The crew of space shuttle Atlantis address the news media after arriving at the Kennedy Space Center on Monday, as lightning flashes across the sky in the background. Photo: NASA TV/Spaceflight Now

Countdown clocks at the Kennedy Space Center began ticking today toward the shuttle Atlantis's blastoff Friday on a mission to outfit and activate the international space station, paving the way for arrival of the lab's first full-time crew in November.

Atlantis's international seven-man crew flew to the Cape from Houston Monday evening, dodging thunderstorms and lightning to arrive around 8 p.m.

"You have no idea, unless you're looking at this storm, how great it is to be here on the ground at KSC. What a trip!" commander Terrence Wilcutt told reporters at the Shuttle Landing Facility. "We're ready to go. We've been training for about seven months and I don't think there's any stone that's been left unturned."

Added crewmate Edward Lu, one of two spacewalkers on the mission: "We're all excited about starting this. And it's not just us, it's all of NASA right now. Because as you all know, there are an awful lot of launches coming up in the next year and space station's really going to be taking shape and we're all looking forward to getting this off on the right foot."

The countdown to launch of the 99th shuttle mission began on time today at 11 a.m. If all goes well, the shuttle will lift off from pad 39B around 8:45:28 a.m. EDT (1245:28 GMT) Friday. The exact launch time -- and the exact duration of the launch window -- will not be determined until after Atlantis is fueled for flight.

The window will open at the moment Earth's rotation carries Atlantis into the plane of the international space station's orbit. To maximize the crew's chances of reaching the station even if one of Atlantis's main engines suffers degraded performance, the window will remain open for just two-and-a-half to roughly five minutes.

Space shuttle Atlantis stands ready for flight enclosed in gantries at launch complex 39B. Photo: NASA TV/Spaceflight Now

The exact duration will be computed based on the effects of winds aloft, the shuttle's fully fueled weight, the station's orbit and other technical factors. Ascent flight director Wayne Hale said he believes the window will end up extending between four and five minutes when all is said and done.

Forecasters, meanwhile, predict generally favorable weather Friday. But there is a 40 percent chance of coastal rain showers that could delay blastoff. The odds improve to 70 percent "go" on Saturday and 80 percent go on Sunday.

But NASA test director Jeff Spaulding said today NASA will only make two back-to-back launch attempts before standing down for 48 hours to top off on-board supplies of liquid oxygen and hydrogen for the shuttle's electricity producing fuel cells.

The only technical concern as the countdown began today was possible lightning damage to a TACAN landing navigation system at the shuttle runway. The extent of the damage is not yet known, but NASA managers are confident the system can be repaired in time to support a launch attempt Friday.

"We're working no significant issues at the pad, all the flight and ground systems appear to be in good shape and we are looking forward to launch on Friday morning," Spaulding said.

See Spaceflight Now's pre -launch timeline for details about Atlantis's countdown to launch.