Short answer: Jacksonville summers trade an hour of afternoon thunder for a green city, and locals plan around it: museums, coffee shops, breweries, bookstores and long lunches, then back outside when it blows through.
The museum move. Art museums, science centers and the historic houses of the older neighborhoods make the classic rainy afternoon. They are calmer on stormy days, and you leave knowing the city a little better.
The long lunch. A summer storm is the best excuse the 904 offers. Pick a spot from /directory/?cat=Food+%26+Drink with a covered patio, order dessert, and let the radar do its thing.
The indoor circuit. Coffee shops for the laptop hours, /directory/?cat=Bars+%26+Nightlife for trivia and pool once evening arrives, and /directory/?cat=Retail+%26+Shopping for the wandering kind of afternoon. The /games page works from the couch, no umbrella required.
With kids. Indoor play spaces, libraries and family spots across /directory/?cat=Kids+%26+Family absorb an astonishing amount of small-person energy on a wet Saturday.
The local secret. Most summer storms pass in under an hour, and the beach after one is empty, cool and gorgeous. Check /events before you write off the evening; the sky usually apologizes by sunset.