Recent northerly flow and an approaching frontal system are driving cooler, wetter conditions across the Marmara region, with multiple models converging on daytime highs near 20°C and overnight lows of 15–18°C for Istanbul on September 28. Persistent cloud cover and moderate-to-heavy rain will limit daytime warming while suppressing radiative cooling at night, keeping the daily minimum well above seasonal extremes. Trader sentiment remains closely balanced between the ≤12°C and ≥22°C outcomes because short-range ensemble spreads still allow for modest forecast shifts in wind direction or precipitation timing that could alter the exact low by a few degrees. Historical September minima average near 17°C, providing context for why mid-range specific-temperature bins trade near even implied odds amid genuine model uncertainty.