Rain begins moving away from Florida today as seen on local radar. Photo: NASA

The shuttle Discovery's countdown to launch on the 100th shuttle mission continues to tick smoothly toward blastoff Thursday night at 9:38:26 p.m. There are no technical problems at pad 39A and the only concern remains the possibility of showers that could cause a delay.

A broad mass of tropical air has soaked the Kennedy Space Center with torrential downpours over the past two days.

But John Weems, a shuttle weather officer with the 45th Space Wing at the Cape Canaveral Air Force Station, is sticking with a 60 percent "go" forecast based on computer models that indicate slightly improving conditions Thursday.

"All the models are in agreement now," he said at today's countdown status briefing. "The clouds are going to start to pull out of the area so now we're worried about where this moisture is really focused.

"I think as this low moves off from the coast, conditions look like they'll be more acceptable than they will be unacceptable."

To reach the international space station, Discovery must take off within five minutes of the moment Earth's rotation carries the launch pad into the plane of the target's orbit. That theoretical window will open at 9:33:27 p.m. Thursday and close at 9:43:26 p.m.

But NASA managers recently decided to target all station assembly missions for launch at the precise moment the pad is in the station's orbital plane. For Discovery's flight, that works out to 9:38:26 p.m.

This "launch-in-plane" strategy minimizes the amount of propellant required and improves the odds of reaching an East Coast runway in the event of a main engine failure early in flight. It also improves the odds of reaching the station if an engine fails to generate full power.

The duration of this "preferred" launch window cannot be computed until the shuttle is fueled for flight. But it will last somewhere between two-and-a-half and five minutes.

The visible satellite image shows the clouds associated with a low pressure system now moving away from the state. Photo: NASA

While NASA managers could elect to launch Discovery Thursday at the beginning of its theoretical 10-minute window to provide a bit more time for bad weather to improve, Weems said the nature of the tropical system affecting Florida makes that an unlikely scenario.

While the weather is expected to change Thursday, it will change slowly and an additional few minutes likely would not make much difference.

Should launch slip to Friday, a different weather system will come into play. A strong cold front is sweeping toward Florida that should reach the area sometime Saturday. Thunderstorms are expected ahead of the front with higher-than-allowable winds behind it.

The forecast for Friday calls for a 60 percent chance of acceptable weather while Saturday's outlook calls for a 60 percent chance of "no-go" conditions, primarily due to high winds.

"For the launch attempt tomorrow, we'll still have a lot of mid-level and low-level type moisture and the possibility of occasional showers," Weems said. "Should we be delayed into Friday, we have the increased chance of thunderstorms forming in advance of that front.

"So we would have a concern over thunderstorms moving from the northwest back into our area. And that would linger right into Saturday. It really depends on the time the front comes through us on Saturday as to what would be the concerns for a Saturday launch."

If the front is just to the north of the launch site as forecasters now expect, "then you would have the increased concern on Saturday for thunderstorms," Weems said. "If the front pushes through us, we would have a concern over some increased wind speeds behind the front."