FRIDAY, AUGUST 25, 2006 1430 GMT (10:30 a.m. EDT) The shuttle Atlantis' countdown continues to tick smoothly toward launch on a space station assembly mission. Liftoff is targeted for 4:30 p.m. Sunday. The latest forecast calls for a 60 percent chance of favorable weather, improving to 80 percent "go" Monday and Teusday. The concern Sunday is for possible afternoon thunderstorms within the launch area. =================================================