U.S. Open organisers were preparing for severe thunderstorms and high winds later in the day as the opening round of the year's second major began under overcast skies at Merion on Thursday.
Little-known American Cliff Kresge struck the first shot on the par-four opening hole and 2012 Masters winner Bubba Watson launched proceedings at the par-four 11th in high-profile company with Dustin Johnson and Belgian Nicolas Colsaerts.
Tournament favourite and world number one Tiger Woods was among the late starters, set to tee off with second-ranked Rory McIlroy and Masters champion Adam Scott on the first tee at 114 p.m. (1714 GMT) in the most eye-catching trio of the day.
However, whether or not Woods and company start on time remains to be seen with a derecho, a widespread windstorm that is associated with a fast-moving band of severe thunderstorms, expected to hit the Philadelphia area on Thursday afternoon.
As much as three inches of rain has been forecast for the day, and Merion Golf Club's iconic par-70 East Course has already been saturated with more than six inches since Friday.
Thankfully, the layout drains extremely well, but the biggest concern for organisers is the green and greenside bunker at the 11th hole, the lowest point on the course.
Extra care has been taken over the location of pin positions on greens more susceptible to puddles while the safety of players and spectators has been paramount in contingency plans.
Fans at Merion have several mouth-watering groupings to watch in the early morning, among them the all-American trio of Phil Mickelson, Steve Stricker and Keegan Bradley who will start at 711 a.m. (1111 GMT) at the first.
Next off on that hole are American world number four Matt Kuchar, fifth-ranked Englishman Justin Rose and sixth-ranked American Brandt Snedeker.
In other high-profile groupings, England's Luke Donald has been paired with fellow former world number ones Lee Westwood of England and Martin Kaymer of Germany while 2003 champion Jim Furyk will be joined by fellow American Zach Johnson and 2010 winner Graeme McDowell.