**Persistent marine layer and onshore westerly flow are holding San Francisco’s overnight minimum near seasonal norms of 56–58 °F.** National Weather Service and private forecasts issued 20–21 August consistently project a Friday low in the upper 50s (most often 57 °F) under mostly cloudy to partly cloudy skies, light-to-moderate west winds, and typical summer marine influence from the cold California Current. Recent observations reinforce this: the 20 August minimum at San Francisco International Airport reached 56–57 °F, and the preceding week showed similar values. No synoptic shift—such as offshore flow or strong high pressure—has appeared to push temperatures appreciably higher or lower. Historical climatology for 21 August (1991–2020 normals ≈ 55.9 °F low) further anchors trader expectations, resulting in the heaviest volume and highest implied probabilities on the 56–59 °F bins. Minor uncertainty remains around exact cloud timing and localized fog depth, which can produce 1–2 °F differences between downtown and airport sensors, but current model consensus keeps the distribution tightly centered on upper-50s readings.