NASA's mission management team cleared the shuttle Atlantis for blastoff Friday on a space station assembly mission delayed three months by hail damage to the ship's external fuel tank. Even as the MMT assessed the shuttle's readiness to fly, thunderstorms rumbled toward the Kennedy Space Center and forecasters predicted possible half-inch hail and wind gusts higher than 50 knots at the Kennedy Space Center.

Engineers were scheduled to pump explosive liquid hydrogen and oxygen into on-board tanks starting at 6:30 p.m. to power Atlantis' electricity producing fuel cells. While stormy weather could cause delays, officials said enough "hold" time was available in the countdown to make up any reasonable amount of lost time.

The shuttle itself is protected at the launch pad by a large rotating gantry that fits around the vehicle, along with sliding panels that shield the orbiter's wings and belly from hail or other wind-borne debris. But the top and back side of the shuttle's external tank are exposed to the elements and the upper section of the tank suffered heavy damage during a freak hail storm Feb. 26.

Work to repair that damage delayed launch from March 15 to this week and while the forecast calls for a 70 percent chance of good weather at launch time Friday, shuttle weather officers predicted afternoon thunderstorms, possibly severe, today and Thursday.

"The top of the tank is exposed, we can't do anything about that," Launch Director Mike Leinbach said when asked about storms approaching this afternoon. "If we get any reports of hail out at the pad, we would do a complete walkdown inspection of the flight hardware and the ground support equipment to make sure we were in good shape.

"I hate predicting the weather myself," he said. "We're going to get what we're going to get and we'll deal with it. And this tank will fly perfectly fine whether we get a little bit more hail or not. We would have to be so unlucky to get more hail on this tank that... but I'd better not go any further than that!"

Amid laughter and nods at an afternoon news conference, MMT Chairman LeRoy Cain joked: "I'd be real happy if we just stopped talking about hail."

Weather aside, there are no technical problems at pad 39A - last used by shuttle Columbia for its final voyage in 2003 - and Cain said earlier concern about apparently out-of-spec hydrogen pressurization lines on a tank scheduled for use by the shuttle Endeavour in August had been resolved with no impact for Atlantis.