3:40 p.m. ET: RNC UPDATE: A GOP source confirms to ABC's Jonathan Karl that Clint Eastwood, the actor, director and former Republican mayor of Carmel, Calif., will make a special cameo appearance at the Republican National Convention before Mitt Romney speaks Thursday night.
2:02 p.m. ET: ABC's Matt Gutman's team on the ground in LaPlace, La., snapped this shot of the incredible flooding in the region:
12:35 p.m. ET: DEVELOPING: Heavy rainfall from Tropical Storm Isaac is threatening a dam in Tangipahoa Parish in Southern Mississippi, leading local officials to order a mass evacuation.
11:40 a.m. ET: Tonight is Mitt Romney's big night, when he will address the Republican National Convention and officially accept the nomination for president.
10:08 a.m. ET: The "Nightline" team's thoughts, prayers and support are with our friend and colleague, "Good Morning America" anchor Robin Roberts, who announced this morning she is starting her medical leave.
"The greatest tragedy is to have the experience and miss the meaning," Roberts said. "I am determined not to miss that meaning because through it I have discovered that I have been given many gifts, gifts that will carry me through this difficult time."