Forecast models from sources like the GFS indicate Istanbul's overnight minimum on September 13 will likely settle near 18–20 °C under partly cloudy skies and light northeasterly flow. Mild maritime air from the Sea of Marmara and limited radiative cooling due to cloud cover and residual moisture keep temperatures from falling sharply, consistent with mid-September climatology where average lows range 17–20 °C. Recent model runs show minor spread in the exact nadir, with some guidance hinting at brief dips near 18 °C while others hold above 19 °C, explaining the tight clustering of market-implied probabilities around the 15–20 °C thresholds and the relatively low odds assigned to extremes above 21 °C or below 16 °C. Updated evening observations and final station readings will resolve the precise value.