Current NWS and model guidance points to overnight lows in the mid-to-upper 60s on September 30 in Chicago, driven by southerly flow advecting milder air and increasing cloud cover that limits radiational cooling. This setup contrasts with the 1991–2020 average low of 52.7°F and historical September minima often dipping into the 40s–50s under clearer, northerly regimes. Key variables differentiating market buckets include wind speeds (stronger mixing raises minima) versus calm conditions favoring drops toward 55°F or below, plus any late-day frontal timing or shower activity that could alter the diurnal range. Traders weigh these against recent warm bias in short-range forecasts, producing closely matched probabilities around the 55°F threshold and 64–65°F range. Updated model runs and NHC-adjacent guidance over the next 48 hours will likely sharpen resolution odds.