This story was updated at 7:40 p.m. EDT.

CAPE CANAVERAL, Fla. - NASA abandoned plans to launch the space shuttle Endeavour for the fourth time in a row on Sunday, this time because of stormy weather that crept too close to an emergency runway.

Endeavour was slated to blast off from a seaside launch pad here at NASA?s Kennedy Space Center at 7:13 p.m. EDT (2313 GMT) after a string of setbacks that have delayed the shuttle?s beleaguered mission by a month.

It was NASA?s second attempt in two days - and fourth try overall - to fly Endeavour after standing down on Saturday due to concerns over potential damage from recent lightning strikes near its launch pad. Endeavour is now slated to launch its crew of seven astronauts to the International Space Station Monday at 6:51 p.m. EDT (2251 GMT), with a 40 percent chance of good weather expected.

Although the weather was not a constraint for Endeavour's actual liftoff, thunderstorms were too close to the Shuttle Landing Facility here, which must remain a viable option in case of a last-minute emergency that forces the shuttle to abort its mission in mid-flight.

That scenario, known as a Return to Launch Site Abort, has never occurred in the 28 years NASA has flown shuttle missions. But a storm cell appeared within 20 miles of the landing site, breaking the flight safety requirement and forcing mission managers to call off today?s launch attempt.

"We?re going to have to declare a scrub today and try to bring the team back for another attempt for tomorrow," NASA launch director Pete Nickolenko told Endeavour?s crew.

"Copy that, we understand and we?ll be ready," shuttle commander Mark Polansky replied.

The weather outlook appears to be declining for the coming days, with dwindling prospects of good weather before July 14, Endeavour's last launch opportunity before it must stand down until July 27.

"It's going to get worse and worse each day," launch weather forecaster Scott McCormick told SPACE.com.

NASA cleared the shuttle to fly this morning after a thorough analysis found no damage incurred by Friday's lightning storm. But Florida's erratic weather cropped up to put a damper on launch hopes again today less than an hour before launch, when the astronauts had already been aboard the orbiter for more than two hours.

"The team's in really good shape and the crew is in good shape," Endeavour's mission management chair Mike Moses said. "It?s just not our day for the weather."

Earlier in June, a gas leak delayed two attempts to launch the shuttle, but that issue has since been repaired.

Endeavour?s STS-127 mission is a 16-day marathon of orbital construction bound or the International Space Station. The shuttle is carrying vital spare parts and a Japanese-built porch for the outpost?s massive Kibo laboratory. The spaceflight will also ferry rookie NASA astronaut Tim Kopra to the station to replace Japanese astronaut Koichi Wakata, who has lived aboard the orbiting lab since late March. Wakata is Japan's first long-duration resident of the station, where he has watched over his country?s $1 billion Kibo laboratory.

Set to ride into orbit aboard Endeavour with Polansky and Kopra are STS-127 pilot Doug Hurley and mission specialists Chris Cassidy, Julie Payette, Tom Marshburn and Dave Wolf. Payette represents the Canadian Space Agency, while the rest are NASA astronauts. Five spacewalks are planned.

Kopra is beginning a long-duration mission to the space station as a flight engineer on the outpost?s six-man Expedition 20 crew. He will join two Russians, another American and astronauts from Belgium and Canada on what is the station?s first full six-person crew.

NASA has until July 14 to launch the shuttle before standing down to allow avoid a traffic conflict with an unmanned Russian cargo ship also due at the space station this month. If the mission is unable to launch by then, NASA can try again on July 27.

SPACE.com is providing continuous coverage of STS-127 with reporter Clara Moskowitz at Cape Canaveral and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed. Live launch coverage will begin at 1:30 p.m. EDT (1730 GMT).