**Recent NWS and model guidance for Chicago shows a cooling trend driven by northeast flow, widespread cloud cover, and periods of showers and thunderstorms associated with a passing system.** Official forecasts from the National Weather Service Chicago office indicate daytime highs near 68°F on September 20 with showers likely, followed by a Sunday night low around 55°F, while other extended outlooks place the daily minimum in the low-to-mid 60s amid variable precipitation timing and intensity. **Market-implied odds cluster tightly around 60–65°F (roughly 80% combined probability across the top three bins) because short-range models differ modestly on the precise overnight minimum.** Key differentiating factors include the degree of cloud cover and precipitation—which limit nocturnal radiative cooling—and the strength of northeasterly advection bringing cooler, more stable air. Clearer breaks could allow temperatures to dip toward the lower 50s, while persistent clouds, rain, or stronger mixing keep readings in the mid-60s. Historical September averages for the date (high ~74°F, low ~57°F) provide context, but the current synoptic setup favors a milder outcome than climatology. **Upcoming NWS forecast updates and new model runs over the next 24–48 hours will likely refine the exact low, especially as the system’s timing and moisture details evolve.** Traders are pricing in this scientific uncertainty rather than committing to extremes.