Unacceptable weather conditions over the launch pad and at the emergency runway at Kennedy Space Center forced NASA to scrub today's liftoff of space shuttle Endeavour. It was a dramatic end to an otherwise smooth countdown.

There was moisture found in clouds over the launch pad, violating the flight through precipitation rule that is in place to protect the shuttle's delicate thermal tiles, just as the countdown was scheduled to resume from the T-minus 9 minute hold. After a brief delay in resuming the countdown, clocks were restarted and taken to the T-minus 5 minute mark where they were stopped again.

While the flight through precipiation launch weather rule constraint was discussed, concerns were raised about rain and low clouds at the Shuttle Landing Facility that Endeavour would use in the event of an engine shutdown in the first minutes of flight. The weather at the runway had been announced "go" for launch, but officials retracted that during the debate on the flight through precipitation situation.

The launch pad weather is governed by the Air Force Range at Cape Canaveral, while the abort runway weather is forecast by meteorologists in Houston's Mission Control. Both groups give their "go/no go" for launch based on the specific areas they are responsible for.

In the end, the weather was simply too dynamic for launch today within the short five-minute window available.

Liftoff is rescheduled for 5:19 p.m. EST Wednesday. Meteorologists are calling for an 80 percent chance of acceptable conditions tomorrow, which is what the forecast predicted today. Showers and clouds again will be the concern.