**Forecast models indicate a cooling trend and light rain or showers arriving in Istanbul on September 22, 2026, shifting daytime highs from the mid-20s°C seen earlier in the week to roughly 19–23°C.** This range aligns with the tightly clustered market probabilities (each of 19–23°C at ~20%), reflecting trader consensus on the timing of a frontal passage while capturing remaining uncertainty in exact peak values. The primary drivers are a shift to northeasterly flow and increased cloud cover ahead of or during light precipitation, which limits daytime heating and promotes modest evaporative cooling. Regional climatology shows September highs normally decline through the month (averaging near 23°C early on, lower later), with typical influence from Black Sea and Mediterranean air masses; the current setup accelerates that seasonal drop by a day or two. Official guidance from sources such as the Met Office and local meteorological services consistently places the maximum in the low-20s°C range under mostly cloudy to overcast skies with scattered showers, though small differences in model timing, rainfall rates, or wind strength can easily swing the daily peak by 1–2°C. Because probabilities remain evenly distributed across 19–23°C and tail outcomes below 18°C or above 24°C stay very low, markets are pricing in modest forecast spread rather than a single dominant scenario. Updated model runs and observational data over the next 24–48 hours will likely narrow this band before resolution.