As of Thursday morning, the weather forecast for Cape Canaveral, Fla., and vicinity called for a chance of thunderstorms developing by launch time on Saturday afternoon.

The concern is upper-level winds that could buffet the shuttle during its liftoff.

"There is a 60-percent chance of (Kennedy Space Center) weather prohibiting launch," said Kathy Winters, a space shuttle weather officer, "which means there's a 40-percent chance that we could be 'go' for launch."

The other pre-launch complication facing NASA involves the sudden resignation earlier this week of Charlie Camarda, one of NASA's top shuttle engineers and a 30-year veteran of the space agency.

Camarda had been director of engineering at the Johnson Space Center in Houston, Texas, and had played a major role on the Mission Management Team that preparing for Discovery's launch.

News media reports alleged that Camarda had been involved in a serious disagreement with Wayne Hale, the NASA's space shuttle program manager, and with Administrator Michael Griffin, about certain safety issues concerning Discovery's large external fuel tank.

Some NASA engineers have been urging Griffin to delay the launch until additional corrections can be made to the system of insulating foam covering the tank.

At a recent briefing for reporters, however, Griffin stressed that he and NASA's senior leadership are confident that insulating foam issues do not pose a direct risk to the Discovery crew during launch.

He also said other NASA engineers have been urging a go-ahead with the shuttle mission - to deliver supplies, building components and a new crewmember to the International Space Station - so they can collect more data about the foam's behavior during the stresses of launch.

Meanwhile, engineers on Thursday removed the shuttle's mid-deck and flight-deck platforms as part of scheduled pre-launch activities. They also completed preparations to load the power reactant storage and distribution system, activated and tested the orbiter's navigation systems, and completed the flight deck preliminary inspections.

Other Thursday tasks included testing the vehicle's pyrotechnic initiator controllers, begin loading cryogenic reactants into Discovery's fuel-cell storage tanks, de-mating the orbiter's mid-body umbilical unit, and closing out certain ground-based equipment.

On Friday, launch personnel will make adjustments to the shuttle's three main engines to prepare for main propellant tanking, fill the launch pad's sound-suppression system water tank, and close out the tail service masts on the mobile launcher platform.

The STS-121 crew includes Lindsey, Pilot Mark Kelly and Mission Specialists Michael Fossum, Lisa Nowak, Stephanie Wilson, Piers Sellers and Thomas Reiter, an astronaut with the European Space Agency. Reiter will remain with the Expedition 13 crew on the station.

Liftoff of Discovery remains set for 3:49 p.m. Eastern Time on Saturday.

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