Residual moisture from a tropical weather system that drenched eastern Florida over the Memorial Day weekend could delay Wednesday’s highly anticipated launch of two NASA astronauts to the International Space Station aboard a SpaceX Falcon 9 rocket. If it is successful, the test flight would restore crewed spaceflight to the U.S. for the first time in nine years, and mark a milestone for a private sector space company.

The rocket launch, set for 4:33 p.m. ET, at the Kennedy Space Center in Cape Canaveral, Florida, will send astronauts Bob Behnken and Doug Hurley aboard SpaceX’s Dragon spacecraft to the Space Station. NASA has been relying on the Falcon 9 rocket to send supplies to the Space Station, but this will be a major test for the Commercial Crew program, through which the space agency is contracting with SpaceX and Boeing to resume crewed spaceflight from U.S. soil after years of hitching a ride aboard Russian rockets.

The tropical weather system unloaded over 7 inches of rain in the Miami area Sunday and Monday, flooding low-lying areas.

In Cape Canaveral, the system produced nearly two inches of rain on Monday and Monday night but weather radar showed much of the rain lifting off to the northeast on Tuesday morning.

By Wednesday, the tropical wave is predicted to be positioned near the Carolinas, but its counter-clockwise circulation could draw moisture back over the Florida Peninsula, allowing scattered thunderstorms to flare up and move toward the vicinity of the Space Coast around the 4:33 p.m. launch time.

SpaceX Demo-2 Preflight

A SpaceX Falcon 9 rocket with the company’s Crew Dragon spacecraft onboard is seen on the launch pad at Launch Complex 39A as preparations continue for the Demo-2 mission at NASA’s Kennedy Space Center in Cape Canaveral, Fla., Monday, May 25. Bill Ingalls/NASA via AP

“On launch day, residual moisture will still be present and mid-level steering flow will be westerly, meaning afternoon convection will travel eastward towards the Space Coast,” the Air Force’s 45th Weather Squadron, wrote in a statement Tuesday morning. “The primary concerns are flight through precipitation, as well as the anvil and cumulus cloud rules associated with the afternoon convection.”

The Weather Squadron, a unit of the Air Force’s 45th Space Wing and responsible for supporting missions at Kennedy Space Center, determined there is a 40% chance that weather conditions will prevent a rocket launch.

This is an improvement from its assessment on Sunday that stated there was a 60% chance of violating weather conditions.

Should storms erupt Wednesday forcing the launch to be scrubbed, SpaceX has scheduled backup launch windows on Saturday at 3:22 p.m. and Sunday at 3 p.m. Weather conditions are forecast to improve marginally with the probability of interference dropping to 30% by Saturday.

In its forecast, the Weather Squadron wrote that although Saturday’s weather will be similar to Wednesday, and the earlier liftoff time gives the launch “a fighting chance” before storms pop up over the peninsula and move toward the coast.

Even if it’s not raining at the launch site, the presence of billowing cumulus clouds or anvil clouds associated with the tops of thunderstorms can pose a danger. When rockets tear through them they can trigger a lightning strike, as happened during Apollo 12 when the Saturn V rocket was hit, causing damage to some non-essential components. The crew was still able to complete the mission to the moon. Cumulus clouds would also subject the rocket to strong updrafts and downdrafts, which could place added stress on the rocket and the Crew Dragon spacecraft.

In addition the the weather on Cape Canaveral, NASA and SpaceX are closely monitoring sea states along the East Coast in the unlikely event the capsule with the two astronauts aboard is forced to abort its launch due to an emergency and ends up in the ocean. While unlikely, high winds and waves could also force officials to scrub the launch.

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