Before the 13-minute warning, your area probably has been under tornado watch for hours, which just means the weather conditions are favorable for producing tornadoes. This is when you should get your "tornado" gear together if they're not in one place already. In other words, you execute step one of the plan. So if you hear the siren, you execute the rest of the plan.

In my case, as soon as I get a watch, I get everything ready -- in addition to my survival kit, I also have my laptop (can't live without it). Then as soon as I get a warning, I head to the shelter area right away as I continue to monitor the weather radio.