NASA is preparing shuttle Endeavor for a June 12 launch to ISS on a mission that will complete construction of Japan's Kibo laboratory. The mission has a three-day launch window; if it doesn't go in that window, the mission must be delayed until next month.
As it has been so many times, Florida's weather is the wild card. NASA is watching possible storms closely. Getting a system as complex as the shuttle ready to launch is difficult enough. Doing that while keeping one eye on the weather has degraded the performance of the program. Of course, one time NASA seemingly risked weather conditions being good enough was in January, 1986, when Challenger was lost. The cold that morning was at least one factor in the tragedy.
Conceiving humanity really opening space until we have a launch system robust enough to operate safely in most weather conditions is tough. The shuttle, early in its design phase, was supposed to do that, but the shuttle that was actually built never came close to such capability. In fact, such a system may still be decades away.