Well, you have four problems. First, due to geographic concentration of refineries on the Gulf coast, and the shutdown of most of them, we have a temporary production halt. The second problem is that gasoline is formulated to last about two weeks and is produced on a "just in time" basis, so there is little reserve. Third problem is it's Labor Day weekend for the whole country, and the local populations in certain areas have increased by a good margin from displaced flood victims. The BIGGEST problem is that people as a large group get panicky and do stupid things at the drop of a hat, particularly if they failed to think their situation through as it was developing. Social media and the news makes panic spread at an astronomical rate, worse than ever in our history.
There are a lot of people who DID flee the storm and went a minimum logical distance, which is SA/Austin/D-FW. Those people need fuel too, along with the rest of us, so demand is up anyway. Couple that with panicked people all filling up an extra 10-20 gallons worth of cans each (or more), and in just a few days the at-the-pump demand may have increased 2-3 times, which is drastic. Even a 20% increase in demand with normal supply is tough. Remember what happened to .22 LR back in 2007?
The reality is there really isn't much of a fuel shortage, just temporary supply interruptions and very high local demand in some parts. Prices will go up, but there's plenty of crude to refine and the local issues will be solved in short order.