Traders assign the highest implied probabilities to a 25–27°C low for Singapore on September 21 because official forecasts from the Meteorological Service Singapore and numerical models consistently project nighttime minima in that narrow range amid prevailing southwest monsoon conditions. Persistent high humidity near 80–90% and variable cloud cover from scattered thundery showers limit radiative cooling, while light southeasterly winds prevent significant drops below the seasonal baseline of roughly 25°C. The tight clustering of odds around these three outcomes reflects genuine forecast uncertainty in the precise overnight minimum, with lower probabilities for 24°C or below reflecting the low historical frequency of such readings in September. Updated model runs and local observations through the evening will be the key near-term catalysts.