Nasa is entering the final stages of preparation for its last ever shuttle mission, but it is keeping a very careful eye on the Florida sky.
The weather over the Space Coast was atrocious on Thursday, with torrential rain falling on the Atlantis orbiter on its pad.
A huge lightning bolt hit the ground close by at one point.
Weather forecasters say conditions do not look good for Atlantis getting away on time at 1126 (1526 GMT) on Friday.
"For launch our main concern is still having showers and thunderstorms in the area, so with that we still have a 70% chance of KSC weather prohibiting launch," said shuttle meteorologist Kathy Winters.
The assessment will be a worry for the hundreds of thousands of people who have been hoping to go down to the Cape Canaveral area to watch the ascent.
Their journey could be wasted. Nasa launch managers are mindful of the impact the big crowds could have on their workers' ability to do their jobs.
If they call off Friday's launch close to lift-off, it is likely another attempt will not be made until Sunday.
"If we get into a scenario where we scrub very late in the count, in order to provide crew rest we may elect to go 48 hours to give our teams time to get home and back to work," said Jeff Spaulding, Nasa test director.
"With the amount of [spectators] we are expecting - upwards of half-a-million to three-quarters-of-a-million folks in the general area - getting home is going to be very challenging."
Four astronauts will ride Atlantis to orbit - Commander Chris Ferguson, Pilot Doug Hurley, and Mission Specialists Sandy Magnus and Rex Walheim.
The primary task of their 12-day sortie is to deliver more than 3.5 tonnes (8,000lb) of supplies to the International Space Station.
A third of the load is food and it will ensure the platform has a year's worth of consumables aboard to sustain its residents.
Friday's ascent would be the 135th and last in the 30-year orbiter programme.
Nasa is committed to retiring its shuttles because the vehicles are too costly to maintain.
The agency believes a more affordable approach to getting astronauts to the ISS can be achieved by contracting out their transport to private companies.
The first of these commercial carriers is expected to enter service sometime in the middle of the decade.