Persistent marine layer and moderate onshore flow from the cool Pacific Ocean are the dominant factors capping daytime warming in San Francisco on August 13, 2026, keeping afternoon highs clustered in the mid-to-upper 60s°F according to National Weather Service and AccuWeather guidance. This temperature inversion limits solar heating while steady southwest winds advect cooler maritime air inland, a classic summer pattern that has produced recent daily maxima of 66–70°F. Current observations show partly to mostly cloudy skies with the marine layer lingering near the coast, suppressing peaks above 70°F. Trader-implied odds align closely with these conditions, favoring 68–69°F as the modal outcome while assigning meaningful probability to 66–67°F if thicker fog persists. No significant model shifts or offshore wind events are indicated that would alter the coastal cooling regime before the daily maximum is recorded.