At G5, induced fields are strong enough that grid operators across a wide map have to manage voltage, and aurora can be seen in places that have no cultural memory of it. Satellites drag, HF radio is unreliable, and pipelines see currents that corrosion engineers write papers about.

NOAA’s geomagnetic scale runs up to G5. The lower steps are common near solar maximum and mostly a matter for operators and for aurora watchers at high latitude. G5 is the top of that scale: extreme. The trouble is not confined to the usual northern belt. On that scale it can reach the Gulf.

The United States has not ridden a modern G5 on today’s inverter-heavy grid. The last storms in that class were earlier eras of the system. That uncertainty should produce drills, not documentaries only.

Inverters, electronic controls, and a grid that leans on devices with software in them are a different machine from the one that met the great storms of earlier eras. That is not a prediction that everything fails. It is a reason the old stories are an incomplete rehearsal. The equipment has changed under the same sun.

What operators do at the top of the scale is the same family of actions as a lesser storm, pushed harder: manage voltage, consider what to take offline, watch harmonics and heating, talk to neighbors on the interconnection. The difference is how much of the map is doing it at once, and how little spare equipment exists if they guess wrong.

You will know it is serious if utilities and emergency managers speak before the sky does. If they do, believe the boring instructions.

Aurora at a latitude that has never seen it is a spectacle and a late cue. The useful cue is the watch and the warning, which can arrive before the sky is worth photographing. Boring instructions are the ones you have already heard: do not run a generator in a closed garage, check the person who needs power for a device, fill water before the system is busy. A G5 does not require a new personality. It requires the ordinary things done early.