineer Tim Kopra, 46.

They will join six men already aboard the station, creating the largest crowd ever aboard the installation at one time. Kopra will replace Japanese astronaut Koichi Wakata aboard the station, and Wakata will return to Earth with the rest of Endeavour's crew at the close of the mission.