**Persistent cloud cover, widespread showers, and breezy northeasterly flow off Lake Michigan are the main factors anchoring trader expectations for Chicago’s September 20 low in the low-to-mid 60s °F.** Official National Weather Service and WeatherBug guidance issued September 19 point to an overnight minimum of roughly 57–61 °F, with the calendar-day low for the 20th most likely falling between 60–65 °F as temperatures remain elevated through the evening before gradual radiational cooling under mostly cloudy skies. The 44 % market weight on 64–65 °F and 24 % on 62–63 °F reflect this consensus, tempered by uncertainty in exact timing of clearing and wind speeds that could either enhance mixing (keeping the low higher) or allow more cooling (pushing it toward 60 °F). Historical September averages near 57 °F provide context, but the current synoptic setup—a slow-moving system delivering 1–3 inches of rain and sustained northeast winds of 15–20 mph with gusts to 30 mph—has kept overnight readings several degrees above normal. No major forecast shifts have occurred in the last 24 hours; the primary variables traders are watching are the next model runs and any rapid clearing that could allow the temperature to dip below 60 °F before midnight.