CAPE CANAVERAL, Fla. -- Stormy skies remain the top concern today as NASA attempts to launch shuttle Endeavour on a mission to deliver new hardware and a fresh, long-term crew to the International Space Station.

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With restricted air space, offshore security zones and increased ground patrols once again in place at the Kennedy Space Center, the space agency can do little but watch and wait to see what Florida's finicky weather has in store during today's terminal countdown.

"The ship's in fine shape. The crew's in great shape. We may end up dodging a few thunderstorms," said Ron Dittemore, NASA's shuttle program director, "but from my perspective it looks great and we're ready to proceed."

Liftoff is set for 7:44 p.m. EDT (2344 GMT), NASA officials said Wednesday.

The exact launch time was kept secret until the day before blast off as part of new security procedures NASA adopted in the wake of the Sept. 11, 2001 terrorist attacks.

Continuing the civilian agency's more restrictive posture in releasing information to the public, NASA TV will not begin live coverage of the events at Launch Complex 39 until 4:30 p.m. EDT (2030 GMT), after the seven-member crew has arrived at the pad and begun boarding Endeavour.

Air Force weather officer Kathy Winters said there is a 40 percent chance of acceptable conditions during the brief, 10-minute launch window. The main concern will be the presence of clouds leftover from afternoon thunderstorms expected in the area.

Storms are expected to form inland over Florida during the day and then gradually move to the east, toward the space center, and dissipate as the afternoon wears on, Winters said.

The problem is likely to be with any high level clouds that detach from an anvil-shaped cell and then drift over the Cape. These so-called thunderstorm debris clouds can remain electrically charged for several hours and spark dangerous lightning if a shuttle flies near or through them.

Such a bolt of lightning could prove catastrophic.

The launch weather rules involving these types of clouds are complicated and take into account if or when the clouds detach, if they are transparent, how long it's been since the most recent lightning strike, the cloud's distance from the shuttle's flight path, the temperature at various altitudes and the amount of electrical charge measured in the air.

Should the weather or an easily solved last-minute technical problem scrub the launch attempt, NASA will try again on Friday, said NASA test director Steve Altemus.

Unfortunately the weather forecast is slightly worse for Friday, with only a 30 percent chance of acceptable conditions as afternoon thunderstorms remain a threat.

If the launch scrubs today and Friday, NASA will wait until Monday for the next attempt, giving the launch team the weekend to top off liquid hydrogen and liquid oxygen supply tanks within Endeavour, Altemus said.

Once airborne the shuttle is to fly a 12-day mission that will feature the installation of a new Mobile Base System for the station's robot arm, the repair of a joint on that same arm, and the unloading of two new racks of science equipment for the station's Destiny laboratory.

An on time launch today would lead to the shuttle's return to Florida on June 11, with a landing about 2:13 p.m. EDT (1813 GMT).