6:30pm ET Wednesday Update: SpaceX took the countdown down to T-30 seconds, but at that time the company had to scrub the launch of its Falcon 9 rocket due to poor weather conditions. Inconveniently, a fairly large thunderstorm complex moved over the launch site just as the clock was counting down.

The company will now try to launch again on Thursday, at 6:01pm ET (22:01 UTC). However, the forecast for Thursday is not particularly great either.

Original post: The rocket is ready. The Dragon spacecraft is ready for its supply run to the International Space Station. But Mother Nature may not be ready for the launch of a Falcon 9 booster on Wednesday evening from Florida.

An unseasonably strong front has pushed into Florida, providing lift for atmospheric moisture and allowing for the development of widespread showers and thunderstorms. As a result, the official forecast for the 6:24pm ET (22:24 UTC) launch time—it is an instantaneous launch window, so the rocket must go at this moment—calls for just a 30% chance of "go" conditions. SpaceX will hope to get lucky and hit a gap in the storms this evening.

There is a backup launch date on Thursday at 6:01pm ET (22:01 UTC), but unfortunately the weather appears to be about the same. If the rocket cannot get off the pad Thursday, the launch will likely slip into early August due to docking and undocking schedules of other visiting vehicles at the International Space Station.

SpaceX last launched nearly a month ago, on June 25, with the third flight of its Falcon Heavy rocket carrying a ride share mission sponsored by the US Air Force. Now, from a different launch pad on the Florida coast, SpaceX will attempt to use its Falcon 9 rocket to launch a Dragon spacecraft bound for the International Space Station with 2.5 metric tons of supplies.

This will actually be the third flight of this Dragon, as this spacecraft has previously flown space station supply missions in April 2015 and December 2017. This is the first time a Dragon has flown three missions into space, but it won't be the last time. Both of SpaceX's upcoming supply missions for NASA, the company's 19th and 20th such flights, will also use twice-flown capsules.

The launch, when it occurs, will be followed by a first stage landing at the company's Landing Zone-1 site along the Florida coast. SpaceX has already flown this rocket once, for a space station supply mission in May, and the company hopes to do so again in December.

Provided the launch is not scrubbed earlier due to weather, the webcast below should begin about 15 minutes before the launch window opens.