Meteorologist Patrick Burke said the weather would be a challenge for Space X on Thursday morning, with only 20 percent of the chance being favorable for launch as severe thunderstorms and gusts are expected to reach southern and central Florida.
"It's almost certain that Cape Canaveral will have thunderstorms and strong winds tomorrow morning," Burke said. "A cold front bringing rain is moving off the coast."
Burke said it might not clear up until Saturday.
On Tuesday, due to technical problems, Space X stopped its planned launch a few minutes before launch, but later the company said the rocket and payload were "in good condition".
Earlier Wednesday, the launch was cancelled due to technical problems with the rocket.
For Musk, this successful launch will be a major victory. Over the years, Musk has been trying to enter the lucrative military space launch market.
Space X said it was the first so-called national security space mission defined by the U.S. military.
In 2016, Space X won an Air Force contract worth $83 million to launch a GPS III satellite that will last 15 years.
Lockheed spokesman Chip Eschenfelder said that under the contract, Lockheed will produce 32 satellites for the Air Force GPS III project, with a total value of $12.6 billion. Space X's launch on Thursday will be the first of them.
Air Force spokesman William Russell said: "Once fully operational, the latest generation of GPS satellites will bring new capabilities to users, including three times accuracy and eight times anti-jamming capability."
The U.S. Air Force said the launch was scheduled for 2014, but was hampered by production delays.
Eschenfelder said the next GPS III satellite would be launched in mid-2019, and subsequent satellites would be tested at the company's Colorado processing facility.