Recent ensemble forecasts from multiple models indicate a daytime maximum of 19–20°C in Istanbul on September 28, as a frontal system and northerly winds advect cooler maritime air, bringing widespread rain and extensive cloud cover that limits solar heating. This places the expected high several degrees below the late-September climatological average of 23–25°C. Trader consensus aligns closely with these outputs, though minor variations in precipitation timing or wind strength could shift the peak by 1°C. Updated guidance from the Turkish State Meteorological Service and global model runs over the next 48 hours will likely refine the precise maximum.