The scariest thing has this taking almost 24hrs to go from vero to Jacksonville. That's a lot of rain in Central Florida. The Sentinel had a great article saying that most subdivisions were built to withstand 7 inches of rain over a 24 hour period. If, and that's a big IF, Frances were to slowly creep over Florida, there would be massive flooding.
Here's a quote:
But double or triple that amount of rain, and streets would flood, and the St. Johns River would rise and back up creeks and tributaries that feed into it.