4:30 p.m. update: Storms and lightning in the proximity of the Kennedy Space Center prevented the launch of the SpaceX Falcon 9 rocket. The next launch attempt is set for Saturday at 3:22 p.m. Due to the forecast for possible showers and storms, weather could again pose challenges.

Three weather rules were violated, which prevented the launch, including the presence of natural lightning and an attached anvil, a towering thunderstorm top that can generate an electric field and trigger lightning when in contact with a rocket’s plume.

The weather showed signs of clearing after the no-go decision was made, but the launch could not wait, due to specific timing requirements necessary for reaching the International Space Station.

Original post

American astronauts will launch into space from U.S. soil for the first time in nearly a decade Wednesday. That is, if weather permits. The chance of thunderstorms that could scrub the planned launch of NASA astronauts Bob Behnken and Doug Hurley to the International Space Station has increased since Tuesday, from 40 to 60 percent.

The predicted storms are expected to be hit or miss. If the mission aboard the SpaceX Falcon 9 rocket can proceed, it will mark a milestone for the private space industry while restoring confidence in the nation’s ability to conduct crewed space flight aboard American-made rockets.

NASA described Wednesday as “not only a big day for our teams — it’s a big day for our country,” in a tweet.

The rocket launch, set for 4:33 p.m. Eastern time, at the Kennedy Space Center in Cape Canaveral, Fla., will send Behnken and Hurley aboard SpaceX’s Crew Dragon spacecraft to the space station. NASA has been relying on the Falcon 9 rocket, which will carry the Crew Dragon, 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 space flight from U.S. soil after years of hitching a ride aboard Russian rockets.

The latest weather updates (last updated at 4:10 p.m.)

A report from a NASA television host indicated that air and sea space around the launch site was clear for launch. However, one weather condition necessary for launch related to “lightning energy dissipation” was being violated.

Weather had been “trending in the right direction” between 3:30 and 3:50 p.m., but just after 4 p.m. it began to trend in the wrong direction, NASA television reported.

A final go-no go decision was expected at 4:13 p.m., 20 minutes before launch.

Weather radar showed a line of showers and thunderstorms beginning to exit the Space Coast from the west. A special marine warning had previously been in effect (until 3:30 p.m.) for coastal waters for heavy showers and storms with wind gusts potentially topping 39 mph. A tornado warning was even in effect until 2:15 p.m. in north central Brevard County, about 20 miles north of Cape Canaveral.

A few pop-up showers and storms trailing this line lingered around the Space Coast and to the west, but radar showed large gaps in precipitation coverage, which could provide an opening for the 4:33 p.m. launch. Still, the presence or proximity of tall cumulus clouds or anvils could potentially pose a problem.

Background on the forecast

The Air Force’s 45th Weather Squadron, responsible for supporting missions at Kennedy Space Center, initially determined there was a 50-percent chance weather would prevent Wednesday’s launch. NASA television announced that probability had increased to 60 percent just after 2 p.m. Wednesday.

“Residual moisture with the passing low pressure system and increased low-level convergence will threaten the Space Coast with showers and thunderstorms this afternoon,” it wrote in its 10 a.m. update.

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

The launch will be aborted if any of a dozen weather rules are violated for various cloud, wind and precipitation scenarios. The majority of the rules relate to the presence of thunderstorms or thunderstorm clouds within 10 nautical miles of the launch site and flight path. These rules were designed to protect against lightning strikes to the in-flight rocket, triggered by the rocket’s interaction with storm clouds or unleashed by the storm itself.

Should storms erupt Wednesday, forcing the launch to be scrubbed, SpaceX has scheduled backup launch windows Saturday at 3:22 p.m. and Sunday at 3 p.m. The weather is forecast to improve marginally over the weekend, with the chance of weather interfering with the launch dropping to 40 percent, according to the Weather Squadron.

Past instances of rockets launching into stormy conditions have led to close calls and disaster. During the Apollo 12 launch, the Saturn V rocket was struck by lightning twice, causing damage to some nonessential components. Nevertheless, the crew was able to complete the mission to the moon.

In 1987, the unmanned Atlas/Centaur-67 rocket was launched into rainy, overcast skies when lightning struck, sending it “tumbling out of control,” according to Reuters. Safety officers were forced to divert and destroy the rocket so its debris would not land in civilian areas.

Atmospheric scientists have learned that rockets generate an electrically conductive exhaust plume that can trigger lightning when passing through a preexisting electric field — usually from clouds containing both water and ice. These are typically tall cumulus clouds or the anvils (or tops) of thunderstorm clouds.

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

He 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.

Through the scheduled launch Wednesday afternoon, the Weather Squadron will relay regular updates directly to SpaceX and NASA, some of which will likely be broadcast during the countdown process.

In addition to the weather on Cape Canaveral, sea states along the East Coast and across the Atlantic toward Ireland are being monitored 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.

Because of Tropical Storm Bertha, some waves off the Southeast coast could reach up to six feet, with wind gusts up to 20 to 30 mph. However, the storm will have moved inland, and its biggest waves will have subsided by the late afternoon launch window. Waves farther north and east along the flight path are predicted to be closer to average, mostly in the range of three to five feet, although an area of higher waves over six feet is possible over the north central Atlantic.

Although high winds and waves along the flight path could force officials to scrub the launch, a NASA television report early Wednesday afternoon stated this was unlikely.

Christian Davenport and Andrew Freedman contributed to this report.