Saturday marked the first day this week that weather did not cause play to be suspended.
Nearly 50 people were treated for heat-related illnesses Saturday, but none were in serious condition, officials said. Most required only some shade and water to be back out on the course.
"This (the weather) has been a very big challenge this week," said Kathy Gordon, a USGA official who has worked 11 U.S. Opens.
Gordon said the weather can best be compared the weather to a bogie rather than a birdie.
"This is the first time we've ever, even in practice rounds, have gone and suspended play every day," she said.
When dark skies roll in, it's up to a on-site meteorologist to make the call whether it is safe to play.
"He (the meteorologist) is in an undisclosed location where he has all kinds of radar and weather devices," said Gordon. "He also has a lightning detection device that he looks at."
USGA officials follow a rule of thumb that if lightning is within 15 miles of a golf course, play must be stopped. Decisions also take into account the distance and severity of storms, said officials.
"We've needed the rain in the area, so we're trying to take a happy spin on it," said Gordon.