I don’t understand. I don’t know what to do. Tantrum.

My son wanted to go outside. So we get dressed and ready (snow clothes) I open up the door so we can go outside. The door he was waiting at. he starts screaming and goes out to the porch and slams his head off the porch. He’s now been having the worst tantrum he’s ever had. He’s been nonstop screaming for 30 minutes. What do I do? I don’t get it. He’s never really been much of a tantrum kind of kid and was usually easy to pull out of it if one started. I’ve read about talking out the problem but how can I when I don’t know what it is? Anytime I say a single word he screams louder. I’ve tried distracting him/ doing something he likes and all that gets is more screams. I’ve even tried turning on his favorite video and that was just more screams. I can’t leave the room because if I do he will slam his head on the ground. Send help and aspirin. (This is the send time since he’s turned 3 that he’s done that. He wanted to go to a store, we went he threw himself down on the ground and started shrieking like I was beating him that one only lasted 5-10 minutes.)