Persistent onshore westerly flow and a variable marine layer continue to anchor San Francisco highs near the long-term August average of 67–68°F, producing tight clustering of market-implied odds around the 66–71°F brackets. National Weather Service guidance and ensemble models show only modest day-to-day variability for August 21, with afternoon sea-breeze reinforcement and limited solar heating under stratus or partial clearing keeping readings from exceeding the upper 60s to low 70s. Recent model runs indicate slight uncertainty in marine-layer depth and onshore wind timing, which can shift the daily maximum by 2–3°F and explains the near-even distribution across the three leading outcomes. Historical analogs confirm that significant offshore flow or clear-sky heating capable of pushing readings to 74°F or higher remains rare this late in the season. Updated NHC and local forecast discussions tomorrow morning will provide the final observational constraints before resolution.