Traders see the lowest temperature in San Francisco on September 17 closely split between the 56–57 °F and 60–61 °F ranges because official observations and model guidance showed a strong marine layer with persistent onshore flow from the Pacific moderating overnight cooling. National Weather Service data recorded a minimum of 63 °F at San Francisco International Airport amid haze and high humidity, well above the 1991–2020 normal of 56 °F, as the layer of stratus and light westerly winds prevented radiational cooling typical of clearer autumn nights. This outcome aligns with historical analogs when coastal upwelling and fog keep lows in the upper 50s to low 60s rather than dipping toward the mid-50s or below. Key variables traders weighed included the depth of the marine layer, wind speeds overnight, and any late-day clearing that could have allowed greater temperature drops, with new observational data from the National Weather Service likely to refine implied probabilities before resolution.