By Jonathan Amos BBC science correspondent, Florida 
It will be the 33rd and final launch of Atlantis The countdown to Friday's final launch of a space shuttle has begun at the Kennedy Space Center (KSC).But the famous clock in front of the spaceport's press centre may struggle to get all the way down to zero.
Forecasters say showers and even storms will be over Florida's Space Coast by the week's end.
When it does fly, the Atlantis orbiter will be hauling more than 3.5 tonnes (8,000lb) of supplies to the International Space Station (ISS).
"I wish I had a better briefing for you," Nasa's shuttle weather officer Kathy Winters told reporters.
"Right now we are going with a 60% chance of KSC weather prohibiting launch due to the potential for showers and isolated thunderstorms in the area."
Huge crowds are expected on the Space Coast to watch the 1126 local time (1526 GMT) lift-off.
The forecast is going to make for some tough decisions on their part. Do they endure the jammed roads and long queues on Friday to get into the best viewing positions, only for the launch to be postponed? Or do they stay away, hoping for a 24-hour delay and much better weather prospects?
Saturday's forecast is better (40% chance of no-go weather conditions); Sunday's even more so (30%)
Winters said early morning conditions on Friday might be fine, but those showers and storms were likely to pop up just around launch time.
The crew arrived at Kennedy on Monday Nasa will continue with its preparations in any case.
The start of the countdown triggers engineers to start working through a pre-planned list of final checks on the shuttle and its associated ground systems.
Nasa Test Director Jeremy Graeber said the team would get on with the job efficiently and would not be distracted by all the emotion swirling around them.
"The team gets into the mode of 'this is launch countdown'. And that's really the focus that everybody has, and the rest of the emotion that comes with the end of the space shuttle programme, I think will kind of roll in when launch is completed and we get past that."
If all goes according to plan, the order to start loading nearly two million litres of liquid hydrogen and liquid oxygen into the shuttle's external tank will go out at around 0200 local time on Friday morning. The crew will then suit up and make the drive down to the launch pad, leaving their quarters just after 0730.
Just four individuals will be flying on the mission. Commander Chris Ferguson, Pilot Doug Hurley, and Mission Specialists Sandy Magnus and Rex Walheim have a busy 12 days in space ahead of them.
Their orbiter has been crammed full of stores, including a year's worth of food for the ISS.
It should give Nasa some room to play with if the new commercial re-supply freighters that are due to replace the shuttle encounter problems with their development.
Friday's ascent will be the 135th and last shuttle launch. It will be the 33rd lift-off for Atlantis.
On its return from orbit, the vehicle will be prepared for public display at the nearby Kennedy Space Center visitor complex.
The other two reusable spaceplanes in Nasa's fleet have already made their swansongs. Discovery completed its last mission on 9 March; Endeavour came home to retirement on 1 June.
It will be some years before the agency can use American systems again to launch astronauts. Private US companies are developing a range of vehicles but they are not expected to enter service until at least the middle of the decade.
Nasa itself intends to divert resources from the shuttle programme into designing a deep-space capsule that could visit the Moon, asteroids or even Mars.
Jonathan.Amos-INTERNET@bbc.co.uk
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