**Forecast consensus centers on an overnight low of 57–59°F for San Francisco on September 19, 2026, aligning with the market’s heaviest weighting on the 58–59°F bin (42.5%) followed by 56–57°F (23%) and 60–61°F (22%).** Official National Weather Service guidance and multiple model runs project partly cloudy to mostly cloudy skies with light to moderate west-southwest winds, keeping the marine layer in place and limiting radiative cooling. This produces minimum temperatures in the upper 50s, consistent with the September climatological average low of ~55.8°F and recent nightly readings of 57–58°F. Primary scientific drivers include the persistent influence of the cold California Current and coastal upwelling, which moderates temperatures even under weak high pressure. No strong offshore flow, clear-sky radiative cooling events, or warm advection are indicated in current model guidance, reducing the probability of readings outside the mid-to-upper 50s. The slight edge to 58–59°F over 56–57°F reflects the latest runs favoring marginally milder conditions from increased cloud cover and a modest marine layer depth. Traders appear to be pricing the high reliability of near-term forecasts for this timeframe, with lower probabilities assigned to outliers that would require unexpected model shifts such as stronger clearing or a surprise wind change. Updated NWS briefings and morning model runs on September 18–19 will be the key near-term catalysts.