Forecast models from the National Weather Service and major ensembles indicate a highest temperature near 70–73°F for San Francisco on August 18, consistent with typical late-summer conditions moderated by the Pacific marine layer. This shallow layer of cool, moist air advected onshore by prevailing westerly flow suppresses daytime warming, especially near the coast and at official reporting sites such as KSFO, while a weak upper-level ridge may allow modest afternoon heating if the inversion lifts slightly. Historical August highs average around 70°F, with rare excursions above 75°F when the marine layer thins or offshore flow strengthens. Recent guidance shows no significant heat advection or fog clearance that would push readings into the mid-70s or higher, keeping trader-implied probabilities concentrated on the 70–73°F bin as the most likely outcome under current atmospheric patterns.