Dark clouds hang over shuttle Discovery at pad 39A today. Photo: NASA TV/Spaceflight Now

With the shuttle Discovery's countdown underway, forecasters are predicting marginally acceptable weather for launch Thursday night at 9:38:26 p.m. There are no technical problems of any significance at pad 39A and NASA test director Steve Altemus said today weather is the only concern as the agency gears up for its 100th shuttle mission.

"The launch team is not working any issues in the count at this point," Altemus said. "The flight and ground systems are performing well and we're all looking forward to putting another piece of the international space station into orbit Thursday night."

Weather permitting, of course. Discovery's launch window will last just two and a half to five minutes - the exact duration will not be computed until Thursday afternoon - giving NASA no time to wait for conditions to improve.

A large area of tropical moisture is moving north across Florida today, soaking the space center with heavy rain. While forecasters expect the clouds to begin thinning out by Thursday afternoon, there is still a 40 percent chance of showers and thick cloud cover at launch time that could force a delay.

The outlook is similar for Friday evening when thunderstorms ahead of an approaching cold front are expected to pound the Kennedy Space Center area. Conditions are expected to be worse Saturday. While the cold front will dry out the air over central Florida, it is expected to produce higher-than-allowable crosswinds at the spaceport's emergency runway. Conditions are 60 percent "no go" Saturday.

"If I had to pick, of the three days I think that the most likely would be Thursday," said John Weems, a shuttle weather officer with the 45th Space Wing at the Cape Canaveral Air Force Station.

But NASA's launch strategy calls for making two back-to-back launch attempts Thursday and Friday, if necessary, and then to stand down a day to give the launch team time to rest. A third launch try would be scheduled for Sunday.

Along with monitoring Florida's weather, NASA managers also are monitoring the progress of Tropical Storm Keith as it heads into the Gulf of Mexico. The storm is now predicted move westward toward the eastern coast of Mexico. Forecasters do not expect the storm to disrupt mission control operations at the Johnson Space Center south of Houston.

The weather will be assessed later today at a standard launch-minus-two-day management review at the Kennedy Space Center. The launch team could be directed to delay or re-order some countdown events to give forecasters more time to monitor Keith's actual progress. But as of this writing, that would not appear to be necessary.

Assuming no changes are ordered, the countdown will resume at 8 p.m. this evening at the T-minus 27-hour mark after a four-hour built-in hold with the loading of liquid hydrogen and oxygen to power the shuttle's electrical generators. The shuttle's aft engine compartment has been closed for flight as have the ship's two 60-foot-long payload bay doors.

"We're clean and green," said launch manager Bill Dowdell.