(CNN) -- The first crewed spaceflight to take off from the US in nearly a decade is scheduled to launch on Saturday.
The latest weather forecast puts the odds of a go or no-go decision at 50/50 -- just as it was on Wednesday, when SpaceX and NASA's first attempt at a launch was scrubbed.
"Saturday and Sunday could turn out to have very similar weather as Wednesday did," said CNN meteorologist Haley Brink, referring to the postponed launch. "We may be waiting on a game-time decision again this weekend."
The next launch opportunity is Saturday at 3:22 p.m. ET, with a backup window of 3 p.m. ET on Sunday.
Scrubbing a launch due to weather is not uncommon.
"Scrubs are part of conducting spaceflight safely and successfully. During my last mission to @Space_Station weather caught us too!" tweeted Bob Behnken, one of the two astronauts who will be aboard SpaceX's Crew Dragon Capsule.
The primary concern is that rain or even thunderstorms could form near the launch site at NASA's Kennedy Space Center in Brevard County, Florida, according to the 45th Space Wing Weather Squadron. The squadron, based at nearby Patrick Air Force Base, provides the weather assessments for launches at the space center. Flight through precipitation, anvil clouds and waves along the flight path will also need to be watched according to NASA.
Experts are wary of lightning
Lightning is one of the biggest concerns for this weekend's planned launch.
Pat Hyland, a research associate for the University of Oklahoma's Cooperative Institute for Mesoscale Meteorological Studies (CIMMS) supporting the NOAA National Severe Storms Laboratory (NSSL), spent several years researching this very topic. During his undergraduate and graduate research, which was funded in part by a NASA EPSCoR grant, he focused on investigating the electric field mill (EFM) network at Kennedy Space Center to determine the false-alarm rate, and to assist with NASA's Lightning Launch Commit Criteria.
"The motivation for the research was in part because studies showed that more than half of lightning casualties resulted from the first or one of the first few cloud-to-ground (CG) flashes in a storm and that significant numbers of casualties resulted from returning to outdoor activities too soon, before lightning had actually ceased," Hyland said. "We were looking for potential patterns in what was happening with the electric field at the surface to hopefully provide guidance for lightning hazard-warning-decision situations, such as what occurs at Kennedy Space Center with their launches."
They also tried to look for patterns in the atmosphere during their rocket-triggered lightning experiments.
"This is yet another reason why, even in the absence of lightning directly around the site, launches can sometimes be scrubbed because rockets can artificially trigger lightning," Hyland said.
For example, lightning in the area will cancel a flight, as will a cloud with a large enough electrical field to produce rocket-triggered lightning. This happens when a giant spark of electricity occurs as a large rocket flies through a strong enough atmospheric electric field.
The electric field needed to induce rocket-triggered lightning is much lower than for natural lightning.
Other hazards could also scrub the launch
Forecasters also have to monitor the winds, and weather downrange of the launchpad.
If there is a sustained wind of 30 mph or more at 162 feet above the launch pad, the mission will be scrubbed.
It's not just the weather at the site of the launch pad. The Weather Squadron also has to monitor the weather downrange because if the Crew Dragon Capsule encounters a problem, it needs to have a safe splashdown location.
A whole team of meteorologists is on hand from the 45th Space Wing and SpaceX to determine if the weather will scrub the launch. As on Wednesday, the decision can be made right up to liftoff.
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