Being prepared is very important, both mentally and physically, and planning ahead will put you ahead of the game. Most people will wait until they know something is coming, and will then hit the stores and start buying up water, snacks, and other supplies. There may not even be enough time for that.

Both my wife and I have small BOBs (bug out bags) in our cars with the most important stuff - flashlight, spare batteries, a couple of glow sticks, matches, knife, multi-tool, Band-Aids, antibacterial cream, baby wipes, painkillers, etc. Mine also has Kahr PM9 in it with a couple of spare magazines. These bags are actually more of a "get-me-back-home" bags. We use Camelbaks, so they also have built-in bladders for fresh water. At home we have a large BOB that is an improved "version" of the above, with a lot more supplies in there - a good fixed blade, walkie-talkies, radio, spare batteries for everything, baby supplies (wipes, diapers, formula, etc.), 3-day supply of MREs for each of us, water filters, a couple of blankets, compass, first aid kit, a few hundred $ in small bills, a sealed pouch with copies of our passports, DLs, SS cards and other important documents, Glock 17 with several spare magazines, and some other stuff that I'm forgetting right now. That's the bag that will be the first thing to grab if we have to leave in a hurry. If we have to bunker-in for something like Katrina (and if there wasn't enough time to leave), there is at least 2-3 weeks supply of bottled water, canned food, sealed snacks, etc. Oh, and plenty of ammo, too, in case those less prepared decide to help themselves with my supplies.