By Paul Rincon
BBC News science reporter, at Kennedy Space Center
Nasa still hopes to launch space shuttle Discovery on Wednesday, despite the storms buffeting Florida as Hurricane Dennis hits the mainland US.
Discovery is still set for a Wednesday launch
At a countdown status briefing, space agency officials re-stated their determination to fly Discovery on the first day of its July launch window.
There is said to be a 30% chance of bad weather stopping the launch on 13 July.
It will be the first shuttle launch since the Columbia disaster in 2003, which killed seven astronauts.
"Discovery is in excellent shape as we continue our preparations in anticipation of Wednesday's launch," Jeff Spaulding, Nasa test director told reporters here at the Kennedy Space Center (KSC) in Florida.
Yet more practice
The US space agency (Nasa) officially set the launch countdown clock ticking on Sunday at 1800 EDT (2300 BST; 2200 GMT) for a scheduled launch at 1551 EDT (2051 BST; 1951 GMT) on Wednesday.
"Our main threat will be thunderstorms," said Kathy Winters, the shuttle weather officer. "With that we will have a 30% chance of KSC weather prohibiting launch."
She added that if launch were to be delayed by 24 hours or 48 hours, the chances of cancelling launch due to high winds would rise to 40%.
DISCOVERY CREW - STS-114
Commander Eileen Collins
Pilot James Kelly
Mission Specialist Andy Thomas
MS Charles Camarda
MS Wendy Lawrence
MS Soichi Noguchi
MS Steve Robinson
Delaying Discovery's launch until after the weekend would do nothing to improve matters, Ms Winters explained, because hurricane centres expect other tropical storms to build over the next few days.
Discovery's seven-strong crew arrived at Cape Canaveral on Saturday evening - a day earlier than expected - because of Hurricane Dennis, which has blown in off the Gulf of Mexico.
Their time before launch on Wednesday will be spent carrying out simulations and, for Discovery's pilot James Kelly and Commander Eileen Collins, trying out difficult landing scenarios in a test aircraft.
While the astronauts are busy with their preparations, engineers will load Discovery with the reactants to provide power while the vehicle is in orbit, conduct communications checkouts and retract the launch pad¿s rotating service structure to ready the vehicle for blast-off.
The shuttle's external tank will be cryogenically loaded with about 500,000 US gallons (two million litres) of propellants on Wednesday at 0600 EDT (11 00 BST; 1000 GMT).
"We're excited to be where we are now,' said Scott Higginbotham, Discovery's payload manager.
"It's taken almost three years for us to put this hardware together, take it apart, put it back together, take it apart."
The STS-114, Return to Flight Mission is due to last 12 days, with a landing set for 25 July at 1100 EDT (1600 BST; 1500 GMT) on the landing strip at the Kennedy Space Center.
It will carry spare parts and other equipment to the International Space Station.