Recent forecasts from multiple models, including ECMWF and GFS runs, indicate an overnight low near 17–18°C for Istanbul on September 21 under mostly clear to partly cloudy skies with light northeast winds. This setup favors moderate radiative cooling, yet the near-even market split between a 15°C-or-below outcome and 25°C-or-higher reflects substantial uncertainty in boundary-layer conditions. Key variables include exact cloud cover timing, which can trap heat and raise minima by several degrees, and any late shifts in wind or advection from the Black Sea. Historical September variability around 16–19°C lows adds context, while the next model update cycle and real-time station observations tonight will likely refine the range before resolution.