A lightning storm, commonly referred to as many players as an "E-Storm" is a rare occurrence that takes place during night; an E-Storm was identified by purplish-whitish lightning bolts across the sky, as well as many light flashes across the entire map.
But it is not just a storm that happens at night.
During an "Electric Storm," gem ores become more common and you have the chance to get an very rare ore under the name of Electrosteel, which has the color and material of a neon white. To gain Electrosteel during an E-Storm, you must have an Angelwood Sapphire pickaxe on a non-spawn island, which includes Fenwick, Freeport, and Perth.
The chance of an E-Storm to happen are very slim, coming in about for a 8% chance every night.