Players practicing Monday morning had to leave the course before 7:00 a.m. when lightning and heavy rain rolled through.
Tiger Woods, a three-time winner of the U.S. Open, was among those chased off the course by the weather.
Woods and others returned when play resumed around 11:00 a.m.
The world's number one golfer practiced on the Merion course twice in the past week, and says it's a challenging course.
That was without the rain.
The 17th fairway at Merion, while beautiful to look, was too soggy to use most of the morning.
This was a day for umbrellas as the persistent precipitation produced mud in some places.
But would-be spectators of today's practice rounds shrugged off the bad weather.
"I'm just excited to be here. Not disappointed. It's golf. It comes and goes, got to roll with the punches. It's weather," Danelle Ohman of Chalfont, Pennsylvania said.
A high-tech tent city near the course was a great spot for a rain delay.
It houses exhibits on all things golf, from putting greens to the science of how a golf course handles heavy rain.
"Part of the engineering and the agronomy behind a golf course is having enough sand and enough drainage for the turf. It will run through. There's a fine line between being able to hold enough water and allowing enough water to run through," John Murray of the Chevron Corporation said.
So as the practicing continues, eyes continue to look upward as clouds loom over Ardmore.
With the practice rounds for the U.S. Open now under way, traffic around the course is starting to back up.
In Haverford Township, Haverford Road from Ardmore Avenue to College Avenue will be closed for the week of the Open.
Drivers should prepare for delays in that area throughout the week and into the weekend.