**Current shallow marine layer and persistent onshore flow are the dominant drivers keeping overnight lows near seasonal norms for San Francisco on August 27.** National Weather Service guidance and recent model runs describe a marine layer roughly 1,000–1,500 feet deep, capped by a temperature inversion that limits radiational cooling while breezy westerly winds (10–20 mph with gusts higher) promote mixing and prevent deeper drops. This setup aligns with historical August averages of 56–58 °F for minimum temperatures and current point forecasts centered in the upper 50s to low 60s. Trader-implied probabilities reflect remaining uncertainty around layer depth, exact wind speeds, and localized microclimate effects across reporting stations, with the 51 °F-or-below outcome edging ahead at 38.5 % amid modest model spread. A trough approaching later in the week is expected to strengthen onshore flow and potentially cool readings further after the 27th, but near-term resolution hinges on the next NWS forecast updates and any overnight observations confirming the inversion strength.