This story was updated at 2:50 p.m. EDT.

CAPE CANAVERAL, Fla. - Poor weather again thwarted the launch of NASA's space shuttle Discovery Sunday following a day of delays due to thunderstorms and thick clouds.  

NASA launch officials scrubbed their second launch attempt in two days for Discovery'sSTS-121 mission at 1:15 p.m. EDT (1715 GMT) after a seesaw of sunshine, rain and thunder here at Kennedy Space Center (KSC).

"We've concluded that we're not going to have a chance to launch today," NASA launch director Michael Leinbach told the STS-121 astronauts aboard Discovery. "We've decided to terminate the count for today and stand down for 48 hours."

The decision, which came after it became clear there would be no chance to loft Discovery today, now pushes the STS-121 launch target to Tuesday, July 4 at2:37:51 p.m. EDT (1837:51 GMT).

"We copy, and looking out the window it doesn't look good today and we think that's a great plan, " STS-121 shuttle commander Steven Lindsey told launch controllers.

A press conference on today's launch scrub is scheduled for 3:00 p.m. EDT (1900 GMT).

Discovery's STS-121 mission is NASA's second shuttle flight since the 2003 Columbia accident. The 12-day spaceflight is expected to deliver vital supplies to the International Space Station (ISS) and a new crewmember - Thomas Reiter of the European Space Agency (ESA) - for the orbital lab. Shuttle astronauts are also expected to test shuttle inspection and repair techniques during the spaceflight.

Stormy countdown

At the start of today's launch attempt, weather forecasters gave the space shot just a30 percent chance of leaving Earth today at 3:26 p.m. EDT (1926 GMT). Clouds, rain or nearby storms violated at least one flight rule - if not more- at one time or another.

"It's almost easier to talk about what's not violating flight rules than what is," NASA launch commentator Bruce Buckingham said early in the day.   

A July 4^thliftoff at 2:37:51 p.m. EDT (1837:51 GMT) on Tuesday carries the best chance of launching Discovery this week, said U.S. Air Force Lt. Kaleb Nordgren, of the45^th Weather Squadron at Cape Canaveral Air Force Base.

After the upcoming U.S. holiday, weather forecasts predict worsening conditions during launch time at KSC, Nordgren added.

Over the next two days, pad workers will top off the super-cold liquid hydrogen and liquid oxygen propellants that will power Discovery's fuel cells during its planned 12-day mission. That activity will allow the shuttle's power system some extra margin for a possible mission extension day during the STS-121spaceflight.

NASA officials and the STS-121 crew are hoping for that 13^th mission day.

The extra time could lead to a third spacewalk, which mission managers pulled off theSTS-121 flight timeline earlier this year, that will test shuttle heat shield repair methods.

Woeful weather

Weather conditions are always a concern during shuttle launches, especially given NASA's slim, 10-minute window to loft orbiters toward the International Space Station (ISS).

NASA mission controllers typically target the middle of Discovery's launch window in order to secure the best optimum flight profile for the two-day ISS trip.

But in away, worrying only about weather is a relief, NASA chief Michael Griffin told SPACE.com.

"It's comforting because we always have the weather, and most of the time we always have something else," Griffin said earlier today. "And as best as we can tell right now we don't have anything else."

NASA astronaut Scott Kelly, a shuttle commander and the identical twin brother ofSTS-121 pilot Mark Kelly, said his brother's enthusiasm remained high despite facing the weather scrubs.

"He's getting ready to go into space, of course he's gung ho," Scott Kelly said of his twin during today's launch countdown.

Scott Kelly, himself, is no stranger to launch scrubs. He served as shuttle pilot during NASA's STS-103 mission to the Hubble Space Telescope in 1999. That mission, also aboard Discovery, suffered several days of delays, including one weather scrub with the crew aboard, before finally reaching orbit.

 "We've launched when we've had a 90 percent of no go and we haven't launched when we've had 10 percent of no go," Kelly said. "You have to be prepared."

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