Blue skies are likely to frame every golf ball that flies high over Oakmont Country Club through the weekend, with low humidity blessing the U.S. Open and its spectators. But just so you know, it's a lot murkier behind the scenes, where local forecasters are looking at a lot of weather systems.

"If one variable changes, it could mess the whole thing up," said Pat Herald, a meteorologist at the National Weather Service. "That's what's difficult about this business."

Practice rounds begin today, leading up to the first round of tournament golf on Thursday morning.

Reacting to the information on his computer, Mr. Herald said the first half of the week "should be dry and fairly seasonable, comfortable." Seasonable is temperatures in the high 70s to low 80s, with nights dipping to the upper 50s. When asked for a seven-day forecast, Mr. Herald was hesitant because of the number of mathematical models that could affect the weather. He said "confidence is very low" for making a forecast that might not have to be changed.

"There are various levels of confidence, and they're all subjective," he said.

But he offered the leading probability of conditions for the championship rounds on Saturday and Sunday.

"The chance of rain at the end of the week is very small for this area, not higher than 30 percent," he said. "There are a lot of systems and they're all showing a variety of solutions."

In meteorologist lingo, "a variety of solutions" means a variety of forecasts is possible, depending on the effect one mathematical model of activity in the atmosphere has on another.