Just for the exercise:
Do your famous Google News search on the Superdome. What day does the Superdome first show up on the news as an evacuation shelter site?
As far as people gravitating to the Convention Center:
Look at the Convention Center's location on a map. Where is it? Note prime evacuation routes, and remember which bridges were blocked by cops. Wonder of wonders, the people living in New Orleans knew where the Convention Center was.
Your post is the first reference I've seen to any "radio announcements."
Your "moving trucks" moving "thousands of people" reference:
At the nearby United Medical Rehab Hospital, 14 patients, 11 staff members and their families awaited their saviors.
Nurse Bernadette Shine said the facility was nearly out of oxygen, and several diabetic patients had been without dialysis for nearly a week. After the fruit cocktail and peanut butter ran out, the staff broke into the candy and drink machines for sugary items to keep patients from going into shock.
"There are people that are not going to make it," Shine said, her voice cracking. "One I've known since I was 10 years old. But we did what we could for them. We did everything we could for them."
After several hours, a small fleet of rented moving trucks showed up to take the people to the downtown convention center so they could be taken out of the city. Police herded people up metal ramps like cattle into the unrefrigerated boxes.