The SpaceX Falcon 9 rocket sits at NASA’s Kennedy Space Center in Florida. (Bill Ingalls/NASA/Reuters)

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 successful, the test flight would restore crewed spaceflight to the United States after nine years and mark a milestone for a private space company.

The rocket launch, set for 4:33 p.m. Eastern time, at the Kennedy Space Center in Cape Canaveral, Fla., will send astronauts Bob Behnken and Doug Hurley aboard SpaceX’s Crew 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 seven 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 counterclockwise 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.

“On launch day, residual moisture will still be present and mid-level steering flow will be westerly, meaning afternoon convection will travel eastward toward 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 “rules” referenced were developed for launches to avoid natural and rocket-triggered lightning strikes, which are possible in the presence of tall cumulus clouds. Numerous “do not launch” rules are in place for various cloud, wind and precipitation scenarios.


Simulated weather radar at 4 p.m. Wednesday from HRRR model. (WeatherBell.com)

On Tuesday, 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 percent chance that weather conditions will violate at least one of the rules and prevent a rocket launch.

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

Captain Jason Fontenot, a meteorologist with the Weather Squadron and spacelift weather operations flight commander, said precipitation in the launch area is his biggest concern but that “inland storms could lead to a scrub if they came close enough.”

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 percent by Saturday.

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

Billowing cumulus clouds or anvil clouds associated with the tops of thunderstorms have a perilous spaceflight history. 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 nonessential components. The crew was able to complete the mission to the moon.

“We have to make sure we stay away from triggered or natural lightning events that come with those different types of clouds,” Fontenot said.

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 to 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 because of an emergency and ends up in the ocean. While unlikely, high winds and waves could also force officials to scrub the launch.

Fontenot said his forecasting team has access to “one of the densest suites of weather sensors anywhere in the world” for monitoring conditions. They include wind sensors, lightning detection systems, a surface electric field system, and radar systems.

The Weather Squadron will release one more public forecast Wednesday morning prior to launch and will then provide regular updates directly to SpaceX and NASA, some of which will likely be broadcast during the countdown process.

Christian Davenport and Andrew Freedman contributed to this report.