Race director Rui Marques issued the signal at 13:17 local time, 2 hours 43 minutes before qualifying, as officials discussed bringing Sunday's 16:00 race start forward.
Forecasts of heavy, persistent thunderstorms could make racing impossible, while US lightning rules require any outdoor event to stop immediately if a strike occurs within eight miles.
A 30-minute restart clock applies after each nearby strike, and any new strike resets it; last year's Miami sprint was also delayed by heavy rain.
Should F1 invest in weather-proofing circuits instead of constantly rescheduling events due to predictable forecasts?
Do F1's wet-weather rules enhance safety or just create new strategic loopholes for teams to exploit?