Persistent thunderstorm activity and associated cloud cover from a moist tropical air mass represent the dominant near-term driver capping Miami highs near the upper 80s on September 21, per National Weather Service guidance and model consensus. Light steering winds and high humidity further limit afternoon heating while scattered to numerous showers reduce solar insolation. Trader emphasis on 90–91 °F (30 % implied probability) versus the 86–89 °F cluster reflects uncertainty in storm timing and coverage; stronger or earlier convection would favor the lower bins, while any unexpected clearing could push readings into the low 90s. Historical late-September baselines hover near 88 °F, but the current pattern of elevated precipitable water and weak pressure gradients supports the market’s broad distribution across the 84–93 °F range. Updated NWS and global model runs over the next 48 hours will refine these variables ahead of market resolution.