You should always have - if possible of course - at least weeks of rice and beans, a water filter (they cost like 30$ for thousands of gallons of good filtering), knowledge of the nearest source of water, cash available on hand. If you can, also, some solar energy and a battery with it. And if in the US (I'm not) I guess a gun. And testing all that beforehand and being familiar with their use is important too (except perhaps the water filter - they're simple to use and last much less longer in storage if used even once, due to the residual water).

While I despise traditional preppers - mostly for their individualism, the best prepping is community - we're entering a planetary cool zone IMO and the basic minimum above is always a good idea if you can afford it. During shit like Katrina or even that recent power outage in Spain, the above is useful. And these events will become more and more common. It's also usually part of the governmental recommendations even in imperial countries, it's just rarely communicated on.

Don't store water in plastic in your car, even food grade translucent HDPE will still allow microorganisms to grow because of the light. Maybe a widemouth glass carboy coated in white food grade silicone?