U.S.G.A. Is Bracing for More Rain at Merion Charlie Riedel/Associated Press Spectators watched golfers on the 17th green during practice Wednesday for the United States Open. ARDMORE, Pa. - United States Golf Association officials lavished praise on that grande dame Merion Golf Club at a news conference on the eve of the United States Open, describing the host course as "a classic beauty" worth celebrating. But in the end, Mother Nature entered the picture, as she has done all year on the PGA Tour. With thunderstorms looming in the forecast and a flood watch in effect in the area starting Thursday morning, weather dominated the conversation Wednesday. Mike Davis, the U.S.G.A. executive director, said Merion could receive anywhere from a quarter of an inch to three inches of rain Thursday afternoon, wreaking havoc with the late starters in the first round. That includes the marquee grouping of Tiger Woods, Rory McIlroy and Adam Scott, which is scheduled to tee off at 1:14 p.m. On Golf Channel on Wednesday, the analyst Frank Nobilo described the forecast as "a little like Armageddon." The course could be rocked by the kind of violent thunderstorms that hit during last June's AT&T National at Congressional Country Club and led to a third round without spectators. "It depends on really what hits us or how lucky or unlucky we are," Davis said. A rainstorm Monday prompted the closing of the 11th hole, which is at the lowest point on the East Course. Davis said the area had been hit by more than six inches of rain in the past four days. Thursday's forecast heightened participants" concerns that mud balls could be what separates the winner from the field because the U.S.G.A. is averse to adopting the lift-clean-and-place rule in its national championships. "We try to go through these scenarios, worst case, how do you handle it," Davis said. A lot of times you just have to see what you're dealing with in terms of how much play do you have left, is it dangerous to spectators, can we get spectators here. So there's so many things that play in.