Data: two carriers if you can (coverage gaps kill chases), a roof-mounted booster if you are serious, and offline maps downloaded before you leave — the storm does not care that your app needs signal. Radio: a NOAA weather radio and, ideally, a ham license — repeaters work when cell towers are overloaded, and Skywarn nets want your reports. Power: a 12V inverter or power station; every device dies at the worst moment otherwise.
Camera: whatever you have, on a mount that does not rattle — rain-X on the windshield does more for your footage than a lens upgrade. Safety: full tank at every stop, paper atlas, first-aid kit, and a hard rule about hail cores: you do not drive into one to save ten minutes.
Argue about the rest in the Storm Chasing board — gear threads are a community tradition for a reason.