**Trader consensus on the San Francisco high temperature for September 17 centers on the narrow 70–73°F range, with 70–71°F holding a slim 43.5% market-implied edge over 72–73°F at 36.5%.** Official National Weather Service guidance and ensemble forecasts indicate a high near 70–73°F under partly cloudy skies, with morning marine-layer stratus potentially delaying warming before southwesterly flow allows limited afternoon clearing. This outcome aligns closely with the long-term September average high of roughly 70.7°F, while recent early-month heat (including a 94°F reading on September 9) has given way to more typical onshore conditions that suppress extremes. Model spread on cloud timing and boundary-layer mixing creates the tight probability split, with lower outcomes (68–69°F) priced at just 11% reflecting limited downside risk from persistent fog or stronger marine influence. Updated NWS and NOAA model runs expected overnight will clarify whether modest warming pushes probabilities toward the upper bin.