By Mark Lamport-Stokes OAKMONT, Pennsylvania, June 16 (Reuters) - U.S. Open organisers were preparing for the likelihood of further rain and thunderstorms as the 116th edition of the year's second major got underway at the treacherous Oakmont Country Club on Thursday. Under clear skies, sectional qualifier Danny McCarthy struck the first shot on the par-four opening hole and fellow American Andres Gonzales began proceedings at the 10th to launch the ninth U.S. Open to be played on the iconic par-70 layout. More than an inch of rain had drenched the course overnight and there was an 80 percent chance of thunderstorms later on Thursday before conditions are expected to improve over the next three days. More...