The space shuttle Atlantis is seen on the launch pad at Kennedy Space Center, Florida on June 17, 2011. (JEAN-LOUIS SANTINI/AFP/GETTY IMAGES)

The official National Weather Service forecast for Cape Canaveral is:

Showers and thunderstorms likely, mainly after noon. Mostly cloudy, with a high near 87. South southwest wind around 10 mph. Chance of precipitation is 70%.

Spectators hoping to catch the launch Friday need to hope that rain holds off until the afternoon. However, those prospects don’t seem terribly high due to the arrival of an ample feed of tropical moisture.

Two high-resolution forecast models simulate moderate to heavy showers producing around 0.25-0.5” of rain near Cape Canaveral betwee 11 a.m. and 2 p.m. on Friday. (NOAA National Weather Service)

In its forecast discussion published early this morning, the National Weather Service Office in Melbourne, Florida noted the tropical wave, arriving from the Atlantic ocean via the easterly trade winds, will merge with a disturbance over the Deep South and mid-Atlantic, shunting south an area of high pressure the might suppress the rain. The consequence?: “...deep south/southwesterly flow across central Florida that will tap deep tropical moisture over the Florida straits/northwest Caribbean.”

An ensemble of forecast models indicate a 50-60 percent chance of measurable convective rain over Cape Canaveral between 11 a.m. and 2 p.m. Friday.

NASA officials are not optimistic about Friday’s launch prospects.

“It’s not looking favorable right now for launch,” said launch weather officer Kathy Winters this morning at a news conference. Winters said there’s a 70% chance weather will prohibit the launch.

If Friday’s launch is canceled, NASA will try again on Saturday and, if necessary, Sunday. Saturday’s launch prospects, however, are also iffy with a 60 percent chance of showers and storms. The best weather conditions are likely to occur on Sunday, when the chance of rain drops to 50 percent.

NASA is expecting between 500,000 and 750,000 spectators for the last-ever mission in the agency’s 30-year space plane program.

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