Weather threatens U.S. astronauts' SpaceX launch from Florida

The projected liftoff has a 60 percent of violating weather constraints because of a thick cloud cover and the likelihood that their SpaceX Falcon 9 rocket would fly through rain, Air Force meteorologists at nearly Patrick Air Force Base said Sunday.

"On launch day, remnant moisture" from a tropical wave will remain in the area, according to the Launch Mission Execution Forecast. "The primary launch weather concerns remain flight through precipitation, the thick cloud layer rule and the cumulus cloud rule associated with the remnant tropical moisture and proximity of [a] developing low."

A launch cannot occur if precipitation is occurring at the launch pad or within the flight path. Similarly a launch generally cannot occur if any part of the planned flight path is through a layer of clouds within 5 nautical miles and is 4,500 feet thick or thicker. Other parameters have to be met, as well.

If the launch is scrubbed, NASA has said, the next attempt to send the Crew Dragon capsule to the International Space Station would come Saturday. Both President Donald Trump and Vice President Mike Pence have announced they would attend the launch Wednesday, and it was not clear whether a postponement would change those plans to attend.

Despite the dire weather forecast, astronauts Doug Hurley and Bob Behnken -- and the mission team -- participated in a "dry dress' rehearsal Saturday in which they donned their black and white spacesuits and made a 20-minute drive in a Tesla Model X to Launch Complex 39A.

Elon Musk, who operates SpaceX, also is the chief executive officer of Tesla.

Hurley and Behnken then took a service tower elevator to the spacecraft access arm gantry and climbed into the capsule. They checked out communication systems, and the hatch was closed. They then went through a run-through with all launch personnel.

According to the space agency, "the rehearsal concluded with the go/no-go poll for Falcon 9 propellant loading, which normally occurs 45 minutes before launch."

A day before the rehearsal, engineers successfully fired the rocket's nine Merlin first-stage engines for seven seconds in what NASA describes as a "critical but routine test."

The liftoff, should it occur Wednesday, is planned for 4:33 p.m. EDT. The Dragon capsule would dock with the International Space Station on Thursday at 11:29 a.m. EDT.