Recent frontal passage and rainfall through September 18 have advected cooler, moist air into the Yangtze valley, elevating boundary-layer humidity and suppressing daytime heating. For the 19th, official guidance from the China Meteorological Administration indicates clearing skies with light northerly flow, promoting modest nocturnal radiative cooling but limited by residual moisture and urban heat-island effects; ensemble forecasts cluster the overnight minimum near 21–23 °C. This narrow range sits between the market’s 17 °C-or-below and 27 °C-or-higher bins, where small differences in cloud cover, wind speed, or dew-point depression can shift the low by 2–4 °C. Traders appear to price residual model spread and the possibility of either stronger post-frontal clearing or lingering low-level moisture as equally plausible.