The trick to spotting twisters, as in "when to worry and when not to", is the color. Once the sky takes on a color that doesn't exist, so much so that nobody has yet managed to describe it adequately but once you see it you know that you're seeing it, you head for cover. Or not. If it doesn't hit you it won't matter since the damaged is extremely localized (as in one side of the street gone, the other side untouched), and if it DOES hit you, you're dead, so why worry?Once we had supercell mesocyclones come up from S.France and I was enjoying a coffee on the balcony and I was checking off the cloud structures and types, only to look up and almost see a funnel start spinning up. Where I live we get very cool clouds.
But that color...